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Two-Level Shape Changes of Polymeric Microcuboids Prepared from Crystallizable Copolymer Networks
(2017)
Polymeric microdevices bearing features like nonspherical shapes or spatially segregated surface properties are of increasing importance in biological and medical analysis, drug delivery, and bioimaging or microfluidic systems as well as in micromechanics, sensors, information storage, or data carrier devices. Here, a method to fabricate programmable microcuboids with shape-memory capability and the quantification of their recovery at different levels is reported. The method uses the soft lithographic technique to create microcuboids with well-defined sizes and surface properties. Microcuboids having an edge length of 25 mu m and a height of 10 mu m were prepared from cross-linked poly[ethylene-co-(vinyl acetate)] (cPEVA) with different vinyl acetate contents and were programmed by compression with various deformation degrees at elevated temperatures. The microlevel shape-recovery of the cuboidal geometry during heating was monitored by optical microscopy (OM) and atomic force microscopy (AFM) studying the related changes in the projected area (PA) or height, while the nanolevel changes of the nanosurface roughness were investigated by in situ AFM. The shape-memory effect at the microlevel was quantified by the recovery ratio of cuboids (R-r,R-micro), while at the. nanolevel, the recovery ratio of the nanoroughness (R-r,R-nano) was measured. The values of R-r,R-micro,,micro could be tailored in a range from 42 +/- 1% to 102 +/- 1% and Rr,nano from 89 +/- 6% to 136 +/- 21% depending on the applied compression ratio and the amount of vinyl acetate content in the cPEVA microcuboids.
Humanoid robots, prosthetic limbs and exoskeletons require soft actuators to perform their primary function, which is controlled movement. In this wont we explored whether crosslinked poly[ethylene-co-(vinyl acetate)] (cPEVA) fibers, with different vinyl acetate (VA) content can serve as torsional fiber actuators. exhibiting temperature controlled reversible rotational changes. Broad melting transitions ranging from 50 to 90 degrees C for cPEVA18-165 or from 40 to 80 degrees C for cPEVA28-165 fibers in combination with complete crystallization at temperatures around 10 degrees C make them suitable actuating materials with adjustable actuation temperature ranges between 10 and 70 degrees C during repetitive cooling and heating. The obtained fibers exhibited a circular cross section with diameters around 0.4 +/- 0.1 mm, while a length of 4 cm was employed for the investigation of reversible rotational actuation after programming by twist insertion using 30 complete rotations at a temperature above melting transition. Repetitive heating and cooling between 10 to 60 degrees C or 70 degrees C of one-end-tethered programmed fibers revealed reversible rotations and torsional force. During cooling 3 +/- 1 complete rotations (Delta theta(r) = + 1080 +/- 360 degrees) in twisting direction were observed, while 4 +/- 1 turns in the opposite direction (Delta theta(r) = - 1440 +/- 1360 degrees) were found during heating. Such torsional fiber actuators, which are capable of approximately one rotation per cm fiber length, can serve as miniaturized rotary motors to provide rotational actuation in futuristic humanoid robots.
The interplay of an enzyme with a multiblock copolymer PDLCL containing two segments of different hydrophilicity and degradability is explored in thin films at the air-water interface. The enzymatic degradation was studied in homogenous Langmuir monolayers, which are formed when containing more than 40 wt% oligo(epsilon-caprolactone) (OCL). Enzymatic degradation rates were significantly reduced with increasing content of hydrophobic oligo(omega-pentadecalactone) (OPDL). The apparent deceleration of the enzymatic process is caused by smaller portion of water-soluble degradation fragments formed from degradable OCL fragments. Beside the film degradation, a second competing process occurs after adding lipase from Pseudomonas cepacia into the subphase, namely the enrichment of the lipase molecules in the polymeric monolayer. The incorporation of the lipase into the Langmuir film is experimentally revealed by concurrent surface area enlargement and by Brewster angle microscopy (BAM). Aside from the ability to provide information about the degradation behavior of polymers, the Langmuir monolayer degradation (LMD) approach enables to investigate polymer-enzyme interactions for non-degradable polymers. (C) 2016 Elsevier Ltd. All rights reserved.
Polyether ether ketone (PEEK) as a high-performance, thermoplastic implant material entered the field of medical applications due to its structural function and commercial availability. In bone tissue engineering, the combination of mesenchymal stem cells (MSCs) with PEEK implants may accelerate the bone formation and promote the osseointegration between the implant and the adjacent bone tissue. In this concept the question how PEEK influences the behaviour and functions of MSCs is of great interest. Here the cellular response of human adipose-derived MSCs to PEEK was evaluated and compared to tissue culture plate (TCP) as the reference material. Viability and morphology of cells were not altered when cultured on the PEEK film. The cells on PEEK presented a high proliferation activity in spite of a relatively lower initial cell adhesion rate. There was no significant difference on cell apoptosis and senescence between the cells on PEEK and TCP. The inflammatory cytokines and VEGF secreted by the cells on these two surfaces were at similar levels. The cells on PEEK showed up-regulated BMP2 and down-regulated BMP4 and BMP6 gene expression, whereas no conspicuous differences were observed in the committed osteoblast markers (BGLAP, COL1A1 and Runx2). With osteoinduction the cells on PEEK and TCP exhibited a similar osteogenic differentiation potential. Our results demonstrate the biofunctionality of PEEK for human MSC cultivation and differentiation. Its clinical benefits in bone tissue engineering may be achieved by combining MSCs with PEEK implants. These data may also provide useful information for further modification of PEEK with chemical or physical methods to regulate the cellular processes of MSCs and to consequently improve the efficacy of MSC-PEEK based therapies.
Degradable multiblock copolymers prepared from equal weight amounts of poly(epsilon-caprolactone)-diol (PCL-diol) and poly[oligo(3S-iso-butylmorpholine-2,5-dione)]-diol (PIBMD-diol), named PCL-PIBMD, provide a phase-segregated morphology. It exhibits a low melting temperature from PCL domains (T-m,T-PCL) of 382 degrees C and a high T-m,T-PIBMD of 170 +/- 2 degrees C with a glass transition temperature (T-g,T-PIBMD) at 42 +/- 2 degrees C from PIBMD domains. In this study, we explored the influence of applying different thermal treatments on the resulting morphologies of solution-cast and spin-coated PCL-PIBMD thin films, which showed different initial surface morphologies. Differential scanning calorimetry results and atomic force microscopy images after different thermal treatments indicated that PCL and PIBMD domains showed similar crystallization behaviors in 270 +/- 30 mu m thick solution-cast films as well as in 30 +/- 2 and 8 +/- 1nm thick spin-coated PCL-PIBMD films. Existing PIBMD crystalline domains highly restricted the generation of PCL crystalline domains during cooling when the sample was annealed at 180 degrees C. By annealing the sample above 120 degrees C, the PIBMD domains crystallized sufficiently and covered the free surface, which restricted the crystallization of PCL domains during cooling. The PCL domains can crystallize by hindering the crystallization of PIBMD domains via the fast vitrification of PIBMD domains when the sample was cooled/quenched in liquid nitrogen after annealing at 180 degrees C. These findings contribute to a better fundamental understanding of the crystallization mechanism of multi-block copolymers containing two crystallizable domains whereby the T-g of the higher melting domain type is in the same temperature range as the T-m of the lower melting domain type. Copyright (c) 2016 John Wiley & Sons, Ltd.
Material surfaces with tailored aerophobicity are crucial for applications where gas bubble wettability has to be controlled, e.g., gas storage and transport, electrodes, bioreactors or medical devices. Here, we present switchable underwater aerophobicity of hydrophobic polymeric substrates, which respond to heat with multilevel micro-and nanotopographical changes. The cross-linked poly[ethylene-co-(vinyl acetate)] substrates possess arrays of microcylinders with a nanorough top surface. It is hypothesized that the specific micro-/nanotopography of the surface allows trapping of a water film at the micro interspace and in this way generates the aerophobic behavior. The structured substrates were programmed to a temporarily stable, nanoscale flat substrate showing aerophilic behavior. Upon heating, the topographical changes caused a switch in contact angle from aerophilic to aerophobic for approaching air bubbles. In this way, the initial adhesion of air bubbles to the programmed flat substrate could be turned into repellence for the recovered substrate surface. The temperature at which the repellence of air bubbles starts can be adjusted from 58 +/- 3 degrees C to 73 +/- 3 degrees C by varying the deformation temperature applied during the temperature-memory programming procedure. The presented actively switching polymeric substrates are attractive candidates for applications, where an on-demand gas bubble repellence is advantageous. (c) 2018 Helmholtz-Zentrum Geesthacht, Zentrum fur Material- und Kustenforschung. Published by Elsevier Ltd.
Material surfaces with tailored aerophobicity are crucial for applications where gas bubble wettability has to be controlled, e.g., gas storage and transport, electrodes, bioreactors or medical devices.
Here, we present switchable underwater aerophobicity of hydrophobic polymeric substrates, which respond to heat with multilevel micro- and nanotopographical changes. The cross-linked poly[ethylene-co-(vinyl acetate)] substrates possess arrays of microcylinders with a nanorough top surface. It is hypothesized that the specific micro-/nanotopography of the surface allows trapping of a water film at the micro interspace and in this way generates the aerophobic behavior. The structured substrates were programmed to a temporarily stable, nanoscale flat substrate showing aerophilic behavior. Upon heating, the topographical changes caused a switch in contact angle from aerophilic to aerophobic for approaching air bubbles. In this way, the initial adhesion of air bubbles to the programmed flat substrate could be turned into repellence for the recovered substrate surface. The temperature at which the repellence of air bubbles starts can be adjusted from 58 ± 3 °C to 73 ± 3 °C by varying the deformation temperature applied during the temperature-memory programming procedure. The presented actively switching polymeric substrates are attractive candidates for applications, where an on-demand gas bubble repellence is advantageous.
Fibrous membranes capable of dynamically responding to external stimuli are highly desirable in textiles and biomedical materials, where adaptive behavior is required to accommodate complex environmental changes. For example, the creation of fabrics with temperature-dependent moisture permeability or self-regulating membranes for air filtration is dependent on the development of materials that exhibit a reversible stimuli-responsive pore size change. Here, by imbuing covalently crosslinked poly(ε-caprolactone) (cPCL) fibrous meshes with a reversible bidirectional shape-memory polymer actuation (rbSMPA) we create a material capable of temperature-controlled changes in porosity. Cyclic thermomechanical testing was used to characterize the mechanical properties of the meshes, which were composed of randomly arranged microfibers with diameters of 2.3 ± 0.6 μm giving an average pore size of approx. 10 μm. When subjected to programming strains of εm = 300% and 100% reversible strain changes of εʹrev = 22% ± 1% and 6% ± 1% were measured, with switching temperature ranges of 10 °C–30 °C and 45 °C–60 °C for heating and cooling, respectively. The rbSMPA of cPCL fibrous meshes generated a microscale reversible pore size change of 11% ± 3% (an average of 1.5 ± 0.6 μm), as measured by scanning electron microscopy. The incorporation of a two-way shape-memory actuation capability into fibrous meshes is anticipated to advance the development and application of smart membrane materials, creating commercially viable textiles and devices with enhanced performance and novel functionality.
Aim: The hydrolytic degradation behavior of degradable aliphatic polyester-based polymers is strongly influenced by the uptake or transport of water into the polymer matrix and also the hydrolysis rate of ester bonds.
Methods: We examined the volumetric swelling behavior of poly[(rac-lactide)-co-glycolide] (PLGA) and PLGA-based polyurethanes (PLGA-PU) with water contents of 0 wt%, 2 wt% and 7 wt% water at 310 K using a molecular modeling approach. Polymer systems with a number average molecular weight of M-n = 10,126 g.mol(-1) were constructed from PLGA with a lactide content of 67 mol%, whereby PLGA-PU systems were composed of five PLGA segments with M-n = 2052 g.mol(-1), which were connected via urethane linkers originated from 2,2,4-trimethyl hexamethylene-1,6-diisocyanate (TMDI), hexamethyl-1,6-diisocyanate (HDI), or L-lysine-1,6-diisocyanate (LDI).
Results: The calculated densities of the dry PLGA-PU systems were found to be lower than for pure PLGA. The obtained volumetric swelling of the PLGA-PU was depending on the type of urethane linker, whereby all swollen PLGA-PUs contained larger free volume distribution compared to pure PLGA. The mean square displacement curves for dry PLGA and PLGA-PUs showed that urethane linker units reduce the mobility of the polymer chains, while an increase in backbone atoms mobility was found, when water was added to these systems. Consequently, an increased water uptake of PLGA-PU matrices combined with a higher mobility of the chain segments should result in an accelerated hydrolytic chain scission rate in comparison to PLGA.
Conclusions: It can be anticipated that the incorporation of urethane linkers might be a helpful tool to adjust the degradation behavior of polyesters.
The thermally induced shape-memory effect of polymers is typically characterized by cyclic uniaxial thermomechanical tests. Here, a molecular-dynamics (MD) simulation approach of such a cyclic uniaxial thermomechanical test is presented for amorphous switching domains of poly(L-lactide) (PLLA). Uniaxial deformation of the constructed PLLA models is simulated with a Parinello-Rahman scheme, as well as a pragmatic geometrical approach. We are able to describe two subsequent test cycles using the presented simulation approach. The obtained simulated shape-memory properties in both test cycles are similar and independent of the applied deformation protocols. The simulated PLLA shows high shape fixity ratios (Rf 94%), but only a moderate shape recovery ratio is obtained (Rr 30%). Finally, the structural changes during the simulated test are characterized by analysis of the changes in the dihedral angle distributions.
Shape-memory properties of polyetherurethane foams prepared by thermally induced phase separation
(2012)
In this study, we report the preparation of two structurally different shape-memory polymer foams by thermally induced phase separation (TIPS) from amorphous polyetherurethanes. Foams with either a homogeneous, monomodal, or with a hierarchically structured, bimodal, pore size distribution are obtained by adoption of the cooling protocol. The shape-memory properties have been investigated for both foam structures by cyclic, thermomechanical experiments, while the morphological changes on the micro scale (pore level) have been compared to the macro scale by an in situ micro compression device experiment. The results show that the hierarchically structured foam achieves higher shape-recovery rates and a higher total recovery as compared to the homogeneous foam, which is due to an increased energy storage capability by micro scale bending of the hierarchically structured foam compared to pure compression of the homogeneous foam.
Shape-Memory Capability of Copolyetheresterurethane Microparticles Prepared via Electrospraying
(2015)
Multifunctional thermo-responsive and degradable microparticles exhibiting a shapememory effect (SME) have attracted widespread interest in biomedicine as switchable delivery vehicles or microactuators. In this work almost spherical solid microparticles with an average diameter of 3.9 +/- 0.9 mm are prepared via electrospraying of a copolyetheresterurethane named PDC, which is composed of crystallizable oligo(p-dioxanone) (OPDO) hard and oligo(e-caprolactone) (OCL) switching segments. The PDC microparticles are programmed via compression at different pressures and their shapememory capability is explored by off-line and online heating experiments. When a low programming pressure of 0.2 MPa is applied a pronounced thermally-induced shape-memory effect is achieved with a shape recovery ratio about 80%, while a high programming pressure of 100 MPa resulted in a weak shape-memory performance. Finally, it is demonstrated that an array of PDC microparticles deposited on a polypropylene (PP) substrate can be successfully programmed into a smart temporary film, which disintegrates upon heating to 60 degrees C.
Advances in the fabrication and characterization of polymeric nanomaterials has greatly advanced the miniaturization of soft actuators, creating materials capable of replicating the functional physical behavior previously limited to the macroscale. Here, we demonstrate how a reversible shape-memory polymer actuation can be generated in a single micro/nano object, where the shape change during actuation of an individual fiber can be dictated by programming using an AFM-based method. Electrospinning was used to prepare poly(epsilon-caprolactone) micro-/nanofibers, which were fixed and crosslinked on a structured silicon wafer. The programming as well as the observation of recovery and reversible displacement of the fiber were performed by vertical three point bending, using an AFM testing platform introduced here. A plateau tip was utilized to improve the stability of the fiber contact and working distance, enabling larger deformations and greater rbSMPA performance. Values for the reversible elongation of epsilon(rev)= 3.4 +/- 0.1% and 10.5 +/- 0.1% were obtained for a single micro (d = 1.0 +/- 0.2 mu m) and nanofiber (d = 300 +/- 100 nm) in cyclic testing between the temperatures 10 and 60 degrees C. The reversible actuation of the nanofiber was successfully characterized for 10 cycles. The demonstration and characterization of individual shape-memory nano and microfiber actuators represents an important step in the creation of miniaturized robotic devices capable of performing complex physical functions at the length scale of cells and structural component of the extracellular matrix.
Classic rotating engines are powerful and broadly used but are of complex design and difficult to miniaturize. It has long remained challenging to make large-stroke, high-speed, high-energy microengines that are simple and robust. We show that torsionally stiffened shape memory nanocomposite fibers can be transformed upon insertion of twist to store and provide fast and high-energy rotations. The twisted shape memory nanocomposite fibers combine high torque with large angles of rotation, delivering a gravimetric work capacity that is 60 times higher than that of natural skeletal muscles. The temperature that triggers fiber rotation can be tuned. This temperature memory effect provides an additional advantage over conventional engines by allowing for the tunability of the operation temperature and a stepwise release of stored energy.
Polymeric materials possessing specific features like programmability, high deformability, and easy processability are highly desirable for creating modern actuating systems. In this study, thermoplastic shape-memory polymer actuators obtained by combining crystallizable poly(epsilon-caprolactone) (PCL) and poly(3S-isobutylmorpholin-2,5-dione) (PIBMD) segments in multiblock copolymers are described. We designed these materials according to our hypothesis that the confinement of glassy PIBMD domains present at the upper actuation temperature contribute to the stability of the actuator skeleton, especially at large programming strains. The copolymers have a phase-segregated morphology, indicated by the well-separated melting and glass transition temperatures for PIBMD and PCL, but possess a partially overlapping T-m of PCL and T-g of PIBMD in the temperature interval from 40 to 60 degrees C. Crystalline PIBMD hard domains act as strong physical netpoints in the PIBMD-PCL bulk material enabling high deformability (up to 2000%) and good elastic recoverability (up to 80% at 50 degrees C above T-m,T-PCL). In the programmed thermoplastic actuators a high content of crystallizable PCL actuation domains ensures pronounced thermoreversible shape changes upon repetitive cooling and heating. The programmed actuator skeleton, composed of PCL crystals present at the upper actuation temperature T-high and the remaining glassy PIBMD domains, enabled oriented crystallization upon cooling. The actuation performance of PIBMD-PCL could be tailored by balancing the interplay between actuation and skeleton, but also by varying the quantity of crystalline PIBMD hard domains via the copolymer composition, the applied programming strain, and the choice of T-high. The actuator with 17 mol% PIBMD showed the highest reversible elongation of 11.4% when programmed to a strain of 900% at 50 degrees C. It is anticipated that the presented thermoplastic actuator materials can be applied as modern compression textiles.
Gelatin-based hydrogels offer various biochemical cues that support encapsulated cells and are therefore suitable as cell delivery vehicles in regenerative medicine. However, besides the biochemical signals, biomechanical cues are crucial to ensure an optimal support of encapsulated cells. Hence, we aimed to correlate the cellular response of encapsulated cells to macroscopic and microscopic elastic properties of glycidylmethacrylate (GMA)-functionalized gelatin-based hydrogels. To ensure that different observations in cellular behavior could be attributed to differences in elastic properties, an identical concentration as well as degree of functionalization of biopolymers was utilized to form covalently crosslinked hydrogels. Elastic properties were merely altered by varying the average gelatin-chain length. Hydrogels exhibited an increased degree of swelling and a decreased bulk elastic modulus G with prolonged autoclaving of the starting solution. This was accompanied by an increase of hydrogel mesh size and thus by a reduction of crosslinking density. Tougher hydrogels retained the largest amount of cells; however, they also interfered with cell viability. Softer gels contained a lower cell density, but supported cell elongation and viability. Observed differences could be partially attributed to differences in bulk properties, as high crosslinking densities interfere with diffusion and cell spreading and thus can impede cell viability. Interestingly, a microscopic elastic modulus in the range of native soft tissue supported cell viability and elongation best while ensuring a good cell entrapment. In conclusion, gelatin-based hydrogels providing a soft tissue-like microenvironment represent adequate cell delivery vehicles for tissue engineering approaches. Copyright (c) 2016 John Wiley & Sons, Ltd.
Soft robots and devices with the advanced capability to perform adaptive motions similar to that of human beings often have stimuli-sensitive polymeric materials as the key actuating component. The external signals triggering the smart polymers’ actuations can be transmitted either via a direct physical connection between actuator and controlling unit (tethered) or remotely without a connecting wire. However, the vast majority of such polymeric actuator materials are limited to one specific type of motion as their geometrical information is chemically fixed. Here, we present magnetically driven nanocomposite actuators, which can be reversibly reprogrammed to different actuation geometries by a solely physical procedure. Our approach is based on nanocomposite materials comprising spatially segregated crystallizable actuation and geometry determining units. Upon exposure to a specific magnetic field strength the actuators’ geometric memory is erased by the melting of the geometry determining units allowing the implementation of a new actuator shape. The actuation performance of the nanocomposites can be tuned and the technical significance was demonstrated in a multi-cyclic experiment with several hundreds of repetitive free-standing shape shifts without losing performance.
Unwanted shrinkage behaviors or failure in structural functions such as mechanical strength or deformability of polymeric products related to their thermomechanical history are a major challenge in production of plastics. Here, we address the question whether we can turn this challenge into an opportunity by creating defined thermomechanical histories in polymers, represented by a specific morphology and nanostructure, to equip polymeric shaped bodies with desired functions, e.g. a temperature-memory, by hot, warm or cold deformation into multiblock copolymers having two partially overlapping melting transitions. A copolyesterurethane named PDLCL, consisting of poly(epsilon-caprolactone) (PCL) and poly(omega-pentadecalactone) (PPDL) crystalline domains, exhibiting a pronounced phase-segregated morphology and partially overlapping melting transitions was selected for this study. Different types of PCL and PPDL crystals as well as distinct degrees of orientation in both amorphous and crystalline domains were obtained after deformation at 20 or 40 degrees C and to a lower extent at 60 degrees C. The generated non-isotropic structures were stable at ambient temperature and represent the different stresses stored. Stress-free heating experiments showed that the relaxation in both amorphous and crystalline phases occurred predominantly with melting of PCL crystals. When the switching temperature, which was similar to the applied deformation temperature (temperature-memory), was exceeded in stress-free heating experiments, the implemented thermomechanical history could be reversed. In contrast, during constant-strain heating to 60 degrees C the generated structural features remained almost unchanged. These findings provide insights about the structure function relation in multiblock copolymers with two crystalline phases exhibiting a temperature-memory effect by implementation of specific thermomechanical histories, which might be a general principle for tailoring other functions like mechanical strength or deformability in polymers. (C) 2016 Elsevier Ltd. All rights reserved.
Temperature-memory technology was utilized to generate flat substrates with a programmable stiffness pattern from cross-linked poly(ethylene-co-vinyl acetate) substrates with cylindrical microstructures. Programmed substrates were obtained by vertical compression at temperatures in the range from 60 to 100 degrees C and subsequent cooling, whereby a flat substrate was achieved by compression at 72 degrees C, as documented by scanning electron microscopy and atomic force microscopy (AFM). AFM nanoindentation experiments revealed that all programmed substrates exhibited the targeted stiffness pattern. The presented technology for generating polymeric substrates with programmable stiffness pattern should be attractive for applications such as touchpads. optical storage, or cell instructive substrates.
Temperature-memory technology was utilized to generate flat substrates with a programmable stiffness pattern from cross-linked poly(ethylene-co-vinyl acetate) substrates with cylindrical microstructures. Programmed substrates were obtained by vertical compression at temperatures in the range from 60 to 100 degrees C and subsequent cooling, whereby a flat substrate was achieved by compression at 72 degrees C, as documented by scanning electron microscopy and atomic force microscopy (AFM). AFM nanoindentation experiments revealed that all programmed substrates exhibited the targeted stiffness pattern. The presented technology for generating polymeric substrates with programmable stiffness pattern should be attractive for applications such as touchpads. optical storage, or cell instructive substrates.
Open porous foams with identical foam density but different pore-size distributions (bimodal or monomodal) are prepared from a shape-memory polyetherurethane (PEU) by thermally induced phase separation. The shape-memory effect of the two PEU foams is explored by cyclic thermomechanical compression tests and microstructural analysis. The obtained results reveal that the PEU foam with a bimodal pore-size distribution exhibits an increased shape-recovery under stress-free conditions, both on the macro- (foam level) as well as the microscale (pore level). While bimodal pore-size distributions induce microscale bending during compression, buckling occurs in foams with monomodal pore-size distributions, leading to both a reduced and delayed shape recovery.
Stem cells are capable of sensing and processing environmental inputs, converting this information to output a specific cell lineage through signaling cascades. Despite the combinatorial nature of mechanical, thermal, and biochemical signals, these stimuli have typically been decoupled and applied independently, requiring continuous regulation by controlling units. We employ a programmable polymer actuator sheet to autonomously synchronize thermal and mechanical signals applied to mesenchymal stem cells (MSC5). Using a grid on its underside, the shape change of polymer sheet, as well as cell morphology, calcium (Ca2+) influx, and focal adhesion assembly, could be visualized and quantified. This paper gives compelling evidence that the temperature sensing and mechanosensing of MSC5 are interconnected via intracellular Ca2+. Up-regulated Ca2+ levels lead to a remarkable alteration of histone H3K9 acetylation and activation of osteogenic related genes. The interplay of physical, thermal, and biochemical signaling was utilized to accelerate the cell differentiation toward osteogenic lineage. The approach of programmable bioinstructivity provides a fundamental principle for functional biomaterials exhibiting multifaceted stimuli on differentiation programs. Technological impact is expected in the tissue engineering of periosteum for treating bone defects.
Stem cells are capable of sensing and processing environmental inputs, converting this information to output a specific cell lineage through signaling cascades. Despite the combinatorial nature of mechanical, thermal, and biochemical signals, these stimuli have typically been decoupled and applied independently, requiring continuous regulation by controlling units. We employ a programmable polymer actuator sheet to autonomously synchronize thermal and mechanical signals applied to mesenchymal stem cells (MSC5). Using a grid on its underside, the shape change of polymer sheet, as well as cell morphology, calcium (Ca2+) influx, and focal adhesion assembly, could be visualized and quantified. This paper gives compelling evidence that the temperature sensing and mechanosensing of MSC5 are interconnected via intracellular Ca2+. Up-regulated Ca2+ levels lead to a remarkable alteration of histone H3K9 acetylation and activation of osteogenic related genes. The interplay of physical, thermal, and biochemical signaling was utilized to accelerate the cell differentiation toward osteogenic lineage. The approach of programmable bioinstructivity provides a fundamental principle for functional biomaterials exhibiting multifaceted stimuli on differentiation programs. Technological impact is expected in the tissue engineering of periosteum for treating bone defects.
Microobjects with programmable mechanical functionality are highly desirable for the creation of flexible electronics, sensors, and microfluidic systems, where fabrication/programming and quantification methods are required to fully control and implement dynamic physical behavior. Here, programmable microcuboids with defined geometries are prepared by a template-based method from crosslinked poly[ethylene-co-(vinyl acetate)] elastomers. These microobjects could be programmed to exhibit a temperature-memory effect or a shape-memory polymer actuation capability. Switching temperaturesT(sw)during shape recovery of 55 +/- 2, 68 +/- 2, 80 +/- 2, and 86 +/- 2 degrees C are achieved by tuning programming temperatures to 55, 70, 85, and 100 degrees C, respectively. Actuation is achieved with a reversible strain of 2.9 +/- 0.2% to 6.7 +/- 0.1%, whereby greater compression ratios and higher separation temperatures induce a more pronounced actuation. Micro-geometry change is quantified using optical microscopy and atomic force microscopy. The realization and quantification of microparticles, capable of a tunable temperature responsive shape-change or reversible actuation, represent a key development in the creation of soft microscale devices for drug delivery or microrobotics.
Dendritic cells (DC) contribute to immunity by presenting antigens to T cells and shape the immune response by the secretion of cytokines. Due to their immune stimulatory potential DC-based therapies are promising approaches to overcome tolerance e.g. against tumors. In order to enforce the immunogenicity of DCs, they have to be matured and activated in vitro, which requires an appropriate cell culture substrate, supporting their survival expansion and activation.
Since most cell culture devices are not optimized for DC growth, it is hypothesized that polymers with certain physicochemical properties can positively influence the DC cultures. With the aim to evaluate the effects that polymers with different chemical compositions have on the survival, the activation status, and the cytokine/chemokine secretion profile of DC, their interaction with polystyrene (PS), polycarbonate (PC), poly(ether imide) (PEI), and poly(styrene-co-acrylonitrile) (PSAN)-based cell culture inserts was investigated. By using this insert system, which fits exactly into 24 well cell culture plates, effects induced from the culture dish material can be excluded. The viability of untreated DC after incubation with the different inserts was not influenced by the different inserts, whereas LPS-activatedDCshowed an increased survival after cultivation on PC, PS, and PSAN compared to tissue culture polystyrene (TCP). The activation status of DC estimated by the expression of CD40, CD80, CD83, CD86 and HLA-DR expression was not altered by the different inserts in untreated DC but slightly reduced when LPS-activated DC were cultivated on PC, PS, PSAN, and PEI compared to TCP. For each polymeric cell culture insert a distinct cytokine profile could be observed.
Since inserts with different chemical compositions of the inserts did not substantially alter the behavior of DC all insert systems could be considered as alternative substrate. The observed increased survival on some polymers, which showed in contrast to TCP a hydrophobic surface, could be beneficial for certain applications such as T cell expansion and activation.
The enzymatic degradation of oligo(epsilon-caprolactone) (OCL) based films at the air-water interface is investigated by Langmuir monolayer degradation (LMD) experiments to elucidate the influence of the molecular architecture and of the chemical structure on the chain scission process. For that purpose, the interactions of 2D monolayers of two star-shaped poly(epsilon-caprolactone)s (PCLs) and three linear OCL based copolyesterurethanes (P(OCL-U)) with the lipase from Pseudomonas cepacia are evaluated in comparison to linear OCL. While the architecture of star-shaped PCL Langmuir layers slightly influences their degradability compared to OCL films, significantly retarded degradations are observed for P(OCL-U) films containing urethane junction units derived from 2, 2 (4), 4-trimethyl hexamethylene diisocyanate (TMDI), hexamethylene diisocyanate (HDI) or lysine ethyl ester diisocyanate (LDI). The enzymatic degradation of the OCL based 2D structures is related to the presence of hydrophilic groups within the macromolecules rather than to the packing density of the film or to the molecular weight. The results reveal that the LMD technique allows the parallel analysis of both the film/enzyme interactions and the degradation process on the molecular level. (C) 2016 Elsevier Ltd. All rights reserved.
Active fibers can serve as artificial muscles in robotics or components of smart textiles. Here, we present an origami hand robot, where single fibers control the reversible movement of the fingers. A recovery/contracting force of 0.2 N with a work capacity of 0.175 kJ kg(-1) was observed in crosslinked poly[ethylene-co-(vinyl acetate)] (cPEVA) fibers, which could enable the bending movement of the fingers by contraction upon heating. The reversible opening of the fingers was attributed to a combination of elastic recovery force of the origami structure and crystallization-induced elongation of the fibers upon cooling.
Polymeric devices capable of releasing submicron particles (subMP) on demand are highly desirable for controlled release systems, sensors, and smart surfaces. Here, a temperature-memory polymer sheet with a programmable smooth surface served as matrix to embed and release polystyrene subMP controlled by particle size and temperature. subMPs embedding at 80 degrees C can be released sequentially according to their size (diameter D of 200 nm, 500 nm, 1 mu m) when heated. The differences in their embedding extent are determined by the various subMPs sizes and result in their distinct release temperatures. Microparticles of the same size (D approximate to 1 mu m) incorporated in films at different programming temperatures T-p (50, 65, and 80 degrees C) lead to a sequential release based on the temperature-memory effect. The change of apparent height over the film surface is quantified using atomic force microscopy and the realization of sequential release is proven by confocal laser scanning microscopy. The demonstration and quantification of on demand subMP release are of technological impact for assembly, particle sorting, and release technologies in microtechnology, catalysis, and controlled release.
Reversible movements of current polymeric actuators stem from the continuous response to signals from a controlling unit, and subsequently cannot be interrupted without stopping or eliminating the input trigger. Here, we present actuators based on cross-linked blends of two crystallizable polymers capable of pausing their movements in a defined manner upon continuous cyclic heating and cooling. This noncontinuous actuation can be adjusted by varying the applied heating and cooling rates. The feasibility of these devices for technological applications was shown in a 140 cycle experiment of free-standing noncontinuous shape shifts, as well as by various demonstrators.
The hierarchical design approach provides various opportunities to adjust the structural performance of polymer materials. Electrospinning processing techniques give access to molecular orientation as a design parameter, which we consider here in view of the shape-memory actuation performance. The aim of this work is to investigate how the reversible strain epsilon'(rev) can be affected by a morphology change from a bulk material to an electrospun mesh. epsilon'(rev) could be increased from 5.5 +/- 0.5% to 15 +/- 1.8% for a blend from a multiblock copolymer with poly(epsilon-caprolactone) (PCL) and poly(L-lactide) (PLLA) segments with oligo(D-lactide) (ODLA). This study demonstrates an effective design approach for enhancing soft actuator performance, which can be broadly applied in soft robotics and medicine.
Temperature-memory polymers remember the temperature, where they were deformed recently, enabled by broad thermal transitions. In this study, we explored a series of crosslinked poly[ethylene-co-(vinyl acetate)] networks (cPEVAs) comprising crystallizable polyethylene (PE) controlling units exhibiting a pronounced temperature-memory effect (TME) between 16 and 99 °C related to a broad melting transition (∼100 °C). The nanostructural changes in such cPEVAs during programming and activation of the TME were analyzed via in situ X-ray scattering and specific annealing experiments. Different contributions to the mechanism of memorizing high or low deformation temperatures (Tdeform) were observed in cPEVA, which can be associated to the average PE crystal sizes. At high deformation temperatures (>50 °C), newly formed PE crystals, which are established during cooling when fixing the temporary shape, dominated the TME mechanism. In contrast, at low Tdeform (<50 °C), corresponding to a cold drawing scenario, the deformation led preferably to a disruption of existing large crystals into smaller ones, which then fix the temporary shape upon cooling. The observed mechanism of memorizing a deformation temperature might enable the prediction of the TME behavior and the knowledge based design of other TMPs with crystallizable controlling units.
Temperature-memory polymers remember the temperature, where they were deformed recently, enabled by broad thermal transitions. In this study, we explored a series of crosslinked poly[ethylene-co-(vinyl acetate)] networks (cPEVAs) comprising crystallizable polyethylene (PE) controlling units exhibiting a pronounced temperature-memory effect (TME) between 16 and 99 °C related to a broad melting transition (∼100 °C). The nanostructural changes in such cPEVAs during programming and activation of the TME were analyzed via in situ X-ray scattering and specific annealing experiments. Different contributions to the mechanism of memorizing high or low deformation temperatures (Tdeform) were observed in cPEVA, which can be associated to the average PE crystal sizes. At high deformation temperatures (>50 °C), newly formed PE crystals, which are established during cooling when fixing the temporary shape, dominated the TME mechanism. In contrast, at low Tdeform (<50 °C), corresponding to a cold drawing scenario, the deformation led preferably to a disruption of existing large crystals into smaller ones, which then fix the temporary shape upon cooling. The observed mechanism of memorizing a deformation temperature might enable the prediction of the TME behavior and the knowledge based design of other TMPs with crystallizable controlling units.
Temperature-memory polymers remember the temperature, where they were deformed recently, enabled by broad thermal transitions. In this study, we explored a series of crosslinked poly[ethylene-co-(vinyl acetate)] networks (cPEVAs) comprising crystallizable polyethylene (PE) controlling units exhibiting a pronounced temperature-memory effect (TME) between 16 and 99 degrees C related to a broad melting transition (similar to 100 degrees C). The nanostructural changes in such cPEVAs during programming and activation of the TME were analyzed via in situ X-ray scattering and specific annealing experiments. Different contributions to the mechanism of memorizing high or low deformation temperatures (T-deform) were observed in cPEVA, which can be associated to the average PE crystal sizes. At high deformation temperatures (>50 degrees C), newly formed PE crystals, which are established during cooling when fixing the temporary shape, dominated the TME mechanism. In contrast, at low T-deform (<50 degrees C), corresponding to a cold drawing scenario, the deformation led preferably to a disruption of existing large crystals into smaller ones, which then fix the temporary shape upon cooling. The observed mechanism of memorizing a deformation temperature might enable the prediction of the TME behavior and the knowledge based design of other TMPs with crystallizable controlling units.
Highly porous poly(ether imide) (PEI) microparticles prepared by a spraying/coagulation process are discussed as candidate adsorber materials for apheresis applications, i.e. removal of uremic toxins from the blood of renal failure patients. PEI particles obtained by the aforementioned procedure can have a broad size distribution with particle diameters ranging from 20 to 800 mu m. In order to further estimate the adsorption behavior of PEI microparticles packed in application relevant apheresis modules, a quantitative information about the relation between particle size and pore morphology is required. In this study, we explored whether the intraparticle porosity of PEI microparticles varies with altering the diameter of the particulate adsorbers. By an analytical wet sieving procedure, the obtained PEI microparticles were separated into five size fractions, which were analyzed by mercury intrusion porosimetry, nitrogen adsorption, and scanning electron microscopy. Mercury intrusion porosimetry revealed for all size fractions high porosity values in the range from 78% to 84% with pore diameters in the range from 10 to 1000nm. A bimodal pore size distribution was found having a first peak at around 100nm, while a second pronounced peak maximum was found at higher pore sizes that increased with raising particle diameter from 300nm for the smallest particle size fraction (50-100 mu m) to 700nm for particles with a diameter of 200 to 250 mu m. Based on these findings, it can be assumed that the main PEI particle size fraction (200-250 mu m) should exhibit the highest adsorption capacity in an apheresis module. Copyright (c) 2016 John Wiley & Sons, Ltd.
Controlling mesenchymal stem cells (MSCs) behavior is necessary to fully exploit their therapeutic potential. Various approaches are employed to effectively influence the migration capacity of MSCs. Here, topographic microstructures with different microscale roughness were created on polystyrene (PS) culture vessel surfaces as a feasible physical preconditioning strategy to modulate MSC migration. By analyzing trajectories of cells migrating after reseeding, we demonstrated that the mobilization velocity of human adipose derived mesenchymal stem cells (hADSCs) could be promoted by and persisted after brief preconditioning with the appropriate microtopography. Moreover, the elevated activation levels of focal adhesion kinase (FAK) and mitogen-activated protein kinase (MAPK) in hADSCs were also observed during and after the preconditioning process. These findings underline the potential enhancement of in vivo therapeutic efficacy in regenerative medicine via transplantation of topographic microstructure preconditioned stem cells.
Non-swelling hydrophobic poly(n-butyl acrylate) network (cPnBA) is a candidate material for synthetic vascular grafts owing to its low toxicity and tailorable mechanical properties. Mesenchymal stem cells (MSCs) are an attractive cell type for accelerating endothelialization because of their superior anti-thrombosis and immune modulatory function. Further, they can differentiate into smooth muscle cells or endothelial-like cells and secret pro-angiogenic factors such as vascular endothelial growth factor (VEGF). MSCs are sensitive to the substrate mechanical properties, with the alteration of their major cellular behavior and functions as a response to substrate elasticity. Here, we cultured human adipose-derived mesenchymal stem cells (hADSCs) on cPnBAs with different mechanical properties (cPnBA250, Young’s modulus (E) = 250 kPa; cPnBA1100, E = 1100 kPa) matching the elasticity of native arteries, and investigated their cellular response to the materials including cell attachment, proliferation, viability, apoptosis, senescence and secretion. The cPnBA allowed high cell attachment and showed negligible cytotoxicity. F-actin assembly of hADSCs decreased on cPnBA films compared to classical tissue culture plate. The difference of cPnBA elasticity did not show dramatic effects on cell attachment, morphology, cytoskeleton assembly, apoptosis and senescence. Cells on cPnBA250, with lower proliferation rate, had significantly higher VEGF secretion activity. These results demonstrated that tuning polymer elasticity to regulate human stem cells might be a potential strategy for constructing stem cell-based artificial blood vessels.
Glycoproteins adsorbing on an implant upon contact with body fluids can affect the biological response in vitro and in vivo, depending on the type and conformation of the adsorbed biomacromolecules. However, this process is poorly characterized and so far not controllable. Here, protein monolayers of high molecular cohesion with defined density are transferred onto polymeric substrates by the Langmuir-Schaefer (LS) technique and were compared with solution deposition (SO) method. It is hypothesized that on polydimethylsiloxane (PDMS), a substrate with poor cell adhesion capacity, the fibronectin (FN) layers generated by the LS and SO methods will differ in their organization, subsequently facilitating differential stem cell adhesion behavior. Indeed, atomic force microscopy visualization and immunofluorescence images indicated that organization of the FN layer immobilized on PDMS was uniform and homogeneous. In contrast, FN deposited by SO method was rather heterogeneous with appearance of structures resembling protein aggregates. Human mesenchymal stem cells showed reduced absolute numbers of adherent cells, and the vinculin expression seemed to be higher and more homogenously distributed after seeding on PDMS equipped with FN by LS in comparison with PDMS equipped with FN by SO. These divergent responses could be attributed to differences in the availability of adhesion molecule ligands such as the Arg-Gly-Asp (RGD) peptide sequence presented at the interface. The LS method allows to control the protein layer characteristics, including the thickness and the protein orientation or conformation, which can be harnessed to direct stem cell responses to defined outcomes, including migration and differentiation. Copyright (c) 2016 John Wiley & Sons, Ltd.
The variation of the molecular architecture of multiblock copolymers has enabled the introduction of functional behaviour and the control of key mechanical properties. In the current study, we explore the synergistic relationship of two structural components in a shape-memory material formed of a multiblock copolymer with crystallizable poly(epsilon-caprolactone) and crystallizable polyfoligo(3S-iso-butylmorpholine-2,5-dione) segments (PCL-PIBMD). The thermal and structural properties of PCL-PIBMD films were compared with PCI.-PU and PMMD-PU investigated by means of DSC, SAXS and WARS measurements. The shape-memory properties were quantified by cyclic, thermomechanical tensile tests, where deformation strains up to 900% were applied for programming PCL-PIBMD films at 50 degrees C. Toluene vapor treatment experiments demonstrated that the temporary shape was fixed mainly by glassy PIBMD domains at strains lower than 600% with the PCL contribution to fixation increasing to 42 +/- 2% at programming strains of 900% This study into the shape-memory mechanism of PCL-PIBMD provides insight into the structure function relation in multiblock copolymers with both crystallizable and glassy switching segments.
The macroscale function of multicomponent polymeric materials is dependent on their phase-morphology. Here, we investigate the morphological structure of a multiblock copolymer consisting of poly(L-lactide) and poly(epsilon-caprolactone) segments (PLLA-PCL), physically cross-linked by stereocomplexation with a low molecular weight poly(D-lactide) oligomer (PDLA). The effects of blend composition and PLLA-PCL molecular structure on the morphology are elucidated by AFM, TEM and SAXS. We identify the formation of a lattice pattern, composed of PLA domains within a PCL matrix, with an average domain spacing d0 = 12 - 19 nm. The size of the PLA domains were found to be proportional to the block length of the PCL segment of the copolymer and inversely proportional to the PDLA content of the blend. Changing the PLLA-PCL / PDLA ratio caused a shift in the melt transition Tm attributed to the PLA stereocomplex crystallites, indicating partial amorphous phase dilution of the PLA and PCL components within the semicrystalline material. By elucidating the phase structure and thermal character of multifunctional PLLA-PCL / PDLA blends, we illustrate how composition affects the internal structure and thermal properties of multicomponent polymeric materials. This study should facilitate the more effective incorporation of a variety of polymeric structural units capable of stimuli responsive phase transitions, where an understanding the phase-morphology of each component will enable the production of multifunctional soft-actuators with enhanced performance.
Multiblock copolymers named PCL-PIBMD consisting of crystallizable poly(epsilon-caprolactone) segments and crystallizable poly[oligo(3S-iso-butylmorpholine-2,5-dione)] segments coupled by trimethyl hexamethylene diisocyanate provide a versatile molecular architecture for achieving shape-memory effects (SMEs) in polymers. The mechanical properties as well as the SME performance of PCL-PIBMD can be tailored by the variation of physical parameters during programming such as deformation strain or applied temperature protocols. In this study, we explored the influence of applying different strain rates during programming on the resulting nanostructure of PCL-PIBMD. Programming was conducted at 50 degrees C by elongation to epsilon(m)=50% with strain rates of 1 or 10 or 50 mmmin(-1). The nanostructural changes were visualized by atomic force microscopy (AFM) measurements and investigated by in situ wide and small angle X-ray scattering experiments. With increasing the strain rate, a higher degree of orientation was observed in the amorphous domains. Simultaneously the strain-induced formation of new PIBMD crystals as well as the fragmentation of existing large PIBMD crystals occurred. The observed differences in shape fixity ratio and recovery stress of samples deformed with various strain rates can be attributed to their different nanostructures. The achieved findings can be relevant parameters for programming the shape-memory polymers with designed recovery forces. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1935-1943
Polymers exhibiting cell-selective effects represent an extensive research field with high relevance for biomedical applications e.g. in the cardiovascular field supporting re-endothelialization while suppressing smooth muscle cell overgrowth. Such an endothelial cell-selective effect could be recently demonstrated for a copolyetheresterurethane (PDC) containing biodegradable poly(p-dioxanone) and poly(epsilon-caprolactone) segments, which selectively enhanced the adhesion of human umbilical vein endothelial cells (HUVEC) while suppressing the attachment of smooth muscle cells (SMC).
In this study we investigated the influence of the fibre orientation (random and aligned) and fibre diameter (2 mu m and 500 nm) of electrospun PDC scaffolds on the adhesion, proliferation and apoptosis of HUVEC and SMC.
Adhesion, viability and proliferation of HUVEC was diminished when the fibre diameter was reduced to a submicron scale, while the orientation of the microfibres did only slightly influence the cellular behaviour. In contrast, a submicron fibre diameter improved SMC viability. In conclusion, PDC scaffolds with micron-sized single fibres could be promising candidate materials for cell-selective stent coatings.
Degradable polymers with a tailorable degradation rate might be promising candidate materials for biomaterial-based cartilage repair. In view of the poor intrinsic healing capability of cartilage, implantation of autologous chondrocytes seeded on a biocompatible slow degrading polymer might be an encouraging approach to improve cartilage repair in the future. This study was undertaken to test if the fiber orientation (random versus aligned) of two different degradable polymers and a polymer intended for long term applications could influence primary articular chondrocytes growth and ultrastructure.
A degradable copoly(ether) esterurethane (PDC) was synthesized via co-condensation of poly(p-dioxanone) diol and poly(epsilon-caprolactone) diol using an aliphatic diisocyanate as linker. Poly(p-dioxanone) (PPDO) was applied as commercially available degradable polymer, while polyetherimide (PEI) was chosen as biomaterial enabling surface functionalization. The fibrous scaffolds of PDC and PPDO were obtained by electrospinning using 1,1,1,3,3,3 hexafluoro-2-propanol (HFP), while for PEI dimethyl acetamide (DMAc) was applied as solvent. Primary porcine articular chondrocytes were seeded at different cell densities on the fibrous polymer scaffolds and analyzed for viability (fluorescein diacetate/ethidiumbromide staining), for type II collagen synthesis (immunolabelling), ultrastructure and orientation on the fibers (SEM: scanning electron microscopy).
Vital chondrocytes adhered on all electrospun scaffolds irrespective of random and aligned topologies. In addition, the chondrocytes produced the cartilage-specific type II collagen on all tested polymer topologies suggesting their differentiated functions. SEM revealed an almost flattened chondrocytes shape on scaffolds with random fiber orientation: whereby chondrocytes growth remained mainly restricted to the scaffold surface. On aligned fibers the chondrocytes exhibited a more spindle-shaped morphology with rougher cell surfaces but only a minority of the cells aligned according to the fibers. As a next step the reduction of the fiber diameter of electrospun scaffolds should be addressed as an important parameter to mimic cartilage ECM structure.
Three oligo[(rac-lactide)-co-glycolide] based polyesterurethanes (OLGA-PUs) containing different diurethane linkers are investigated by the Langmuir monolayer technique and compared to poly[(rac-lactide)-co-glycolide] (PLGA) to elucidate the influence of the diurethane junction units on hydrophilicity and packing motifs of these polymers at the air-water interface. The presence of diurethane linkers does not manifest itself in the Langmuir layer behavior both in compression and expansion experiments when monomolecular films of OLGA-PUs are spread on the water surface. However, the linker retard the evolution of morphological structures at intermediate compression level under isobaric conditions (with a surface pressure greater than 11 mN m(-1)) compared to the PLGA, independent on the chemical structure of the diurethane moiety. The layer thicknesses of both OLGA-PU and PLGA films decrease in the high compression state with decreasing surface pressure, as deduced from ellipsometric data. All films must be described with the effective medium approximation as water swollen layers.
Aim: Multifunctional polymer-based biomaterials, which combine degradability with a shape-memory capability and in this way enable the design of actively moving implants such as self-anchoring implants or controlled release systems, have been recently introduced. Of particular interest are approved degradable polymers such as poly(L-lactide) (PLLA), which can be easily functionalized with a shape-memory effect. In the case of semicrystalline PLLA, the glass transition can be utilized as shape-memory switching domain.
Methods: In this work we applied a fully atomistic molecular dynamics simulation to study the shape-memory behavior of PLLA. A heating-deformation-cooling programming procedure was applied to atomistic PLLA packing models followed by a recovery module under stress-free conditions allowing the shape recovery. The recovery was simulated by heating the samples from T-low = 250 K to T-high = 500 K with different heating rates beta of 125, 40 and 4 K.ns(-1).
Results: We could demonstrate that the obtained strain recovery rate (R-r) was strongly influenced by the applied simulation time and heating rate, whereby R-r values in the range from 46% to 63% were achieved. On its own the application of a heating rate of 4 K.ns(-1) enabled us to determine a characteristic switching temperature of T-sw = 473 K for the modeled samples.
Conclusions: We anticipate that the atomistic modeling approach presented should be capable of enabling further study of T-sw with respect to the molecular structure of the investigated SMP and therefore could be applied in the context of design and development of new shape-memory (bio) materials.
A multiblock copolymer termed as PCL-PIBMD, consisting of crystallizable poly(epsilon-caprolactone) (PCL) segments and crystallizable poly(3S-isobutyl-morpholine-2,5-dione) (PIBMD) segments, has been reported as a material showing a thermally-induced shape-memory effect. While PIBMD crystalline domains act as netpoints to determine the permanent shape, both PCL crystalline domains and PIBMD amorphous domains, which have similar transition temperatures (T-trans) can act as switching domains. In this work, the influence of the deformation temperature (T-deform = 50 or 20 degrees C), which was above or below T-trans, on the structural changes of PCL-PIBMD during uniaxial deformation and the shapememory properties were investigated. Furthermore, the relative contribution of crystalline PCL and PIBMD amorphous phases to the fixation of the temporary shape were distinguished by a toluene vapor treatment approach. The results indicated that at 50 degrees C, both PCL and PIBMD amorphous phases can be orientated during deformation, resulting in thermally-induced crystals of PCL domains and joint contribution to the switching domains. In contrast at 20 degrees C, the temporary shape was mainly fixed by PCL crystals generated via strain-induced crystallization.
Background: Magnetic composites of thermosensitive shape-memory polymers (SMPs) and magnetite nanoparticles (MNPs) allow noncontact actuation of the shape-memory effect in an alternating magnetic field. In this study, we investigated whether the magnetic heating capability of cross-linked poly(epsilon-caprolactone)/MNP composites (cPCLC) could be improved by covalent coating of MNPs with oligo(epsilon-caprolactone) (OCL).
Methods: Two different types of cPCLC containing uncoated and OCL-coated MNP with identical magnetite weight content were prepared by thermally induced polymerization of poly(epsilon-caprolactone) diisocyanatoethyl methacrylate. Both cPCLCs exhibited a melting transition at T-m = 48 degrees C, which could be used as switching transition.
Results: The dispersion of the embedded nanoparticles within the polymer matrix could be substantially improved, when the OCL-coated MNPs were used, as visualized by scanning electron microscopy. We could further demonstrate that in this way the maximal achievable bulk temperature (T-bulk) obtained within the cPCLC test specimen in magnetic heating experiments at a magnetic field strength of H = 30 kA.m(-1) could be increased from T bulk = 48 degrees C to T bulk = 74 degrees C.
Guidance of postinfarct myocardial remodeling processes by an epicardial patch system may alleviate the consequences of ischemic heart disease. As macrophages are highly relevant in balancing immune response and regenerative processes their suitable instruction would ensure therapeutic success. A polymeric mesh capable of attracting and instructing monocytes by purely physical cues and accelerating implant degradation at the cell/implant interface is designed. In a murine model for myocardial infarction the meshes are compared to those either coated with extracellular matrix or loaded with induced cardiomyocyte progenitor cells. All implants promote macrophage infiltration and polarization in the epicardium, which is verified by in vitro experiments. 6 weeks post-MI, especially the implantation of the mesh attenuates left ventricular adverse remodeling processes as shown by reduced infarct size (14.7% vs 28-32%) and increased wall thickness (854 mu m vs 400-600 mu m), enhanced angiogenesis/arteriogenesis (more than 50% increase compared to controls and other groups), and improved heart function (ejection fraction = 36.8% compared to 12.7-31.3%). Upscaling as well as process controls is comprehensively considered in the presented mesh fabrication scheme to warrant further progression from bench to bedside.
Poly(ether imide) (PEI), which can be chemically functionalized with biologically active ligands, has emerged as a potential biomaterial for medical implants. Electrospun PEI scaffolds have shown advantageous properties, such as enhanced endothelial cell adherence, proliferation and low platelet adhesion in in vitro experiments. In this study, the in vivo behaviour of electrospun PEI scaffolds and PEI films was examined in a murine subcutaneous implantation model. Electrospun PEI scaffolds and films were surgically implanted subcutaneously in the dorsae of mice. The surrounding subcutaneous tissue response was examined via histopathological examination at 7 and 28days after implantation. No serious adverse events were observed for both types of PEI implants. The presence of macrophages or foreign body giant cells in the vicinity of the implants and the formation of a fibrous capsule indicated a normal foreign body reaction towards PEI films and scaffolds. Capsule thickness and inflammatory infiltration cells significantly decreased for PEI scaffolds during days 7-28 while remaining unchanged for PEI films. The infiltration of cells into the implant was observed for PEI scaffolds 7days after implantation and remained stable until 28days of implantation. Additionally some, but not all, PEI scaffold implants induced the formation of functional blood vessels in the vicinity of the implants. Conclusively, this study demonstrates the in vivo biocompatibility of PEI implants, with favourable properties of electrospun PEI scaffolds regarding tissue integration and wound healing.
The implementation of shape-memory effects (SME) in polymeric micro- or nano-objects currently relies on the application of indirect macroscopic manipulation techniques, for example, stretchable molds or phantoms, to ensembles of small objects. Here, we introduce a method capable of the controlled manipulation and SME quantification of individual micro- and nano-objects in analogy to macroscopic thermomechanical test procedures. An atomic force microscope was utilized to address individual electro-spun poly(ether urethane) (PEU) micro- or nanowires freely suspended between two micropillars on a micro-structured silicon substrate. In this way, programming strains of 10 +/- 1% or 21 +/- 1% were realized, which could be successfully fixed. An almost complete restoration of the original free-suspended shape during heating confirmed the excellent shape-memory performance of the PEU wires. Apparent recovery stresses of sigma(max,app)=1.2 +/- 0.1 and 33.3 +/- 0.1MPa were obtained for a single microwire and nanowire, respectively. The universal AFM test platform described here enables the implementation and quantification of a thermomechanically induced function for individual polymeric micro- and nanosystems.
Fibrous shape-memory polymer (SMP) scaffolds were investigated considering the fiber as basic microstructural feature. By reduction of the fiber diameter in randomly oriented electrospun polyetherurethane (PEU) meshes from the micro-to the nano-scale, we observed changes in the molecular orientation within the fibers and its impact on the structural and shape-memory performance. It was assumed that a spatial restriction by reduction of the fiber diameter increases molecular orientation along the orientation of the fiber. The stress-strain relation of random PEU scaffolds is initially determined by the 3D arrangement of the fibers and thus is independent of the molecular orientation. Increasing the molecular orientation with decreasing single fiber diameter in scaffolds composed of randomly arranged fibers did not alter the initial stiffness and peak stress but strongly influenced the elongation at break and the stress increase above the Yield point. Reduction of the single fiber diameter also distinctly improved the shape-memory performance of the scaffolds. Fibers with nanoscale diameters (< 100 nm) possessed an almost complete shape recovery, high recovery stresses and fast relaxation kinetics, while the shape fixity was found to decrease with decreasing fiber diameter. Hence, the fiber diameter is a relevant design parameter for SMP.
Crosslinking of thermoplastics is a versatile method to create crystallizable polymer networks, which are of high interest for shape-memory actuators. Here, crosslinked poly(epsilon-caprolactone) thermosets (cPCLs) were prepared from linear starting material, whereby the amount of extractable polymer was varied. Fractions of 5-60 wt % of non-crosslinked polymer chains, which freely interpenetrate the crosslinked network, were achieved leading to differences in the resulting phase of the bulk material. This can be described as "sponge-like" with open or closed compartments depending on the amount of interpenetrating polymer. The crosslinking density and the average network chain length remained in a similar range for all network structures, while the theoretical accessible volume for reptation of the free polymer content is affected. This feature could influence or introduce new functions into the material created by thermomechanical treatment. The effect of interpenetrating PCL in cPCLs on the reversible actuation was analyzed by cyclic, uniaxial tensile tests. Here, high reversible strains of up to Delta epsilon = 24% showed the enhanced actuation performance of networks with a non-crosslinked PCL content of 30 wt % resulting from the crystal formation in the phase of the non-crosslinked PCL and co-crystallization with network structures. Additional functionalities are reprogrammability and self-healing capabilities for networks with high contents of extractable polymer enabling reusability and providing durable actuator materials.
BACKGROUND: The formation of a functionally-confluent endothelial cell (EC) monolayer affords proliferation of EC, which only happens in case of appropriate migratory activity. AIM OF THE STUDY: The migratory pathway of human umbilical endothelial cells (HUVEC) was investigated on different polymeric substrates. MATERIAL AND METHODS: Surface characterization of the polymers was performed by contact angle measurements and atomic force microscopy under wet conditions. 30,000 HUVEC per well were seeded on polytetrafluoroethylene (PTFE) (theta(adv) = 119 degrees +/- 2 degrees), on low-attachment plate LAP (theta(adv) = 28 degrees +/- 2 degrees) and on polystyrene based tissue culture plates (TCP, theta(adv) = 22 degrees +/- 1 degrees). HUVEC tracks (trajectories) were recorded by time lapse microscopy and the euclidean distance (straight line between starting and end point), the total distance and the velocities of HUVEC not leaving the vision field were determined. RESULTS: On PTFE, 42 HUVEC were in the vision field directly after seeding. The mean length of single migration steps (SML) was 6.1 +/- 5.2 mu m, the mean velocity (MV) 0.40 +/- 0.3 mu m.min(-1) and the complete length of the trajectory (LT) was 710 +/- 440 mu m. On TCP 82 HUVEC were in the vision field subsequent to seeding. The LT was 840 +/- 550 mu m, the SML 6.1 +/- 5.2 mu m and the MV 0.44 +/- 0.3 mu m.min(-1). The trajectories on LAP differed significantly in respect to SML (2.4 +/- 3.9 mu m, p <0.05), the MV (0.16 +/- 0.3 mu m.min(-1), p <0.05) and the LT (410 +/- 300 mu m, p <0.05), compared to PTFE and TCP. Solely on TCP a nearly confluent EC monolayer developed after three days. While on TCP diffuse signals of vinculin were found over the whole basal cell surface organizing the binding of the cells by focal adhesions, on PTFE vinculin was merely arranged at the cell rims, and on the hydrophilic material (LAP) no focal adhesions were found. CONCLUSION: The study revealed that the wettability of polymers affected not only the initial adherence but also the migration of EC, which is of importance for the proliferation and ultimately the endothelialization of polymer-based biomaterials.
Cardiovascular metallic stents established in clinical application are typically coated by a thin polymeric layer on the stent struts to improve hemocompatibility, whereby often a drug is added to the coating to inhibit neointimal hyperplasia. Besides such thin film coatings recently nano/microfiber coated stents are investigated, whereby the fibrous coating was applied circumferential on stents. Here, we explored whether a thin fibrous encasement of metallic stents with preferentially longitudinal aligned fibers and different local fiber densities can be achieved by electrospinning. An elastic degradable copolyetheresterurethane, which is reported to selectively enhance the adhesion of endothelial cells, while simultaneously rejecting smooth muscle cells, was utilized for stent coating. The fibrous stent encasements were microscopically assessed regarding their single fiber diameters, fiber covered area and fiber alignment at three characteristic stent regions before and after stent expansion. Stent coatings with thicknesses in the range from 30 to 50 mu m were achieved via electrospinning with 1,1,1,3,3,3-hexafluoro-2-propanol (HFP)-based polymer solution, while a mixture of HFP and formic acid as solvent resulted in encasements with a thickness below 5 mu m comprising submicron sized single fibers. All polymeric encasements were mechanically stable during expansion, whereby the fibers deposited on the struts remained their position. The observed changes in fiber density and diameter indicated diverse local deformation mechanisms of the microfibers at the different regions between the struts. Based on these results it can be anticipated that the presented fibrous encasement of stents might be a promising alternative to stents with polymeric strut coatings releasing anti-proliferative drugs. Copyright (c) 2015 John Wiley & Sons, Ltd.
Electrical actuation of coated and composite fibers based on poly[ethylene-co-(vinyl acetate)]
(2020)
Robots are typically controlled by electrical signals. Resistive heating is an option to electrically trigger actuation in thermosensitive polymer systems. In this study electrically triggerable poly[ethylene-co-(vinyl acetate)] (PEVA)-based fiber actuators are realized as composite fibers as well as polymer fibers with conductive coatings. In the coated fibers, the core consists of crosslinked PEVA (cPEVA), while the conductive coating shell is achieved via a dip coating procedure with a coating thickness between 10 and 140 mu m. The conductivity of coated fibers sigma = 300-550 S m(-1) is much higher than that of the composite fibers sigma = 5.5 S m(-1). A voltage (U) of 110 V is required to heat 30 cm of coated fiber to a targeted temperature of approximate to 65 degrees C for switching in less than a minute. Cyclic electrical actuation investigations reveal epsilon '(rev) = 5 +/- 1% reversible change in length for coated fibers. The fabrication of such electro-conductive polymeric actuators is suitable for upscaling so that their application potential as artificial muscles can be explored in future studies.
Electrical actuation of coated and composite fibers based on poly[ethylene-co-(vinyl acetate)]
(2020)
Robots are typically controlled by electrical signals. Resistive heating is an option to electrically trigger actuation in thermosensitive polymer systems. In this study electrically triggerable poly[ethylene-co-(vinyl acetate)] (PEVA)-based fiber actuators are realized as composite fibers as well as polymer fibers with conductive coatings. In the coated fibers, the core consists of crosslinked PEVA (cPEVA), while the conductive coating shell is achieved via a dip coating procedure with a coating thickness between 10 and 140 mu m. The conductivity of coated fibers sigma = 300-550 S m(-1) is much higher than that of the composite fibers sigma = 5.5 S m(-1). A voltage (U) of 110 V is required to heat 30 cm of coated fiber to a targeted temperature of approximate to 65 degrees C for switching in less than a minute. Cyclic electrical actuation investigations reveal epsilon '(rev) = 5 +/- 1% reversible change in length for coated fibers. The fabrication of such electro-conductive polymeric actuators is suitable for upscaling so that their application potential as artificial muscles can be explored in future studies.
Remaining uremic toxins in the blood of chronic renal failure patients represent one central challenge in hemodialysis therapies. Highly porous poly(ether imide) (PEI) microparticles have been recently introduced as candidate absorber materials, which show a high absorption capacity for uremic toxins and allow hydrophilic surface modification suitable for minimization of serum protein absorption. In this work, the effects of extracts prepared from PEI microparticles modified by nucleophilic reaction with low molecular weight polyethylene imine (Pei) or potassium hydroxide (KOH), on human monocytic (THP-1) cells are studied. The obtained results suggested that the extracts of Pei and KOH modified PEI absorbers have no negative effect on THP-1 cell viability and do not initiate the critical differentiation towards macrophages. The extracts did not enhance transcript or protein levels of investigated proinflammatory markers in THP-1 cells, namely, TNF alpha, MCP1, IL6 and IL8. Based on these findings such modified PEI microparticles should be qualified for further pre-clinical evaluation i.e. in an in vivo animal experiment.
A high cell viability of around 99 +/- 18% and 99 +/- 5% was observed when THP-1 cells were cultured in the presence of aqueous extracts of the PEI microparticles in medium A and medium B respectively. The obtained microscopic data suggested that PEI particle extracts have no significant effect on cell death, oxidative stress or differentiation to macrophages. It was further found that the investigated proinflammatory markers in THP-1 cells were not up-regulated. These results are promising with regard to the biocompatibility of PEI microparticles and in a next step the hemocompatibility of the microparticles will be examined.
Soft actuator performance can be tuned by chemistry or mechanical manipulation, but this adjustability is limited especially in view of their growing technological relevance. Inspired from textile engineering, we designed and fabricated fiber mesh actuators and introduced new features like anisotropic behavior and soft-tissue like elastic deformability. Design criteria for the meshes are the formation of fiber bundles, the angle between fiber bundles in different stacked layers and covalent crosslinks forming within and between fibers at their interfacial contact areas. Through crosslinking the interfiber bond strength increased from a bond transmitting neither axial nor rotational loads (pin joint) to a bond strength capable of both (welded joint). For non-linear elastic stiffening, stacked fiber bundles with four embracing fibers were created forming microstructural rhombus shapes. Loading the rhombus diagonally allowed generation of “soft tissue”-like mechanics. By adjustment of stacking angles, the point of strong increase in stress is tuned. While the highest stresses are observed in aligned and crosslinked fiber mats along the direction of the fiber, the strongest shape-memory actuation behavior is found in randomly oriented fiber mats. Fiber mesh actuators controlled by temperature are of high significance as soft robot skins and as for active patches supporting tissue regeneration.
Langmuir monolayer degradation (LMD) experiments with polymers possessing outstanding biomedical application potential yield information regarding the kinetics of their hydrolytic or enzymatic chain scission under well-defined and adjustable degradation conditions. A brief review is given of LMD investigations, including the author's own work on 2-dimensional (2D) polymer systems, providing chain scission data, which are not disturbed by simultaneously occurring transport phenomena, such as water penetration into the sample or transport of scission fragments out of the sample.
A knowledge-based approach for the description and simulation of polymer hydrolytic and enzymatic degradation based on a combination of fast LMD experiments and computer simulation of the water penetration is briefly introduced. Finally, the advantages and disadvantages of this approach are discussed.
The endothelialization of synthetic surfaces applied as cardiovascular implant materials is an important issue to ensure the anti-thrombotic quality of a biomaterial. However, the rapid and constant development of a functionallycon-fluent endothelial cell monolayer is challenging. In order to investigate the compatibility of potential implant materials with endothelial cells several in vitro studies are performed. Here, glass and tissue culture plates (TCP) are often used as reference materials for in vitro pre-testing. However, a direct comparison of both substrates is lacking. Therefore, a comparison of study results is difficult, since results are often related to various reference materials. In this study, the endothelialization of glass and TCP was investigated in terms of adherence, morphology, integrity, viability and function using human umbilical vein endothelial cells (HUVEC). On both substrates an almost functionally confluent HUVEC monolayer was developed after nine days of cell seeding with clearly visible cell rims, decreased stress fiber formation and a pronounced marginal filament band. The viability of HUVEC was comparable for both substrates nine days after cell seeding with only a few dead cells. According to that, the cell membrane integrity as well as the metabolic activity showed no differences between TCP and glass. However, a significant difference was observed for the secretion of IL-6 and IL-8. The concentration of both cytokines, which are associated with migratory activity, was increased in the supernatant of HUVEC seeded on TCP. This result matches well with the slightly increased number of adherent HUVEC on TCP. In conclusion, these findings indicate that both reference materials are almost comparable and can be used equivalently as control materials in in vitro endothelialization studies.
In biomaterial development, the design of material surfaces that mimic the extra-cellular matrix (ECM) in order to achieve favorable cellular instruction is rather challenging. Collagen-type IV (Col-IV), the major scaffolding component of Basement Membranes (BM), a specialized ECM with multiple biological functions, has the propensity to form networks by self-assembly and supports adhesion of cells such as endothelial cells or stem cells. The preparation of biomimetic Col-IV network-like layers to direct cell responses is difficult. We hypothesize that the morphology of the layer, and especially the density of the available adhesion sites, regulates the cellular adhesion to the layer. The Langmuir monolayer technique allows for preparation of thin layers with precisely controlled packing density at the air-water (A-W) interface. Transferring these layers onto cell culture substrates using the Langmuir-Schafer (LS) technique should therefore provide a pathway for preparation of BM mimicking layers with controlled cell adherence properties. In situ characterization using ellipsometry and polarization modulation-infrared reflection absorption spectroscopy of Col-IV layer during compression at the A-W interface reveal that there is linear increase of surface molecule concentration with negligible orientational changes up to a surface pressure of 25 mN m(-1). Smooth and homogeneous Col-IV network-like layers are successfully transferred by LS method at 15 mN m(-1) onto poly(ethylene terephthalate) (PET), which is a common substrate for cell culture. In contrast, the organization of Col-IV on PET prepared by the traditionally employed solution deposition method results in rather inhomogeneous layers with the appearance of aggregates and multilayers. Progressive increase in the number of early adherent mesenchymal stem cells (MSCs) after 24 h by controlling the areal Col-IV density by LS transfer at 10, 15 and 20 mN m(-1) on PET is shown. The LS method offers the possibility to control protein characteristics on biomaterial surfaces such as molecular density and thereby, modulate cell responses.
A series of multiblock copolymers (PDLCL) synthesized from oligo(omega-pentadecalactone) diol (OPDL) and oligo(epsilon-caprolactone) diol (OCL), which are linked by 2,2(4), 4-trimethyl-hexamethylene diisocyanate (TMDI), is investigated by the Langmuir monolayer technique at the air-water interface. Brewster angle microscopy (BAM) and spectroscopic ellipsometry are employed to characterize the polymer film morphologies in situ. PDLCL containing >= 40 wt% OCL segments form homogeneous Langmuir monofilms after spreading. The film elasticity modulus decreases with increasing amounts of OPDL segments in the copolymer. In contrast, the OCL-free polyesterurethane OPDL-TMDI cannot be spread to monomolecular films on the water surface properly, and movable slabs are observed by BAM even at low surface pressures. The results of the in situ morphological characterization clearly show that essential information concerning the reliability of Langmuir monolayer degradation (LMD) experiments cannot be obtained from the evaluation of the pi-A isotherms only. Consequently, in situ morphological characterization turns out to be indispensable for characterization of Langmuir layers before LMD experiments.
A versatile strategy to integrate multiple functions in a polymer based material is the formation of polymer networks with defined nanostructures. Here, we present synthesis and comprehensive characterization of covalently surface functionalized magnetic nanoparticles (MNPs) comprising a bi-layer oligomeric shell, using Sn(Oct)(2) as catalyst for a two-step functionalization. These hydroxy-terminated precursors for degradable magneto-and thermo-sensitive polymer networks were prepared via two subsequent surfaceinitiated ring-opening polymerizations (ROPs) with omega-pentadecalactone and e-caprolactone. A two-step mass loss obtained in thermogravimetric analysis and two distinct melting transitions around 50 and 85 degrees C observed in differential scanning calorimetry experiments, which are attributed to the melting of OPDL and OCL crystallites, confirmed a successful preparation of the modified MNPs. The oligomeric coating of the nanoparticles could be visualized by transmission electron microscopy. The investigation of degrafted oligomeric coatings by gel permeation chromatography and H-1-NMR spectroscopy showed an increase in number average molecular weight as well as the presence of signals related to both of oligo(omega-pentadecalactone) (OPDL) and oligo(e-caprolactone) (OCL) after the second ROP. A more detailed analysis of the NMR results revealed that only a few.-pentadecalactone repeating units are present in the degrafted oligomeric bi-layers, whereby a considerable degree of transesterification could be observed when OPDL was polymerized in the 2nd ROP step. These findings are supported by a low degree of crystallinity for OPDL in the degrafted oligomeric bi-layers obtained in wide angle X-ray scattering experiments. Based on these findings it can be concluded that Sn(Oct)(2) was suitable as catalyst for the preparation of nanosized bi-layered coated MNP precursors by a two-step ROP.
An atomistic molecular dynamics simulation approach is applied to model the influence of urethane linker units as well as the addition of water molecules on the simulated shape-memory properties of poly[(rac-lactide)-co-glycolide] (PLGA) and PLGA-based copolyester urethanes comprising different urethane linkers. The shape-memory performance of these amorphous packing models is explored in a simulated heating-deformation-cooling-heating procedure. Depending on the type of incorporated urethane linker, the mechanical properties of the dry copolyester urethanes are found to be significantly improved compared with PLGA, which can be attributed to the number of intermolecular hydrogen bonds between the urethane units. Good shape-memory properties are observed for all the modeled systems. In the dry state, the shape fixation is found to be improved by implementation of urethane units. After swelling of the copolymer models with water, which results in a reduction of their glass transition temperatures, the relaxation kinetics during unloading and shape recovery are found to be substantially accelerated.
An ellipsometric approach towards the description of inhomogeneous polymer-based Langmuir layers
(2016)
The applicability of nulling-based ellipsometric mapping as a complementary method next to Brewster angle microscopy (BAM) and imaging ellipsometry (IE) is presented for the characterization of ultrathin films at the air-water interface. First, the methodology is demonstrated for a vertically nonmoving Langmuir layer of star-shaped, 4-arm poly(omega-pentadecalactone) (PPDL-D4). Using nulling-based ellipsometric mapping, PPDL-D4-based inhomogeneously structured morphologies with a vertical dimension in the lower nm range could be mapped. In addition to the identification of these structures, the differentiation between a monolayer and bare water was possible. Second, the potential and limitations of this method were verified by applying it to more versatile Langmuir layers of telechelic poly[(rac-lactide)-co-glycolide]-diol (PLGA). All ellipsometric maps were converted into thickness maps by introduction of the refractive index that was derived from independent ellipsometric experiments, and the result was additionally evaluated in terms of the root mean square roughness, R-q. Thereby, a three-dimensional view into the layers was enabled and morphological inhomogeneity could be quantified.
Uremia is a phenomenon caused by retention of uremic toxins in the plasma due to functional impairment of kidneys in the elimination of urinary waste products. Uremia is presently treated by dialysis techniques like hemofiltration, dialysis or hemodiafiltration. However, these techniques in use are more favorable towards removing hydrophilic than hydrophobic uremic toxins. Hydrophobic uremic toxins, such as hydroxy hipuric acid (OH-HPA), phenylacetic acid (PAA), indoxyl sulfate (IDS) and p-cresylsulfate (pCRS), contribute substantially to the progression of chronic kidney disease (CKD) and cardiovascular disease. Therefore, objective of the present study is to test adsorption capacity of highly porous microparticles prepared from poly(ether imide) (PEI) as an alternative technique for the removal of uremic toxins. Two types of nanoporous, spherically shaped microparticles were prepared from PEI by a spraying/coagulation process. PEI particles were packed into a preparative HPLC column to which a mixture of the four types of uremic toxins was injected and eluted with ethanol. Eluted toxins were quantified by analytical HPLC. PEI particles were able to adsorb all four toxins, with the highest affinity for PAA and pCR. IDS and OH-HPA showed a partially non-reversible binding. In summary, PEI particles are interesting candidates to be explored for future application in CKD.