@article{BaeckemoLiuLendlein2021, author = {B{\"a}ckemo, Johan Dag Valentin and Liu, Yue and Lendlein, Andreas}, title = {Bio-inspired and computer-supported design of modulated shape changes in polymer materials}, series = {MRS communications / a publication of the Materials Research Society}, volume = {11}, journal = {MRS communications / a publication of the Materials Research Society}, number = {4}, publisher = {Springer}, address = {Berlin}, issn = {2159-6867}, doi = {10.1557/s43579-021-00056-6}, pages = {462 -- 469}, year = {2021}, abstract = {The Venus flytrap is a fascinating plant with a finely tuned mechanical bi-stable system, which can switch between mono- and bi-stability. Here, we combine geometrical design of compliant mechanics and the function of shape-memory polymers to enable switching between bi- and mono-stable states. Digital design and modelling using the Chained Beam Constraint Model forecasted two geometries, which were experimentally realized as structured films of cross-linked poly[ethylene-co-(vinyl acetate)] supported by digital manufacturing. Mechanical evaluation confirmed our predicted features. We demonstrated that a shape-memory effect could switch between bi- and mono-stability for the same construct, effectively imitating the Venus flytrap.}, language = {en} } @article{IzraylitLiuTarazonaetal.2021, author = {Izraylit, Victor and Liu, Yue and Tarazona, Natalia A. and Machatschek, Rainhard Gabriel and Lendlein, Andreas}, title = {Crystallization and degradation behaviour of multiblock copolyester blends in Langmuir monolayers}, series = {MRS communications / a publication of the Materials Research Society}, volume = {11}, journal = {MRS communications / a publication of the Materials Research Society}, number = {6}, publisher = {Springer}, address = {Berlin}, issn = {2159-6859}, doi = {10.1557/s43579-021-00107-y}, pages = {850 -- 855}, year = {2021}, abstract = {Supporting the wound healing of soft tissues requires fixation devices becoming more elastic while degrading. To address this unmet need, we designed a blend of degradable multiblock copolymers, which is cross-linked by PLA stereocomplexation combining two soft segments differing substantially in their hydrolytic degradation rate. The degradation path and concomitant structural changes are predicted by Langmuir monolayer technique. The fast hydrolysis of one soft segment leads to a decrease of the total polymer mass at constant physical cross-linking density. The corresponding increase of the average spacing between the network nodes suggests the targeted increase of the blend's flexibility.}, language = {en} } @article{KruegerGengeDietzeYanetal.2018, author = {Kr{\"u}ger-Genge, Anne and Dietze, Stefanie and Yan, Wan and Liu, Yue and Fang, Liang and Kratz, Karl and Lendlein, Andreas and Jung, Friedrich}, title = {Endothelial cell migration, adhesion and proliferation on different polymeric substrates}, series = {Clinical hemorheology and microcirculation : blood flow and vessels}, volume = {70}, journal = {Clinical hemorheology and microcirculation : blood flow and vessels}, number = {4}, publisher = {IOS Press}, address = {Amsterdam}, issn = {1386-0291}, doi = {10.3233/CH-189317}, pages = {511 -- 529}, year = {2018}, abstract = {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.}, language = {en} } @phdthesis{Liu2021, author = {Liu, Yue}, title = {Polymeric objects switchable between two shapes}, school = {Universit{\"a}t Potsdam}, pages = {xv, 73}, year = {2021}, abstract = {As the ongoing trend of developing smart materials that can reversibly switch geometry stimulated by environmental control addressed increasing attention in many research fields, especially for biomedical or soft robotic applications. Shape-memory polymers (SMPs), which can change shape, stiffness, size, and structure when exposed to an external stimulus, are intensively explored as encouraging material candidates for achieving multifunctionality, and for miniaturizing into micro-components to expand the applications. Besides, the geometrical design has gained growing attention for creating engineering applications, such as bi-stable mechanisms, and has the potential to be explored by implementing SMP for new functions. In this context, this thesis aimed to develop smart micro-/nano-objects based on SMP and explore new functions by geometrical design using SMP. Here, two types of stimuli-responsive objects capable of one-way temperature-memory effect (TME) or free-standing reversible actuation e.g., micro/nanofibers (i) and microcuboids (ii) at different aspects were explored. At first, it was hypothesized that the advanced atomic force microscopy (AFM) platform can be established to study individual polymeric micro-/nanofibers (i) in terms of incorporation and characterization of a reversible shape-memory actuation capability. Crystallizable material was chosen for preparing the fibers and the molecular alignment within the fibers among different diameters will influence the crystallization-induced elongation during cooling that determined the reversible effect. For the second type, microcuboids (ii), it was hypothesized that a programming and quantification approach can be developed to enable the realization and characterization of a one-way micro-TME and micro-shape-memory polymer actuation (SMPA) in microcuboids. The responsive temperature of one-way shape transformation can be tuned by programming temperature (Tp) and the separation temperature (Tsep) for post-programming can influence the actuation. Finally, a geometrical design with bi-stability was combined with SME to create new functions of shape actuation. It was hypothesized that the predicted bi-stable or mono-stable structures can be achieved with the aid of digital fabrication methods. Using shape-memory effect (SME), the alteration of bi-stable and mono-stable can initiate shape transformation with a larger magnitude and higher energy output. In the first part, the method to quantify the reversible SMPA of a single micro/nano crystallizable fiber with geometry change during the actuation was explored. Electrospinning was used to prepare poly (ε-caprolactone) (PCL) micro/nanofiber with different diameters, which were fixed by UV glue and crosslinked on the structured silicon wafer. Using AFM, the programming, as well as the observation of recovery and reversible displacement of the fiber, were performed by vertical three-point bending at the free suspended part. A plateau tip was chosen to achieve stable contact and longer working distance for performing larger deformation, enabling intensified reversible SMPA of single fibers. In this way, programming strains of 39 ± 1\% or 46 ± 1\% were realized for fiber with a diameter of 1 ± 0.2 µm and 300 ± 50 nm, which were bent at 80 °C and fixed at 10 °C. Values for the reversible elongation of εrev = 3.4 ± 0.1\% and 10.5 ± 0.1\% were obtained for a single micro and nanofiber respectively between 10 and 60 °C. The higher actuation effect observed for nanofiber demonstrated that the highly compact and oriented crystallites in nanofibers, which determined the pronounced εrev compared to the thick microfibers. Besides, a stable reversible actuation of a nanofiber can be tracked by AFM tip up to 10 cycles, indicating a sustainable application can be achieved on the fiber actuators. The findings obtained for cPCL micro-/nano-fibers will help design and evaluate the next generation polymeric microactuators or micromanipulators. The second part of the thesis studies the shape-memory effect (SME) of a single individual SMP micro-object by controlling deformation temperatures during programming and actuation temperatures during reversible change. In this work, microcuboids of crosslinked poly[ethylene-co-(vinyl acetate)] (cPEVA) elastomers with 18 wt\% vinyl acetate (VA) contents were successfully prepared by template-based replication from polydimethylsiloxane (PDMS) mold. The micro-TME and micro-SMPA were observed and studied based on micro-geometry change using optical microscopy (OM) and AFM. Different switching temperatures of shape recovery were achieved from 55 °C to 86 °C by tuning Tp from 55 °C to 100 °C, indicating a successful implementation of micro-TME on individual microcuboid. For micro-SMPA functionalization, microcuboids were deformed by compression at 100 °C and the change in single particle height was monitored during cyclic heating and cooling between various Tseps from 60 °C to 85 °C and 20 °C. The micro-SMPA on a single microcuboid was achieved with a reversible strain in the range of 2 to 7\%, whereby higher compression ratio CR and Tsep induced prominent reversible strain. The results achieved in this work demonstrated the successful functionalization of microcuboids with different SMEs by controlling temperatures during programming and actuation processes. Based on these achievements, such micro-objects can be further designed as on demand switchable microactuators or release systems with adjustable working temperatures. In the last part of the work, a new function of shape-memory polymeric bi-stable 3D structured film was designed and fabricated. The SME and geometrical design of compliant mechanics were merged to enable switching between bi-stable and mono-stable states, which generate snap movement that mimics the Venus flytrap. A truncated tetrahedron structure with a slope angle as a tunable parameter to alter the bi-stability was chosen for the study to combine with SME. It was anticipated that the structured film designed with a slope angle of 30° exhibited mono-stable behavior, and such a structure with a slope angle of 45° exhibited bi-stable behavior. Then the structured SMP film of designed mono-stable shape was successfully fabricated using soft lithography based on 3D printed master molds supported from digital manufacturing. The structured mold was also used in programming the SMP film into the structure with a higher slope angle to attain bi-stability. Finally, the switching between bi-stable and mono-stable states was successfully realized using SME, which introduces snapping movement triggered by heat. The implementation of compliant mechanisms by the SME increased the magnitude of thermally induced reconfiguration without additional external force. To sum up, the results of the thesis support the development of smart objects capable of one-way micro-TME, free-standing reversible actuation, or bi-stability mediated shape-memory reconfiguration. Electrospinning and template-based method were used for fabrication with good control of geometry and low size dispersity. Microscopy methods especially the AFM platform with decent sensitivity was developed for implementation as well as characterization of SME on individual micro-/nanoobjects. Implementation of bi-stability improves the shape transformation amplitude of thermally triggered SMP. These findings can give novel insights for designing polymer-based actuators or soft robotics.}, language = {en} } @article{LiuGouldKratzetal.2020, author = {Liu, Yue and Gould, Oliver E. C. and Kratz, Karl and Lendlein, Andreas}, title = {Shape-memory actuation of individual micro-/nanofibers}, series = {MRS Advances}, volume = {5}, journal = {MRS Advances}, number = {46-47}, publisher = {Cambridge Univ. Press}, address = {New York}, issn = {2059-8521}, doi = {10.1557/adv.2020.276}, pages = {2391 -- 2399}, year = {2020}, abstract = {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.}, language = {en} } @article{LiuGouldKratzetal.2022, author = {Liu, Yue and Gould, Oliver E. C. and Kratz, Karl and Lendlein, Andreas}, title = {On demand sequential release of (sub)micron particles controlled by size and temperature}, series = {Small : nano micro}, volume = {18}, journal = {Small : nano micro}, number = {5}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1613-6810}, doi = {10.1002/smll.202104621}, pages = {8}, year = {2022}, abstract = {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.}, language = {en} } @article{LiuGouldRudolphetal.2020, author = {Liu, Yue and Gould, Oliver E. C. and Rudolph, Tobias and Fang, Liang and Kratz, Karl and Lendlein, Andreas}, title = {Polymeric microcuboids programmable for temperature-memory}, series = {Macromolecular materials and engineering}, volume = {305}, journal = {Macromolecular materials and engineering}, number = {10}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1438-7492}, doi = {10.1002/mame.202000333}, pages = {7}, year = {2020}, abstract = {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.}, language = {en} } @article{LiuRazzaqRudolphetal.2017, author = {Liu, Yue and Razzaq, Muhammad Yasar and Rudolph, Tobias and Fang, Liang and Kratz, Karl and Lendlein, Andreas}, title = {Two-Level Shape Changes of Polymeric Microcuboids Prepared from Crystallizable Copolymer Networks}, series = {Macromolecules : a publication of the American Chemical Society}, volume = {50}, journal = {Macromolecules : a publication of the American Chemical Society}, publisher = {American Chemical Society}, address = {Washington}, issn = {0024-9297}, doi = {10.1021/acs.macromol.6b02237}, pages = {2518 -- 2527}, year = {2017}, abstract = {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.}, language = {en} } @article{WangKratzBehletal.2015, author = {Wang, Weiwei and Kratz, Karl and Behl, Marc and Yan, Wan and Liu, Yue and Xu, Xun and Baudis, Stefan and Li, Zhengdong and Kurtz, Andreas and Lendlein, Andreas and Ma, Nan}, title = {The interaction of adipose-derived human mesenchymal stem cells and polyether ether ketone}, series = {Clinical hemorheology and microcirculation : blood flow and vessels}, volume = {61}, journal = {Clinical hemorheology and microcirculation : blood flow and vessels}, number = {2}, publisher = {IOS Press}, address = {Amsterdam}, issn = {1386-0291}, doi = {10.3233/CH-152001}, pages = {301 -- 321}, year = {2015}, abstract = {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.}, language = {en} } @article{ZhangLiuMachatscheketal.2022, author = {Zhang, Shanshan and Liu, Yue and Machatschek, Rainhard Gabriel and Lendlein, Andreas}, title = {Ultrathin collagen type I films formed at the air-water interface}, series = {MRS advances : a journal of the Materials Research Society (MRS)}, volume = {7}, journal = {MRS advances : a journal of the Materials Research Society (MRS)}, number = {4}, publisher = {Springer Nature Switzerland AG}, address = {Cham}, issn = {2059-8521}, doi = {10.1557/s43580-021-00160-8}, pages = {56 -- 62}, year = {2022}, abstract = {Collagen-based biomaterials with oriented fibrils have shown great application potential in medicine. However, it is still challenging to control the type I collagen fibrillogenesis in ultrathin films. Here, we report an approach to produce cohesive and well-organized type I collagen ultrathin films of about 10 nm thickness using the Langmuir-Blodgett technique. Ellipsometry, rheology, and Brewster angle microscopy are applied to investigate in situ how the molecules behave at the air-water interface, both at room temperature and 37 degrees C. The interfacial storage modulus observed at room temperature vanishes upon heating, indicating the existence and disappearance of the network structure in the protein nanosheet. The films were spanning over holes as large as 1 mm diameter when transferred at room temperature, proving the strong cohesive interactions. A highly aligned and fibrillar structure was observed by atomic force microscopy (AFM) and optical microscopy.}, language = {en} }