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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.
The tissue integration of synthetic polymers can be promoted by displaying RGD peptides at the biointerface with the objective of enhancing colonization of the material by endogenous cells. A firm but flexible attachment of the peptide to the polymer matrix, still allowing interaction with receptors, is therefore of interest. Here, the covalent coupling of flexible physical anchor groups, allowing for temporary immobilization on polymeric surfaces via hydrophobic or dipole-dipole interactions, to a RGD peptide was investigated. For this purpose, a stearate or an oligo(ethylene glycol) (OEG) was attached to GRGDS in 51-69% yield. The obtained RGD linker constructs were characterized by NMR, IR and MALDI-ToF mass spectrometry, revealing that the commercially available OEG and stearate linkers are in fact mixtures of similar compounds. The RGD linker constructs were co-electrospun with poly(p-dioxanone) (PPDO). After electrospinning, nitrogen could be detected on the surface of the PPDO fibers by X-ray photoelectron spectroscopy. The nitrogen content exceeded the calculated value for the homogeneous material mixture suggesting a pronounced presentation of the peptide on the fiber surface. Increasing amounts of RGD linker constructs in the electrospinning solution did not lead to a detection of an increased amount of peptide on the scaffold surface, suggesting inhomogeneous distribution of the peptide on the PPDO fiber surface. Human adipose-derived stem cells cultured on the patches showed similar viability as when cultured on PPDO containing pristine RGD. The fully characterized RGD linker constructs could serve as valuable tools for the further development of tissue-integrating polymeric scaffolds. Copyright (c) 2016 John Wiley & Sons, Ltd.
A unique fabrication process of low molar mass, crystalline polypeptoid fibers is described. Thermoresponsive fiber mats are prepared by electrospinning a homogeneous blend of semicrystalline poly(N-(n-propyl) glycine) (PPGly; 4.1 kDa) with high molar mass poly(ethylene oxide) (PEO). Annealing of these fibers at approximate to 100 degrees C selectively removes the PEO and produces stable crystalline fiber mats of pure PPGly, which are insoluble in aqueous solution but can be redissolved in methanol or ethanol. The formation of water-stable polypeptoid fiber mats is an important step toward their utilization in biomedical applications such as tissue engineering or wound dressing.
Nanofibrous mats are interesting scaffold materials for biomedical applications like tissue engineering due to their interconnectivity and their size dimension which mimics the native cell environment. Electrospinning provides a simple route to access such fiber meshes. This thesis addresses the structural and functional control of electrospun fiber mats. In the first section, it is shown that fiber meshes with bimodal size distribution could be obtained in a single-step process by electrospinning. A standard single syringe set-up was used to spin concentrated poly(ε-caprolactone) (PCL) and poly(lactic-co-glycolic acid) (PLGA) solutions in chloroform and meshes with bimodal-sized fiber distribution could be directly obtained by reducing the spinning rate at elevated humidity. Scanning electron microscopy (SEM) and mercury porosity of the meshes suggested a suitable pore size distribution for effective cell infiltration. The bimodal fiber meshes together with unimodal fiber meshes were evaluated for cellular infiltration. While the micrometer fibers in the mixed meshes generate an open pore structure, the submicrometer fibers support cell adhesion and facilitate cell bridging on the large pores. This was revealed by initial cell penetration studies, showing superior ingrowth of epithelial cells into the bimodal meshes compared to a mesh composed of unimodal 1.5 μm fibers. The bimodal fiber meshes together with electrospun nano- and microfiber meshes were further used for the inorganic/organic hybrid fabrication of PCL with calcium carbonate or calcium phosphate, two biorelevant minerals. Such composite structures are attractive for the potential improvement of properties such as stiffness or bioactivity. It was possible to encapsulate nano and mixed sized plasma-treated PCL meshes to areas > 1 mm2 with calcium carbonate using three different mineralization methods including the use of poly(acrylic acid). The additive seemed to be useful in stabilizing amorphous calcium carbonate to effectively fill the space between the electrospun fibers resulting in composite structures. Micro-, nano- and mixed sized fiber meshes were successfully coated within hours by fiber directed crystallization of calcium phosphate using a ten-times concentrated simulated body fluid. It was shown that nanofibers accelerated the calcium phosphate crystallization, as compared to microfibers. In addition, crystallizations performed at static conditions led to hydroxyapatite formations whereas in dynamic conditions brushite coexisted. In the second section, nanofiber functionalization strategies are investigated. First, a one-step process was introduced where a peptide-polymer-conjugate (PLLA-b-CGGRGDS) was co-spun with PLGA in such a way that the peptide is enriched on the surface. It was shown that by adding methanol to the chloroform/blend solution, a dramatic increase of the peptide concentration at the fiber surface could be achieved as determined by X-ray photoelectron spectroscopy (XPS). Peptide accessibility was demonstrated via a contact angle comparison of pure PLGA and RGD-functionalized fiber meshes. In addition, the electrostatic attraction between a RGD-functionalized fiber and a silica bead at pH ~ 4 confirmed the accessibility of the peptide. The bioactivity of these RGD-functionalized fiber meshes was demonstrated using blends containing 18 wt% bioconjugate. These meshes promoted adhesion behavior of fibroblast compared to pure PLGA meshes. In a second functionalization approach, a modular strategy was investigated. In a single step, reactive fiber meshes were fabricated and then functionalized with bioactive molecules. While the electrospinning of the pure reactive polymer poly(pentafluorophenyl methacrylate) (PPFPMA) was feasible, the inherent brittleness of PPFPMA required to spin a PCL blend. Blends and pure PPFPMA showed a two-step functionalization kinetics. An initial fast reaction of the pentafluorophenyl esters with aminoethanol as a model substance was followed by a slow conversion upon further hydrophilization. This was analysed by UV/Vis-spectroscopy of the pentaflurorophenol release upon nucleophilic substitution with the amines. The conversion was confirmed by increased hydrophilicity of the resulting meshes. The PCL/PPFPMA fiber meshes were then used for functionalization with more complex molecules such as saccharides. Aminofunctionalized D-Mannose or D-Galactose was reacted with the active pentafluorophenyl esters as followed by UV/Vis spectroscopy and XPS. The functionality was shown to be bioactive using macrophage cell culture. The meshes functionalized with D-Mannose specifically stimulated the cytokine production of macrophages when lipopolysaccharides were added. This was in contrast to D-Galactose- or aminoethanol-functionalized and unfunctionalized PCL/PPFPMA fiber mats.