Materials Science and Technology of Polymers at the University of Twente

Polymer Micro- and Nanofibers as Functional Material Platforms

An intrinsic and very relevant property of polymers is that they can be spun or pulled to provide stable fibers. This has been recognized early in the course of the development of polymer technology, and resulting fibers have been used in many fields from textiles to reinforcing constituents in composites. If further progress is to be made with polymer fibers, then these should be (chemically or physically) functionalized, their morphology should be controlled across the length scales, and the fibers should then be integrated in complex constructs. The aim of this PhD Thesis has been the development of new polymer functional micro- and nanofibers for potential applications as responsive membranes, reactive porous systems, actuators, and functional biomedical constructs. Special emphasis has been laid on combining chemistry, innovative fiber spinning techniques, fiber functionalization, and eventually, fiber assembly.

Functional alginate core-shell hydrogel microfibers were prepared by a novel procedure utilizing microfluidics. The structure, dimensions, and properties of these fibers could be controlled by the synthetic procedures used to make the spun polymers, and by the spinning process itself. The other approach used in this Thesis included making fibers by electrospinning, which provided fibrous mats for further functionalization, as well as yielded oriented fiber bundles for preparing fibrous materials with architecture control. Functional polymer nanofibers were synthesized by surface-initiated ATRP, followed by the fabrication of electrospun porous membranes featuring brominated macroinitiators. These micro- and nanofibers with large specific surface area provided abundant functional sites at their surface. Fibrous architectures for actuator applications are important additions to this field as related platforms exhibit faster response compared to the bulk materials due to their large surface-to-volume ratio, when (for example) they are used in redox responsive actuators.  As the next step towards complexity, “Janus constructs” were prepared in this Thesis, offering sandwich structures with an adhesive side and the other side displaying protein antifouling, using functionalized electrospun mats. The ability to incorporate microfibers and microtubes (spun by microfluidics) in a hydrogel matrix between the two confining mat surfaces has been demonstrated. Due to the mild conditions in the preparation of the hydrogel microtubes, these 3D constructs have great potential to act as carriers of biofactors (or cells) in biomedical engineering.

In Chapter 1, a scope of this work, a brief introduction into the topics discussed in this Thesis, and the motivation for the reported research are presented.

In Chapter 2, a brief summary of synthesis and preparation of polymer micro- and nanofibers were presented, based on the recent literature. Focus has been laid on functional polymer micro- and nanofibers fabricated by the two techniques utilized in this work, i.e. electrospinning and microfluidics. Regarding applications, we paid particular attention to the structural biomedical field. Modification approaches and the applications of the micro- and nanofibers were subsequently discussed.

In Chapter 3, the preparation, characterization and applications of core-shell alginate hybrid microfibers is described. The fibers were prepared under mild conditions by microfluidics. The hydrogel microfibers with high elasticity and fast actuation were used to fabricate “smart” structures like microfiber bundles and a thermoresponsive bending “Janus film”. Hydrogel microfiber bundles could lift up to 180 times the weight of their own upon the action of external stimuli, like temperature. The Janus films designed by thermoresponsive P(NIPAM-HEMA)/alginate microfibers and PVA hydrogel had response time of 1.6 s when the temperature increased to above LCST. The core layer in the core-shell microfibers can be potentially loaded with biofactors to incorporate biological functions in the structure. This capability of loading biofactors made these core-shell microfibers interesting for the construction of 3D multi-functional platforms.

In Chapter 4, Pd nanoparticle loading of gel-brush grafted polymer nanofibers in membranes was achieved and used in flow-through catalysis. The membranes featuring PHEMA gel-brush layers were fabricated by surface-initiated ATRP. Pd nanoparticle loading was obtained by in-situ reduction of Pd2+, initially coordinated to carboxyl groups in the brush, in aqueous Pd(NO3)2 electrolytes. The catalytic efficiency of reducing 4-nitrophenol to 4-aminophenol in continuous flow-through catalysis was more than 99.9% at 0.1 mL/min flow rate and no more processing steps for the separation of the catalysts from the products was needed.

In Chapter 5, thermoresponsive membranes with switchable wettability are introduced. These membranes were synthesized by grafting PNIPAM brushes at the surface of nanofibers in porous electrospun membranes featuring brominated macroinitiators. The thermal responsivity of PNIPAM brushes in the membranes allowed us to control the pore size and the wettability of the membranes by temperature. These hybrid membranes were able to separate oil/water emulsions stabilized by Tween 80 at 25 °C with a high separation efficiency of 92 % and with the significantly lower separation efficiency of 25 % at 50 °C, i.e. the membrane showed thermoresponsive behavior. An explanation for the behavior observed is offered.

In Chapter 6, antifouling electrospun porous membranes are described. These membranes were prepared by grafting hydrophilic POEGMA brushes, featuring brominated PCL macroinitiators. Adhesive electrospun porous membranes were fabricated by modification of POEGMA brushes using RGD peptides. Biocompatible hydrogels were selected (in combination with microfibers described in Chapter 3) to assemble a 3D “Janus” sandwich platform. This integrated multi-functional platform can (in principle) be used in controlling the utilization of cells for releasing biofactors, packaged in the microfibers in the middle layer e.g. for regenerative medicine (“liver patch”). The focus however has been laid on the fabrication and characterization of the materials platform in this Thesis, thus biomedical assessments have not been reported here.

In Chapter 7, an actuator including redox responsive PFS-PEG based hydrogel nanofibers is introduced, obtained in an aligned organization by electrospinning. (PFS belongs to the class of organometallic polymers featuring ferrocene in their main chain, thus these macromolecules exhibit redox responsivity.) The PFS-PEG based hydrogel fibers showed enhanced mechanical properties in redox processes. Corresponding fiber bundles, when used as a “redox energy driven motor”, could lift up objects with weights that are 1000 times higher than their own. (Here we define motors as constructs, which convert energy into work; in our case the energy comes from the redox process.) In addition, the PFS-PEG hydrogel nanofiber mats were used to in-situ to reduce silver nanoparticles on their surface from electrolyte solutions due to the redox responsivity of PFS. The PFS-PEG based hydrogel mats featuring Ag nanoparticles showed excellent inhibition to S. aureus and E. coli. This redox-responsive nanofiber mats thus could be utilized as antibacterial membranes.

Following the research described in previous Chapters, Chapter 8 provides an outlook for further studies and applications towards the complexity and integration of functional fibers in multi-functional constructs.