Biocompatible Silk/Polymer Energy Harvesters Using Stretched Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) Nanofibers

Hierarchically patterned protein scaffolds with nano-fibrillar and micro-lamellar structures modulate neural stem cell homing and promote neuronal differentiation

Biophysical factors are essential in cell survival and behaviors, but constructing a suitable 3D microenvironment for the recruitment of stem cells and exerting their physiological functions remain a daunting challenge. Here, we present a novel silk fibroin (SF)-based fabrication strategy to develop hierarchical microchannel scaffolds for biomimetic nerve microenvironments in vitro. We first modulated the formation of SF nanofibers (SFNFs) that mimic the nanostructures of the native extracellular matrix (ECM) by using graphene oxide (GO) nanosheets as templates. Then, SFNF-GO systems were shaped into 3D porous scaffolds with aligned micro-lamellar structures by freeze-casting. The interconnected microchannels successfully induced cell infiltration and migration to the SFNF-GO scaffolds' interior. Meanwhile, the nano-fibrillar structures and the GO component significantly induced neural stem cells (NSCs) to differentiate into neurons within a short timeframe of 14 d. Importantly, these 3D hierarchical scaffolds induced a mild inflammatory response, extensive cell recruitment, and effective stimulation of NSC neuronal differentiation when implanted in vivo. Therefore, these SFNF-GO lamellar scaffolds with distinctive nano-/micro-topographies hold promise in the fields of nerve injury repair and regenerative medicine.

Electrospun Piezoelectric Scaffold with External Mechanical Stimulation for Promoting Regeneration of Peripheral Nerve Injury

Safe and efficient provision of electrical stimulation (ES) for nerve repair and regeneration is a problem that needs to be addressed. In this study, a silk fibroin/poly(vinylidene fluoride-co-hexafluoropropylene)/Ti₃C₂T_x (SF/PVDF-HFP/MXene) composite scaffold with piezoelectricity was developed by electrospinning technology. MXene was loaded to the scaffold to enhance the piezoelectric properties (Output voltage reaches up to 100 mV), mechanical properties, and antibacterial activity. Cell experiments demonstrated piezoelectric stimulation under external ultrasonication for promoting the growth and proliferation of Schwann cells (SCs) cultured on this electrospun scaffold. Further in vivo study with rat sciatic nerve injury model revealed that the SF/PVDF-HFP/MXene nerve conduit could induce the proliferation of SCs, enhance the elongation of axon, and promote axonal myelination. Under the piezoelectric effect of this nerve scaffold, the rats with regenerative nerve exhibited a favorable recovery effect of motor and sensory function, indicating a safe and feasible method of using this SF/PVDF-HFP/MXene piezoelectric scaffold for ES provision in vivo.

Impact of Electrospun Piezoelectric Core-Shell PVDFhfp/PDMS Mesh on Tenogenic and Inflammatory Gene Expression in Human Adipose-Derived Stem Cells: Comparison of Static Cultivation with Uniaxial Cyclic Tensile Stretching

Specific microenvironments can trigger stem cell tenogenic differentiation, such as specific substrates or dynamic cell cultivation. Electrospun meshes composed by core–shell fibers (random or aligned; PDMS core; piezoelectric PVDFhfp shell) were fabricated by coaxial electrospinning. Elastic modulus and residual strain were assessed. Human ASCs were seeded on such scaffolds either under static conditions for 1 week or with subsequent 10% dynamic stretching for 10,800 cycles (1 Hz, 3 h), assessing load elongation curves in a Bose^® bioreactor system. Gene expression for tenogenic expression, extracellular matrix, remodeling, pro-fibrotic and inflammatory marker genes were assessed (PCR). For cell-seeded meshes, the E modulus increased from 14 ± 3.8 MPa to 31 ± 17 MPa within 3 h, which was not observed for cell-free meshes. Random fibers resulted in higher tenogenic commitment than aligned fibers. Dynamic cultivation significantly enhanced pro-inflammatory markers. Compared to ASCs in culture flasks, ASCs on random meshes under static cultivation showed a significant upregulation of Mohawk, Tenascin-C and Tenomodulin. The tenogenic commitment expressed by human ASCs in contact with random PVDFhfp/PDMS paves the way for using this novel highly elastic material as an implant to be wrapped around a lacerated tendon, envisioned as a functional anti-adhesion membrane.

Fabrication of Piezoelectric Electrospun Termite Nest-like 3D Scaffolds for Tissue Engineering

A high piezoelectric coefficient polymer and biomaterial for bone tissue engineering- poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-has been successfully fabricated into 3D scaffolds using the wet electrospinning method. Three-dimensional (3D) scaffolds have significant advantages for tissue engineering applications. Electrospinning is an advanced method and can fabricate 3D scaffolds. However, it has some limitations and is difficult to fabricate nanofibers into 3D shapes because of the low controllability of porosity and internal pore shape. The PVDF-HFP powders were dissolved in a mixture of acetone and dimethylformamide with a ratio of 1:1 at various concentrations of 10, 13, 15, 17, and 20 wt%. However, only the solutions at 15 and 17 wt% with optimized electrospinning parameters can be fabricated into biomimetic 3D shapes. The produced PVDF-HFP 3D scaffolds are in the cm size range and mimic the structure of the natural nests of termites of the genus Apicotermes. In addition, the 3D nanofiber-based structure can also generate more electrical signals than the conventional 2D ones, as the third dimension provides more compression. The cell interaction with the 3D nanofibers scaffold was investigated. The in vitro results demonstrated that the NIH 3T3 cells could attach and migrate in the 3D structures. While conventional electrospinning yields 2D (flat) structures, our bio-inspired electrospun termite nest-like 3D scaffolds are better suited for tissue engineering applications since they can potentially mimic native tissues as they have biomimetic structure, piezoelectric, and biological properties.

Electrostrictive and Structural Properties of Poly(Vinylidene Fluoride-Hexafluoropropylene) Composite Nanofibers Filled with Polyaniline (Emeraldine Base)

Previous studies have reported that poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymers can exhibit large electrostrictive strains depending on the filler. This work examines the electrostrictive and structural properties of P(VDF-HFP) nanofibers modified with conductive polymer polyaniline (PANI). The P(VDF-HFP)/PANI composite nanofibers were prepared by an electrospinning method with different PANI concentrations (0, 0.5, 1, 1.5, 3 and 5 wt.%). The average diameter, water contact angle and element were analyzed by SEM, WCA and EDX, respectively. The crystalline, phase structure and mechanical properties were investigated by XRD, FTIR and DMA, respectively. The dielectric properties and electrostrictive behavior were also studied. The results demonstrated that the composite nanofibers exhibited uniform fibers without any bead formation, and the WCA decreased with increasing amount of PANI. However, a high dielectric constant and electromechanical response were obtained. The electrostrictive coefficient, crystalline, phase structure, dielectric properties and interfacial charge distributions increased in relation to the PANI content. Moreover, this study indicates that P(VDF-HFP)/PANI composite nanofibers may represent a promising route for obtaining electrostrictive composite nanofibers for actuation applications, microelectromechanical systems and sensors based on electrostrictive phenomena.

Fluorinated nanotubes: synthesis and self-assembly of cyclic peptide-poly(vinylidene fluoride) conjugates

The synthesis of cyclic peptide-poly(vinylidene fluoride) (CP-PVDF) conjugates comprising (d-alt-l)-cyclopeptides as building blocks and their self-assembly into tube-like structures is described. By growing two PVDF polymeric chains from opposite sides of a preassembled cyclic-peptide macro-chain transfer agent, a PVDF-CP-PVDF conjugate was prepared. This "grafting-from" strategy, allowed the synthesis of the conjugate with high purity and using facile purification steps. The controlled self-assembly of the conjugate from DMF or DMSO solutions was carried out by addition of THF. This triggered the aggregation process that led to formation of tube-like structures. The mean length and width of the PVDF-CP-PVDF tubes were measured using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Surprisingly, the self-assembly of the CP-PVDF conjugates in DMF/THF allowed the preparation of long (up to 25 μm) tube-like structures. The formation of such long tubular peptide-polymer aggregates, based on the stacking of cyclopeptides, is unprecedented and is believed to rely on synergetic effects between the stacking of the cyclic peptide and the interactions of the fluoropolymer-peptide conjugates.

Fabrication and application of biocompatible nanogenerators

As a new sustainable energy source, ubiquitous mechanical energy has received great attention and was successfully harvested by different types of nanogenerators. Among them, biocompatible nanogenerators are of particular interests due to their potential for biomedical applications. In this review, we provide an overview of the recent achievements in the fabrication and application of biocompatible nanogenerators. The development process and working mechanism of nanogenerators are introduced. Different biocompatible materials for energy harvesting, such as amino acids, peptide, silk protein, and cellulose, are discussed and compared. We then discuss different applications of biocompatible nanogenerators. We conclude with the challenges and potential research directions in this emerging field.

Thermal Conductivity of Protein-Based Materials: A Review

Fibrous proteins such as silks have been used as textile and biomedical materials for decades due to their natural abundance, high flexibility, biocompatibility, and excellent mechanical properties. In addition, they also can avoid many problems related to traditional materials such as toxic chemical residues or brittleness. With the fast development of cutting-edge flexible materials and bioelectronics processing technologies, the market for biocompatible materials with extremely high or low thermal conductivity is growing rapidly. The thermal conductivity of protein films, which is usually on the order of 0.1 W/m·K, can be rather tunable as the value for stretched protein fibers can be substantially larger, outperforming that of many synthetic polymer materials. These findings indicate that the thermal conductivity and the heat transfer direction of protein-based materials can be finely controlled by manipulating their nano-scale structures. This review will focus on the structure of different fibrous proteins, such as silks, collagen and keratin, summarizing factors that can influence the thermal conductivity of protein-based materials and the different experimental methods used to measure their heat transfer properties.

Protein-Based Fiber Materials in Medicine: A Review

Fibrous materials have garnered much interest in the field of biomedical engineering due to their high surface-area-to-volume ratio, porosity, and tunability. Specifically, in the field of tissue engineering, fiber meshes have been used to create biomimetic nanostructures that allow for cell attachment, migration, and proliferation, to promote tissue regeneration and wound healing, as well as controllable drug delivery. In addition to the properties of conventional, synthetic polymer fibers, fibers made from natural polymers, such as proteins, can exhibit enhanced biocompatibility, bioactivity, and biodegradability. Of these proteins, keratin, collagen, silk, elastin, zein, and soy are some the most common used in fiber fabrication. The specific capabilities of these materials have been shown to vary based on their physical properties, as well as their fabrication method. To date, such fabrication methods include electrospinning, wet/dry jet spinning, dry spinning, centrifugal spinning, solution blowing, self-assembly, phase separation, and drawing. This review serves to provide a basic knowledge of these commonly utilized proteins and methods, as well as the fabricated fibers’ applications in biomedical research.

Ultra-Stretchable Piezoelectric Nanogenerators via Large-Scale Aligned Fractal Inspired Micro/Nanofibers

Stretchable nanogenerators that directly generate electricity are promising for a wide range of applications in wearable electronics. However, the stretchability of the devices has been a long-standing challenge. Here we present a newly-designed ultra-stretchable nanogenerator based on fractal-inspired piezoelectric nanofibers and liquid metal electrodes that can withstand strain as large as 200%. The large-scale fractal poly(vinylidene fluoride) (PVDF) micro/nanofibers are fabricated by combination of helix electrohydrodynamic printing (HE-Printing) and buckling-driven self-assembly. HE-Printing exploits "whipping/buckling" instability of electrospinning to deposit serpentine fibers with diverse geometries in a programmable, accurately positioned, and individually-controlled manner. Self-organized buckling utilizes the driven force from the prestrained elastomer to assemble serpentine fibers into ultra-stretchable fractal inspired architecture. The nanogenerator with embedded fractal PVDF fibers and liquid-metal microelectrodes demonstrates high stretchability (>200%) and electricity (currents >200 nA), it can harvest energy from all directions by arbitrary mechanical motion, and the rectified output has been applied to charge the commercial capacitor and drive LEDs, which enables wearable electronics applications in sensing and energy harvesting.