Electrically conductive polymer composites composed of polycaprolactone fumarate and polypyrrole (PCLF-PPy) have been developed for nerve regeneration applications. Here we report the synthesis and characterization of PCLF-PPy and in vitro studies showing PCLF-PPy materials support both PC12 cell and dorsal root ganglia (DRG) neurite extension. PCLF-PPy composite materials were synthesized by polymerizing pyrrole in pre-formed PCLF scaffolds (Mn 7,000 or 18,000 g mol−1) resulting in interpenetrating networks of PCLF-PPy. Chemical compositions and thermal properties were characterized by ATR-FTIR, XPS, DSC, and TGA. PCLF-PPy materials were synthesized with five different anions (naphthalene-2-sulfonic acid sodium salt (NSA), dodecylbenzenesulfonic acid sodium salt (DBSA), dioctyl sulfosuccinate sodium salt (DOSS), potassium iodide (I), and lysine) to investigate effects on electrical conductivity and to optimize chemical composition for cellular compatibility. PCLF-PPy materials have variable electrical conductivity up to 6 mS cm−1 with bulk compositions ranging from 5 to 13.5 percent polypyrrole. AFM and SEM characterization show microstructures with a root mean squared (RMS) roughness of 1195 nm and nanostructures with RMS roughness of 8 nm. In vitro studies using PC12 cells and DRG show PCLF-PPy materials synthesized with NSA or DBSA support cell attachment, proliferation, neurite extension, and are promising materials for future studies involving electrical stimulation.
Electrically Conductive; Polypyrrole; Nerve; PCLF
A variety of cell types respond to electrical stimuli, accordingly many conducting polymers (CPs) have been used as tissue engineering (TE) scaffolds, one such CP is polypyrrole (PPy). PPy is a well studied biomaterial with potential TE applications due to its electrical conductivity and many other beneficial properties. Combining its characteristics with an elastomeric material, such as polyurethane (PU), may yield a hybrid scaffold with electrical activity and significant mechanical resilience. Pyrrole was in situ polymerized within a PU emulsion mixture in weight ratios of 1:100, 1:20, 1:10 and 1:5, respectively. Morphology, electrical conductivity, mechanical properties and cytocompatibility with C2C12 myoblast cells were characterized. The polymerization resulted in a composite with a principle base of PU interspersed with an electrically percolating network of PPy nanoparticles. As the mass ratio of PPy to PU increased so did electrical conductivity of the composites. In addition, as the mass ratio of PPy to PU increased, stiffness of the composite increased while maximum elongation length decreased. Ultimate tensile strength was reduced by approximately 47% across all samples with the addition of PPy to the PU base. Cytocompatibility assay data indicated no significant cytotoxic effect from the composites. Static cellular seeding of C2C12 cells and subsequent differentiation showed myotube formation on the composite materials.
Tissue Engineering; Polyurethane; Polypyrrole; Myoblast; Electrical Conduction
Electrically conducting polymers such as polypyrrole (PPy) are important biomaterials in neural engineering applications, including neural probes, nerve conduits, and scaffolds for tissue and nerve regeneration. Surface modification of these polymers can introduce other valuable characteristics for neural interfacing in addition to electrical conductivity, such as topographical features and chemical bioactivity. Here, the patterning of PPy to create topographical cues for cells is reported. In particular, 1 and 2 µm wide PPy microchannels are fabricated using electron-beam (e-beam) lithography and electropolymerization. A systematic analysis of parameters controlling PPy micropatterning is performed, and finds that microchannel depth, roughness, and morphology are highly dependent on the e-beam writing current, polymerization current, PPy/dopant concentrations, and the polymerization time. Embryonic hippocampal neurons cultured on patterned PPy polarize (i.e., defined an axon) faster on this modified material, with a twofold increase in the number of cells with axons compared to cells cultured on unmodified PPy. These topographical features also have an effect on axon orientation but do not have a significant effect on overall axon length. This is the first investigation that studies controlled PPy patterning with small dimensions (i.e., less than 5 µm) for biological applications, which demonstrates the relevance of expanding microelectronic materials and techniques to the biomedical field.
Polycaprolactone fumarate (PCLF) is a cross-linkable derivate of polycaprolactone diol that has been shown to be an effective nerve conduit material that supports regeneration across segmental nerve defects and has warranted future clinical trials. Degradation of the previously studied PCLF (PCLFDEG) releases toxic small molecules of diethylene glycol used as the initiator for the synthesis of polycaprolactone diol. In an effort to eliminate this toxic degradation product we present a strategy for the synthesis of PCLF from either propylene glycol (PCLFPPD) or glycerol (PCLFGLY). PCLFPPD is linear and resembles the previously studied PCLFDEG, while PCLFGLY is branched and exhibits dramatically different material properties. The synthesis and characterization of their thermal, rheological, and mechanical properties are reported. The results show that the linear PCLFPPD has material properties similar to the previously studied PCLFDEG. The branched PCLFGLY exhibits dramatically lower crystalline properties resulting in lower rheological and mechanical moduli, and is therefore a more compliant material. In addition, the question of an appropriate FDA approvable sterilization method is addressed. This study shows that autoclave sterilization on PCLF materials is an acceptable sterilization method for cross-linked PCLF and has minimal effect on the PCLF thermal and mechanical properties.
Polycaprolactone fumarate; polyester; sterilization; nerve regeneration
In an effort of achieving suitable biomaterials for peripheral nerve regeneration, we present a material design strategy of combining a crystallite-based physical network and a crosslink-based chemical network. Biodegradable polymer disks and conduits have been fabricated by photo-crosslinking three poly(ε-caprolactone fumarate)s (PCLF530, PCLF1250, and PCLF2000), which were synthesized from the precursor poly(ε-caprolactone) (PCL) diols with nominal molecular weights of 530, 1250, and 2000 g.mol−1, respectively. Thermal properties such as glass transition temperature (Tg), melting temperature (Tm), and crystallinity of photo-crosslinked PCLFs were examined and correlated with their rheological and mechanical properties. Furthermore, in vitro degradation of uncrosslinked and crosslinked PCLFs in PBS crosslinked PCLFs in 1 N NaOH aqueous solution at 37 °C was studied. In vitro cytocompatibility, attachment, and proliferation of Schwann cell precursor line SPL201 cells on three PCLF networks were investigated. Crosslinked PCLF2000 with the highest crystallinity and mechanical properties was found to best support cell attachment and proliferation. Using a new photo-crosslinking method, single-lumen crosslinked PCLF nerve conduits without defects were fabricated in a glass mold. Crosslinked PCLF2000 nerve conduits were selected for evaluation in a 1-cm gap rat sciatic nerve model. Histological evaluation demonstrated that the material was biocompatible with sufficient strength to hold sutures in place after 6 and 17 weeks of implantation. Nerve cable with myelinated axons was found in the crosslinked PCLF2000 nerve conduit.
Poly(ε-caprolactone fumarate); Photo-crosslinking; Peripheral nerve regeneration; Cell responses
We present a material design strategy of combining crystallinity and crosslinking to control the mechanical properties of polymeric biomaterials. Three polycaprolactone fumarates (PCLF530, PCLF1250, and PCLF2000) synthesized from the precursor polycaprolactone (PCL) diols with nominal molecular weights of 530, 1250, and 2000 g.mol-1, respectively, were employed to fabricate polymer networks via photo-crosslinking process. Five different amounts of photo-crosslinking initiator were applied during fabrication in order to understand the role of photoinitiator in modulating the crosslinking characteristics and physical properties of PCLF networks. Thermal properties such as glass transition temperature (Tg), melting temperature (Tm), and degradation temperature (Td) of photo-crosslinked PCLFs were examined and correlated with their rheological and mechanical properties.
Polycaprolactone fumarate; Photo-crosslinking; Mechanical Properties
Numerous regenerating tissues respond favorably to electrical stimulation, creating a need for a bioactive conducting platform for tissue engineering applications. The drive for biosensors and electrode coatings further requires control of the surface properties of promising conductive materials such as polypyrrole. Here we present carboxy-endcapped polypyrrole (PPy-α-COOH), a unique bioactive conducting polymer with a carboxylic acid layer, composed of a polypyrrole (PPy) surface modified with pyrrole-α-carboxylic acid (Py-α-COOH). This unique structure is simple to produce, provides a stable bioactive surface via covalent bonds, and preserves bulk properties such as electrical conductivity and mechanical integrity. The chemical structure of this polymer composite was characterized by angle-resolved X-ray photoelectron spectroscopy (XPS), which demonstrated the presence of carboxylic acid functionality on the top surface of conductive PPy. A four-point probe test was used to verify the similar conductivity of PPy-α-COOH compared to that of standard PPy. To demonstrate the potential to influence cellular activity, the carboxylic acid monolayer surface was grafted with the cell-adhesive Arg-Gly-Asp (RGD) motif. Human umbilical vein endothelial cells (HUVECs) cultured on RGD-modified PPy-α-COOH demonstrated significantly higher adhesion and spreading than on the negative controls PPy-α-COOH and unmodified PPy.
Electroactive polymers such as polypyrrole (PPy) are highly attractive for a number of biomedical applications, including their use as coatings for electrodes or neural probes and as scaffolds to induce tissue regeneration. Surface modification of these materials with biological moieties is desired to enhance the biomaterial-tissue interface and to promote desired tissue responses. Here, we present the synthesis and physicochemical characterization of poly(1-(2-carboxyethyl)pyrrole) (PPyCOOH), a PPy derivative that contains a chemical group that can be easily modified with biological moieties at the N-position of polymer backbone. FTIR, XPS, and fluorescence microscopy were used to demonstrate the successful incorporation of carboxylic acid (-COOH) functionality into PPy materials, and a four-point probe analysis was used to demonstrate electrical conductivity in the semiconductor range. Human umbilical vascular endothelial cells (HUVECs) cultured on PPyCOOH films surface modified with the cell-adhesive Arg-Gly-Asp (RGD) motif demonstrated improved attachment and spreading. Thus, PPyCOOH could be useful in developing PPy composites that contain a variety of biological molecules as bioactive conducting platforms for specific biomedical purposes.
Electrospinning is a promising approach to create nanofiber structures that are capable of supporting adhesion and guiding extension of neurons for nerve regeneration. Concurrently, electrical stimulation of neurons in the absence of topographical features also has been shown to guide axonal extension. Therefore, the goal of this study was to form electrically conductive nanofiber structures and to examine the combined effect of nanofiber structures and electrical stimulation. Conductive meshes were produced by growing polypyrrole (PPy) on random and aligned electrospun poly(lactic-co-glycolic acid) (PLGA) nanofibers, as confirmed by scanning electron micrographs and X-ray photon spectroscopy. PPy-PLGA electrospun meshes supported the growth and differentiation of rat pheochromocytoma 12 (PC12) cells and hippocampal neurons comparable to non-coated PLGA control meshes, suggesting that PPy-PLGA may be suitable as conductive nanofibers for neuronal tissue scaffolds. Electrical stimulation studies showed that PC12 cells, stimulated with a potential of 10 mV/cm on PPy-PLGA scaffolds, exhibited 40–50% longer neurites and 40–90% more neurite formation compared to unstimulated cells on the same scaffolds. In addition, stimulation of the cells on aligned PPy-PLGA fibers resulted in longer neurites and more neurite-bearing cells than stimulation on random PPy-PLGA fibers, suggesting a combined effect of electrical stimulation and topographical guidance and the potential use of these scaffolds for neural tissue applications.
polypyrrole; nanofibers; nerve tissue engineering; electrical stimulation; PC12 cells; hippocampal neurons
Biomaterials that present multiple stimuli are attractive for a number of biomedical applications. In particular, electrical and biological cues are important factors to include in interfaces with neurons for applications such as nerve conduits and neural probes. Here, we report the combination of these two stimuli, by immobilizing nerve growth factor (NGF) on the surface of the electrically conducting polymer polypyrrole (PPy). NGF was immobilized using an intermediate linker provided by a layer of polyallylamine conjugated to an arylazido functional group. Upon exposure to UV light and activation of the azido groups, NGF was fixed to the substrate. Three different surface concentrations were obtained (0.21–0.98 ng/mm2) and similar levels of neurite extension were observed on immobilized NGF as with soluble NGF. Additionally, electrical stimulation experiments were conducted with the modified polymer and revealed a 50% increase in neurite outgrowth in PC12 cells compared to experiments without electrical stimulation. This novel modification of PPy provides both electrical and biological stimulation, by presenting tethered growth factors and only producing a small decrease in the material's properties (conductivity ~10 S cm−1) when compared to other modification techniques (conductivity ~10−3–10−6 S cm−1.
nerve growth factor; neural cell; surface grafting; electrical stimulation; protein immobilization
Polypyrrole (PPy) is a biocompatible, electrically conductive polymer that has great potential for battery, sensor, and neural implant applications. Its amorphous structure and insolubility, however, limit the experimental techniques available to study its structure and properties at the atomic level. Previous theoretical studies of PPy in bulk are also scarce. Using ab initio calculations, we have constructed a molecular mechanics force field of chloride-doped PPy (PPyCl) and undoped PPy. This model has been designed to integrate into the OPLS force field, and parameters are available for the Gromacs and TINKER software packages. Molecular dynamics (MD) simulations of bulk PPy and PPyCl have been performed using this force field, and the effects of chain packing and electrostatic scaling on the bulk polymer density have been investigated. The density of flotation of PPyCl films has been measured experimentally. Amorphous X-ray diffraction of PPyCl was obtained and correlated with atomic structures sampled from MD simulations. The force field reported here is foundational for bridging the gap between experimental measurements and theoretical calculations for PPy based materials.
polypyrrole; conducting polymers; molecular mechanics; characterization; quantum mechanics; force field; parameters
Background and the purpose of the study
Biodegradable Poly(caprolactone fumarate) (PCLF) has been used as bioresorbable sutures. In this study, doxorubicin HCl (Dox) loaded PCLF nanoparticles were prepared and characterized.
Material and methods
PCLFs were synthesized by polycondensation of PCL diols (Mws of 530, 1250 and 2000) with fumaryl chloride. The degradation of PCLF in NaOH, water and phosphate buffer saline (PBS), was determined in terms of changes in Mw. Nanoparticles (NPs) were prepared by two methods. In microemulsion polymerization method, dichloromethane containing PCLF and photoinitiator were combined with the water containing surfactants and then the mixture was placed under light for crosslinking. In nanoprecipitation method, the organic solvent containing PCLF was poured into the stirring water. The effect of several variables including concentration of PCLF, polyvinyl alcohol (PVA), Dox and Trypan blue (Trb) and the Mw of PCLF and PVA on NP size and loading were evaluated.
PCLF 530, 1250 and 2000 in PBS or water were not degraded over 28 days. Nanoprecipitaion method gave spherical (revealed by SEM images) stable NPs of about 225 with narrow size distribution and a zeta potential of −43 mV. The size of NP increased significantly by increase in Mw or concentration of PCLF. Although PVA was not necessary for formation of NPs, but it decreased with NP size. Dox loading and EE were 2.5–6.8% and 15–20%, respectively. Increasing the drug concentration increased the drug loading (DL) and NP size. The entrapment efficiency (EE) for Trb ranged from 1% for PCLF530 to 6% for PCLF2000. An increase in PCLF concentration resulted in an increase in EE. Dox and Trb release showed a burst followed by 80% and 78% release during 3 and 4 days respectively.
PCLF possessed suitable characteristics for preparation of nanoparticulate drug delivery system such as desired NP size, stability and degradation time. Although PCLF530 NPs were the smallest, but their DL were lower than PCLF1250 and 2000 NPs.
PCLF nanoparticles; Copolymer molecular weight; Nanoprecipitation method
Electrically conductive polymers, such as polypyrrole (PPy), show promise for modifying the dimensions and properties of micro- and nanoscale structures. Mechanisms for controlling the formation of PPy films of nanoscale thickness were evaluated by electrochemically synthesizing and examining PPy films on planar gold electrodes under a variety of growth conditions. Tunable PPy coatings were then deposited by electropolymerization on the sidewalls of individual, electrically addressable carbon nanofibers (CNFs). The ability to modify the physical size of specific nanofibers in controllable fashion is demonstrated. The biocompatibility, potential for chemical functionalization, and ability to effect volume changes of this nanocomposite can lead to advanced functionality, such as specific, nanoscale valving of materials and morphological control at the nanoscale.
Carbon nanofibers; Polypyrrole; Electropolymerization; Surface modification; Nanomaterial
A novel self-crosslinkable and biodegradable macromer poly(caprolactone fumarate) (PCLF) has been developed for guided bone regeneration. This macromer is a copolymer of fumaryl chloride, which contains double bonds for in-situ crosslinking, and poly(ε-caprolactone) that has a flexible chain to facilitate self-crosslinkability. PCLF was characterized with Fourier transform infrared (FTIR) spectroscopy, 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and gel permeation chromatography (GPC). Porous scaffolds were fabricated with sodium chloride particles as the porogen and a chemical initiation system. The PCLF scaffolds were characterized with scanning electron microscopy (SEM) and micro-computed tomography (micro-CT). The cytotoxicity and in vivo biocompatibility of PCLF were also assessed. Our results suggest that this novel copolymer, PCLF, is an injectable, self-crosslinkable, and biocompatible macromer that may be potentially used as a scaffold for tissue engineering applications.
Polypyrrole (PPy) is an inherently conducting polymer that has shown great promise for biomedical applications within the nervous system. However, to effectively use PPy as a biomaterial implant, it is important to understand and reproducibly control the electrical properties, physical topography, and surface chemistry of the polymer. Although there is much research published on the use of PPy in various applications, there is no systematic study linking the methodologies used for PPy synthesis to PPy’s basic polymeric properties (e.g., hydrophilicity, surface roughness), and to the biological effects these properties have on cells. Electrochemically synthesized PPy films differ greatly in their characteristics depending on synthesis parameters such as dopant, substrate, and thickness, among other parameters. In these studies, we have used three dopants (chloride (Cl), tosylate (ToS), polystyrene sulfonate (PSS)), two substrates (gold and indium tin oxide-coated glass), and a range of thicknesses, to measure and compare the biomedically-important characteristics of surface roughness, contact angle, conductivity, dopant stability, and cell adhesion (using PC-12 cells and Schwann cells). As predicted, we discovered large differences in roughness depending on the dopant used and the thickness of the film, while substrate choice had little effect. From contact angle measurements, PSS was found to yield the most hydrophilic material, most likely because of free charges from the long PSS chains exposed on the surface of the PPy. ToS-doped PPy films were tenfold more conductive than Cl- or PSS-doped films. X-ray photoelectron spectroscopy studies were used to evaluate dopant concentrations of PPy films stored in water and phosphate buffered saline over 14 days, and conductance studies over the same timeframe measured electrical stability. PSS proved to be the most stable dopant, though all films experienced significant decay in conductivity and dopant concentration. Cell adhesion studies demonstrated the dependence of cell outcome on film thickness and dopant choice. The strengths and weaknesses of different synthesis parameters, as demonstrated by these experiments, are critical design factors that must be leveraged when designing biomedical implants. The results of these studies should provide practical insight to researchers working with conducting polymers, and particularly PPy, on the relationships between synthesis parameters, polymeric properties, and biological compatibility.
polypyrrole; conducting polymers; electrochemical synthesis; characterization; biocompatibility; conductivity
The transected rat thoracic (T9/10) spinal cord model is a platform for quantitatively compa0ring biodegradable polymer scaffolds. Schwann cell-loaded scaffolds constructed from poly (lactic co-glycolic acid) (PLGA), poly(ε-caprolactone fumarate) (PCLF), oligo(polyethylene glycol) fumarate (OPF) hydrogel or positively charged OPF (OPF+) hydrogel were implanted into the model. We demonstrated that the mechanical properties (3-point bending and stiffness) of OPF and OPF+ hydrogels closely resembled rat spinal cord. After one month, tissues were harvested and analyzed by morphometry of neurofilament-stained sections at rostral, midlevel, and caudal scaffold. All polymers supported axonal growth. Significantly higher numbers of axons were found in PCLF (P < 0.01) and OPF+ (P < 0.05) groups, compared to that of the PLGA group. OPF+ polymers showed more centrally distributed axonal regeneration within the channels while other polymers (PLGA, PCLF and OPF) tended to show more evenly dispersed axons within the channels. The centralized distribution was associated with significantly more axons regenerating (P < 0.05). Volume of scar and cyst rostral and caudal to the implanted scaffold was measured and compared. There were significantly smaller cyst volumes in PLGA compared to PCLF groups. The model provides a quantitative basis for assessing individual and combined tissue engineering strategies.
OPF; PLGA; PCLF; axon regeneration; spinal cord injury; Schwann cell
We have prepared conductive core-sheath nanofibers via a combination of electrospinning and aqueous polymerization. Specifically, nanofibers electrospun from poly(ε-caprolactone) (PCL) and poly(L-lactide) (PLA) were employed as templates to generate uniform sheaths of polypyrrole (PPy) via in situ polymerization. These conductive core-sheath nanofibers offer a unique system for studying the synergistic effect of different cues on neurite outgrowth in vitro. We found that explanted dorsal root ganglia (DRG) adhered well to the conductive core-sheath nanofibers and generated neurites across the surface when there was a nerve growth factor in the medium. Furthermore, the neurites could be oriented along one direction and enhanced by 82% in terms of maximum length when uniaxially aligned conductive core-sheath nanofibers are compared with their random counterparts. Electrical stimulation, when applied through the mats of conductive core-sheath nanofibers, was found to further increase the maximum length of neurite for random and aligned samples by 83% and 47%, respectively, relative to the controls without electrical stimulation. Combined together, these results suggest the potential use of the conductive core-sheath nanofibers as scaffolds in applications such as neural tissue engineering.
Two poly(ε-caprolactone fumarate)s (PCLFs) with distinct physical properties have been employed to prepare nanocomposites with hydroxyapatite (HA) nanoparticles via photo-crosslinking. The two PCLFs are PCLF530 and PCLF2000, named after their precursor PCL diol molecular weight of 530 and 2000 g.mol-1, respectively. Crosslinked PCLF530 is amorphous while crosslinked PCLF2000 is semi-crystalline with a melting temperature (Tm) of ∼40 °C and a crystallinity of 40%. Consequently, the rheological and mechanical properties of crosslinked PCLF2000 are significantly greater than those of crosslinked PCLF530. Structural characterizations and physical properties of both series of crosslinked PCLF/HA nanocomposites with HA compositions of 0%, 5%, 10%, 20%, and 30% have been investigated. By adding HA nanoparticles, crosslinked PCLF530/HA nanocomposites demonstrate enhanced rheological and mechanical properties while the enhancement in compressive modulus is less prominent in crosslinked PCLF2000/HA nanocomposites. In vitro cell attachment and proliferation have been performed using rat bone marrow stromal cells (BMSCs) and correlated with the material properties. Cell attachment and proliferation on crosslinked PCLF530/HA nanocomposite disks have been enhanced strongly with increasing the HA composition. However, surface morphology and surface chemistry such as composition, hydrophilicity, and the capability of adsorbing protein cannot be used to interpret the cell responses on different samples. Instead, the role of surface stiffness in regulating cell responses can be supported by the correlation between the change in compressive modulus and BMSC proliferation on these two series of crosslinked PCLFs and PCLF/HA nanocomposites.
Polycaprolactone fumarate (PCLF); Hydroxyapatite (HA); Nanocomposite; Photo-crosslinking; Bone marrow stromal cell responses
Polypyrrole (PPy) is a conjugated polymer that displays particular electronic properties including conductivity. In biomedical applications, it is usually electrochemically generated with the incorporation of any anionic species including also negatively charged biological macromolecules such as proteins and polysaccharides to give composite materials. In biomedical research, it has mainly been assessed for its role as a reporting interface in biosensors. However, there is an increasing literature on the application of PPy as a potentially electrically addressable tissue/cell support substrate. Here, we review studies that have considered such PPy based conducting polymers in direct contact with biological tissues and conclude that due to its versatile functional properties, it could contribute to a new generation of biomaterials.
biomaterials; conducting polymers; polypyrrole; tissue; cells
Polypyrrole (PPy) and polypyrrole-carboxylic functionalized multi wall carbon nanotube composites (PPy/f-MWCNT) were synthesized by in situ chemical oxidative polymerization of pyrrole on the carbon nanotubes (CNTs). The structure of the resulting complex nanotubes was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The effects of f-MWCNT concentration on the electrical properties of the resulting composites were studied at temperatures between 100 K and 300 K. The Hall mobility and Hall coefficient of PPy and PPy/f-MWCNT composite samples with different concentrations of f-MWCNT were measured using the van der Pauw technique. The mobility decreased slightly with increasing temperature, while the conductivity was dominated by the gradually increasing carrier density.
polypyrrole; carbon nanotube; Hall effect; conductivity
Sensorineural hearing loss is associated with gradual degeneration of spiral ganglion neurons (SGNs), compromising hearing outcomes with cochlear implant use. Combination of neurotrophin delivery to the cochlea and electrical stimulation from a cochlear implant protects SGNs, prompting research into neurotrophin-eluting polymer electrode coatings. The electrically conducting polypyrrole/para-toluene sulfonate containing neurotrophin-3 (Ppy/pTS/NT3) was applied to 1.7 mm2 cochlear implant electrodes. Ppy/pTS/NT3-coated electrode arrays stored 2 ng NT3 and released 0.1 ng/day with electrical stimulation. Guinea pigs were implanted with Ppy/pTS or Ppy/pTS/NT3 electrode arrays two weeks after deafening via aminoglycosides. The electrodes of a subgroup of these guinea pigs were electrically stimulated for 8 hr/day for 2 weeks. There was a loss of SGNs in the implanted cochleae of guinea pigs with Ppy/pTS-coated electrodes indicative of electrode insertion damage. However, guinea pigs implanted with electrically stimulated Ppy/pTS/NT3-coated electrodes had lower electrically-evoked auditory brainstem response thresholds and greater SGN densities in implanted cochleae compared to non-implanted cochleae and compared to animals implanted with Ppy/pTS-coated electrodes (p<0.05). Ppy/pTS/NT3 did not exacerbate fibrous tissue formation and did not affect electrode impedance. Drug-eluting conducting polymer coatings on cochlear implant electrodes present a clinically viable method to promote preservation of SGNs without adversely affecting the function of the cochlear implant.
The purpose of this study is to optimize poly(3,4,-ethylenedioxythiophene) (PEDOT) polymerization into decellular nerve scaffolding for interfacing to peripheral nerves. Our ultimate aim is to permanently implant highly conductive peripheral nerve interfaces between amputee, stump, nerve fascicles and prosthetic electronics. Decellular nerve (DN) scaffolds are an FDA approved biomaterial (Axogen ) with the flexible tensile properties needed for successful permanent coaptation to peripheral nerves. Biocompatible, electroconductive, PEDOT facilitates electrical conduction through PEDOT coated acellular muscle. New electrochemical methods were used to polymerize various PEDOT concentrations into DN scaffolds without the need for a final dehydration step. DN scaffolds were then tested for electrical impedance and charge density. PEDOT coated DN scaffold materials were also implanted as 15–20mm peripheral nerve grafts. Measurement of in-situ nerve conduction immediately followed grafting. DN showed significant improvements in impedance for dehydrated and hydrated, DN, polymerized with moderate and low PEDOT concentrations when they were compared with DN alone (a ≤ 0.05). These measurements were equivalent to those for DN with maximal PEDOT concentrations. In-situ, nerve conduction measurements demonstrated that DN alone is a poor electro-conductor while the addition of PEDOT allows DN scaffold grafts to compare favorably with the “gold standard”, autograft (Table 1). Surgical handling characteristics for conductive hydrated PEDOT DN scaffolds were rated 3 (pliable) while the dehydrated models were rated 1 (very stiff) when compared with autograft ratings of 4 (normal). Low concentrations of PEDOT on DN scaffolds provided significant increases in electro active properties which were comparable to the densest PEDOT coatings. DN pliability was closely maintained by continued hydration during PEDOT electrochemical polymerization without compromising electroconductivity.
poly(3,4,-ethylenedioxythiophene); peripheral nerve; decellular nerve; nerve conduction
Poly(ε-caprolactone fumarate) (PCLF) scaffold formulations were assessed as a delivery system of recombinant human bone morphogenetic protein (rhBMP-2) for bone tissue engineering. The formulations included PCLF with combinations of poly(vinyl alcohol) (PVA) and hydroxyapatite (HA). The assessments included in vitro and in vivo assays. In vitro assays validated cell attachment using a pre-osteoblast cell line (MC3T3-E1). Additionally, in vitro release profiles of rhBMP-2 from PCLF scaffolds were determined up to 21 days. Data suggested PCLF incorporated with PVA and HA accelerated rhBMP-2 release and the released protein was bioactive. For the in vivo study, a critical sized defect (CSD) model in a rabbit calvaria was used to test PCLF scaffolds. At 6 weeks post-implantation, significantly more bone formation was measured in PCLF scaffolds containing rhBMP-2 than in scaffolds without rhBMP-2. In conclusion, we demonstrated PCLF delivered biologically active rhBMP-2, promoted bone healing in a CSD and has potential as a bone tissue engineering scaffold.
poly(ε-caprolactone fumarate); three-dimensional scaffold; rabbit calvarial critical sized defect; rhBMP-2; bone tissue engineering
Aiming to achieve suitable polymeric biomaterials with controlled physical properties for hard and soft tissue replacements, we have developed a series of blends consisting of two photo-crosslinkable polymers: polypropylene fumarate (PPF) and polycaprolactone fumarate (PCLF). Physical properties of both uncrosslinked and UV crosslinked PPF/PCLF blends with PPF composition ranging from 0% to 100% have been investigated extensively. It has been found that the physical properties such as thermal, rheological, and mechanical properties could be modulated efficiently by varying the PPF composition in the blends. Thermal properties including glass transition temperature (Tg) and melting temperature (Tm) have been correlated with their rheological and mechanical properties. Surface characteristics such as surface morphology, hydrophilicity and the capability of adsorbing serum protein from culture medium have also been examined for the crosslinked polymer and blend discs. For potential applications in bone and nerve tissue engineering, in vitro cell studies including cytotoxicity, cell adhesion, and proliferation on crosslinked discs with controlled physical properties have been performed using rat bone marrow stromal cells and SPL201 cells, respectively. In addition, the role of mechanical properties such as surface stiffness in modulating cell responses has been emphasized using this model blend system.
Photo-crosslinking; Polymer blends; Poly(propylene fumarate) (PPF); Poly(caprolactone fumarate) (PCLF); Controlled physical properties; Cell responses
We have developed a new fabrication technique to create three-dimensional (3D) porous poly(ε-caprolactone fumarate) (PCLF) scaffolds using hydrogel microparticle porogens, as an alternative to overcome certain limitations of traditional scaffold fabrication techniques such as a salt leaching method. Both natural hydrogel, gelatin, and synthetic hydrogel, poly(ethylene glycol) sebacic acid diacrylate, were used as porogens to fabricate 3D porous PCLF scaffolds. Hydrogel microparticles were prepared by a single emulsion technique with the particle size in the range of 100–500 μm after equilibrium in water. The pore size distribution, porosity, pore interconnectivity, and spatial pore heterogeneity of the 3D PCLF scaffolds were assessed using micro-computed tomography and imaging analysis. Scaffolds fabricated with the hydrogel porogens had higher porosity and pore interconnectivity as well as more homogeneous spatial pore distribution, compared to the scaffolds made from the salt leaching process. Compressive moduli of the scaffolds were also measured and showed that lower porosity yielded greater modulus of the scaffolds. Overall, the new fabrication technology using hydrogel porogens may be beneficial for certain tissue engineering applications.