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
Electrically conductive hydrogel composites consisting of oligo(polyethylene glycol) fumarate (OPF) and polypyrrole (PPy) were developed for applications in nerve regeneration. OPF-PPy scaffolds were synthesized using three different anions: naphthalene-2-sulfonic acid sodium salt (NSA), dodecylbenzenesulfonic acid sodium salt (DBSA), and dioctyl sulfosuccinate sodium salt (DOSS). Scaffolds were characterized by ATR-FTIR, XPS, AFM, dynamic mechanical analysis, electrical resistivity measurements, and swelling experiments. OPF-PPy scaffolds were shown to consist of up to 25 mol% polypyrrole with a compressive modulus ranging from 265 to 323 kPa and a sheet resistance ranging from 6 to 30 × 103 Ohms/square. In vitro studies using PC12 cells showed OPF-PPy materials had no cytotoxicity and PC12 cells showed distinctly better cell attachment and an increase in the percent of neurite bearing cells on OPF-PPy materials compared to OPF. The neurite lengths of PC12 cells were significantly higher on OPF-PPyNSA and OPF-PPyDBSA. These results show that electrically conductive OPF-PPy hydrogels are promising candidates for future applications in nerve regeneration.
hydrogel; electrical; conductive; nerve; tissue 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
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
The significant drawbacks and lack of success associated with current methods to treat critically sized nerve defects have led to increased interest in neural tissue engineering. Conducting polymers show great promise due to their electrical properties, and in the case of polypyrrole (PPY), its cell compatibility as well. Thus, the goal of this study is to synthesize a conducting composite nerve conduit with PPY and poly(D, L-lactic acid) (PDLLA), assess its ability to support the differentiation of rat pheochromocytoma 12 (PC12) cells in vitro, and determine its ability to promote nerve regeneration in vivo. Different amounts of PPY (5%, 10%, and 15%) are used to synthesize the conduits resulting in different conductivities (5.65, 10.40, and 15.56 ms/cm, respectively). When PC12 cells are seeded on these conduits and stimulated with 100 mV for 2 h, there is a marked increase in both the percentage of neurite-bearing cells and the median neurite length as the content of PPY increased. More importantly, when the PPY/PDLLA nerve conduit was used to repair a rat sciatic nerve defect it performed similarly to the gold standard autologous graft. These promising results illustrate the potential that this PPY/PDLLA conducting composite conduit has for neural tissue engineering.
Tissue Engineering; Nerve Regeneration; Conducting Polymer; Peripheral Nerve; Animal Model
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.
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
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
A novel strategy for affinity-based surface modification of the conducting polymer, polypyrrole, (PPy), has been developed. A 12-amino acid peptide (THRTSTLDYFVI, hereafter denoted T59) was previously identified via the phage display technique. This peptide non-covalently binds to the chlorine-doped conducting polymer polypyrrole (PPyCl). Studies have previously shown that conductive polymers have promising application in neural electrodes, sensors, and for improving regeneration and healing of peripheral nerves and other tissues. Thus, the strong and specific attachment of bio-active molecules to the surface of PPy using the T59 affinity peptide is an exciting new approach to enhance the bioactivity of electrically active materials for various biomedical applications. We demonstrate this by using T59 as a tether to modify PPyCl with the laminin fragment IKVAV to enhance cell interactions, as well as with the so-called stealth molecule poly(ethylene glycol; PEG) to decrease cell interactions. Using these two modification strategies, we were able to control cell attachment and neurite extension on the PPy surface, which is critical for different applications (i.e., the goal for tissue regeneration is to enhance cell interactions, whereas the goal for electrode and sensor applications is to reduce glial cell interactions and thus decrease scarring). Significantly, the conductivity of the PPyCl surface was unaffected by this surface modification technique, which is not the case with other methods that have been explored to surface modify conducting polymers. Finally, using subcutaneous implants, we confirmed that the PPyCl treated with the T59 peptide did not react in vivo differently than untreated PPyCl.
((Conducting Polymer; Surface Modification; Tissue Engineering; Functional Coatings; Neural Electrodes))
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
Nerve guidance conduits (NGCs) are FDA-approved devices used to bridge gaps across severed nerve cables and help direct axons sprouting from the proximal end toward the distal stump. In this paper we present the development of a novel electrically conductive, biodegradable NGC made from a polypyrrole-block-polycaprolactone (PPy-PCL) copolymer material laminated with poly(lactic-co-glycolic acid) (PLGA). The PPy-PCL has a bulk conductivity ranging 10–20 S/cm and loses 40 wt% after 7 months under physiologic conditions. Dorsal root ganglia (DRG) grown on flat PPy-PCL/PLGA material exposed to direct current electric fields (EF) of 100 mV/cm for 2 h increased axon growth by 13% (± 2%) towards either electrode of a 2-electrode setup, compared to control grown on identical substrates without EF exposure. Alternating current increased axon growth by 21% (± 3%) without an observable directional preference, compared to the same control group. The results from this study demonstrate PLGA-coated PPy-PCL is a unique biodegradable material that can deliver substrate EF stimulation to improve axon growth for peripheral nerve repair.
degradeable polypyrrole; nerve conduit; electric field; cell stimulation; nerve regeneration
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
An electrically conductive polypyrrole (PPy) doped with a bioactive agent is an emerging functional biomaterial for tissue engineering. We therefore used chondroitin sulfate (CS)-doped PPy coating to modify initially electrically insulating polylactide resulting in novel osteogenic scaffolds. In situ chemical oxidative polymerization was used to obtain electrically conductive PPy coating on poly-96L/4D-lactide (PLA) nonwoven scaffolds. The coated scaffolds were characterized and their electrical conductivity was evaluated in hydrolysis. The ability of the coated and conductive scaffolds to enhance proliferation and osteogenic differentiation of human adipose stem cells (hASCs) under electrical stimulation (ES) in three-dimensional (3D) geometry was compared to the noncoated PLA scaffolds. Electrical conductivity of PPy-coated PLA scaffolds (PLA-PPy) was evident at the beginning of hydrolysis, but decreased during the first week of incubation due to de-doping. PLA-PPy scaffolds enhanced hASC proliferation significantly compared to the plain PLA scaffolds at 7 and 14 days. Furthermore, the alkaline phosphatase (ALP) activity of the hASCs was generally higher in PLA-PPy seeded scaffolds, but due to patient variation, no statistical significance could be determined. ES did not have a significant effect on hASCs. This study highlights the potential of novel PPy-coated PLA scaffolds in bone tissue engineering.
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
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.
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.
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
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
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.
Electrically conductive and biologically active scaffolds are desirable for enhancing adhesion, proliferation and differentiation of a number of cell types such as neurons. Hence, the incorporation of neuroactive molecules into electroconductive polymers via a specific and stable method is essential for neuronal tissue engineering applications. Traditional conjugation approaches dramatically impair conductivities and/or stabilities of the scaffolds and ligands. In this study, we developed copolymers (PPy-NSE) of N-hydroxyl succinimidyl ester pyrrole and regular pyrrole, which can be immobilized with nerve growth factor (NGF) without significantly hindering electroconductivity. The presence of active ester groups was confirmed using reflectance infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) from the copolymers prepared from different monomer compositions. We selected PPy-NSE50 (polymerized from a 50 : 50 monomer ratio of pyrrole : pyrrole-NSE) for further modification with NGF because this copolymer retains good conductivity (approx. 8 S cm−1) and presents active ester groups for NGF immobilization. We tethered NGF on the PPy-NSE50 surface, and found that PC12 cells extended neurites similarly to cells cultured in NGF-containing medium. XPS and enzyme-linked immunosorbent assay confirmed that NGF immobilized via the active ester on the PPy-NSE50 film was stable for up to 5 days in phosphate-buffered saline solution. Also, application of an external electrical potential to NGF-immobilized PPy films did not cause a significant release of NGF nor reduce their neurotrophic activity. This novel scaffold, providing electroconductive and neurotrophic activities, has potential for neural applications, such as tissue engineering scaffolds and biosensors.
polypyrrole; nerve growth factor; PC12 cells; nerve tissue engineering
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
AIM: To study the therapeutic effect of three tube-guides with electrical conductivity associated to mesenchymal stem cells (MSCs) on neuro-muscular regeneration after neurotmesis.
METHODS: Rats with 10-mm gap nerve injury were tested using polyvinyl alcohol (PVA), PVA-carbon nanotubes (CNTs) and MSCs, and PVA-polypyrrole (PPy). The regenerated nerves and tibialis anterior muscles were processed for stereological studies after 20 wk. The functional recovery was assessed serially for gait biomechanical analysis, by extensor postural thrust, sciatic functional index and static sciatic functional index (SSI), and by withdrawal reflex latency (WRL). In vitro studies included cytocompatibility, flow cytometry, reverse transcriptase polymerase chain reaction and karyotype analysis of the MSCs. Histopathology of lung, liver, kidneys, and regional lymph nodes ensured the biomaterials biocompatibility.
RESULTS: SSI remained negative throughout and independently from treatment. Differences between treted groups in the severity of changes in WRL existed, showing a faster regeneration for PVA-CNTs-MSCs (P < 0.05). At toe-off, less acute ankle joint angles were seen for PVA-CNTs-MSCs group (P = 0.051) suggesting improved ankle muscles function during the push off phase of the gait cycle. In PVA-PPy and PVA-CNTs groups, there was a 25% and 42% increase of average fiber area and a 13% and 21% increase of the “minimal Feret’s diameter” respectively. Stereological analysis disclosed a significantly (P < 0.05) increased myelin thickness (M), ratio myelin thickness/axon diameter (M/d) and ratio axon diameter/fiber diameter (d/D; g-ratio) in PVA-CNT-MSCs group (P < 0.05).
CONCLUSION: Results revealed that treatment with MSCs and PVA-CNTs tube-guides induced better nerve fiber regeneration. Functional and kinematics analysis revealed positive synergistic effects brought by MSCs and PVA-CNTs. The PVA-CNTs and PVA-PPy are promising scaffolds with electric conductive properties, bio- and cytocompatible that might prevent the secondary neurogenic muscular atrophy by improving the reestablishment of the neuro-muscular junction.
Nerve regeneration; Neurotmesis; Tube-guides; Mesenchymal stem cells; Carbon nanotubes; Functional assessment; Gait biomechanical analysis; Histomorfometry; Polyvinyl alcohol
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
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
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.