Polyvinylidene fluoride (PVDF) electrospun fibers were used as a model substrate for deposition of polypyrrole (PPy) coating in which biotin was incorporated via chemical polymerization. Electrospun PVDF fibers were collected on a rotating drum following a technique adapted from the literature (Yee et al., 2007; Cozza et al., 2013). Briefly, a 20% PVDF solution was prepared by dissolving PVDF pellets (molecular weight: 275,000) in dimethylformamide (DMF) and acetone (5:4 v/v ratio). The polymer-containing solution in a syringe (needle gauge: 18) was placed in the syringe pump (Pump 11 Elite, Harvard Apparatus, Holliston, MA) of the electrospinning set-up. In order to obtain aligned fibers, the rotating drum of the electrospinning set-up was placed 30 cm away from the polymer-containing syringe; and the liquid was delivered at the flow rate 0.6 ml/h. Electrospinning was accomplished at an applied voltage of 20 kV and a rotating drum speed of 400 rpm. The PVDF fibers thus obtained were then maintained under vacuum for 24 h (to accelerate solvent evaporation).

Distilled pyrrole (15 mM) was mixed with an aqueous solution (1 mM) of sodium p-toluenesulfate (SPTS; 95%) and biotin (50 mM) in pure ethanol under nitrogen. In order to optimize the coating process, the PVDF fibers were soaked in this solution for 30 min at room temperature followed by ultrasonication for 5 min (to remove trapped air bubbles), prior to addition of ferric chloride solution (4 mM), and continuous shaking at 4 °C for preselected time intervals of 1, 6, 12, 18 and 24 h to produce various PPy coatings. The polymerization process was terminated at the prescribed time points by rinsing the coated PVDF fibers with copious amount of a water: ethanol (30:70 v/v) solution to remove unreacted chemicals and PPy debris. All chemicals and reagents used in the aforementioned processes were purchased from Sigma- Aldrich (MO, United States).

Scanning Tunneling Electron Microscopy (STEM) model S5500 (Hitachi High-Tech, Schaumburg, IL) was used to examine the morphology of the pure PVDF electrospun fibers and the five groups of PPy-coated PVDF fibers after polymerization for the various preselected time intervals. All specimens were coated with silver-palladium and imaged under 30 kV applied voltage at 3,000x and 15,000x magnifications. The collected micrographs were then analyzed to quantify orientation distribution of electrospun PVDF fibers, before and after PPy coating, using ImageJ (v1.8.0, NIH Image, Bethesda, MD) to evaluate the impact of PPy polymerization on the alignment of the PVDF fibers.

The hydrophilicity of electrospun fiber mats (both PVDF and the five groups of PPy coated PVDF fibers) was determined using the contact angle sessile drop method at room temperature. A water droplet (2 µL in volume) was introduced on the flat surface of mats and the contact angle was recorded using the VCA Optima System (AST Products Inc., Billerica, MA). In addition, the impact of PPy debris, produced during the polymerization, on wettability of the PPy coated PVDF fibers was studied by sonication (15 min at room temperature) of the samples to remove the debris, followed by comparison of the contact angle between the pre-sonication and post-sonication PVDF and PPy coated PVDF fiber samples. This analysis was conducted for four samples per group to quantify the wettability properties of the samples.

Uncoated PVDF fibers, PPy-coated PVDF fibers doped with biotin, and PPy-coated PVDF fibers without biotin were examined using Attenuated Total Reflection Fourier Transform IR (ATR-FTIR; Tensor 27; Bruker Optics, Billerica, MA) in order to evaluate the relative fraction of the polar β phase (β ratio) of the PVDF and to confirm incorporation of biotin, a co-doping agent, in the polymerized PPy. The β ratio of PVDF electrospun fibers was calculated using the equation: f ( β ) = A β 1.26   A α + A β , where A_α is the absorbance for the infrared absorption band at 530 cm⁻¹ (representing CF₂ bending of the α-phase) and A_β is the absorbance for the infrared absorption band at 840 cm⁻¹ (representing CH₂ bending of the β -phase) (Yee et al., 2007). The absorbance values A_α and A_β were calculated from the FTIR spectra (acquired at a resolution of 4 cm⁻¹ and a scan number of 16) for the respective samples.

Samples were dried in a lyophilizer overnight and cut into 10 mm × 5 mm sizes (n = 6 samples per group). Uniaxial elastic modulus was measured in the direction aligned with fiber orientation using a UStretch testing system (CellScale Biomaterials Testing, Ontario, Canada) at room temperature. The length between the clamps was set to 5 mm, and the thickness of each sample was measured using a micrometer prior to testing. Samples were stretched uniaxially to failure and the slope of the load-displacement graph was used to calculate the elastic modulus.

The local elastic modulus of the fibers was measured with a microindenter (Piuma Chiaro, Optics11, Amsterdam, The Netherlands). A probe with a tip radius of 21 µm and cantilever spring constant of 3.88 N/m was used to measure the elastic modulus of the electrospun fibers. Dry samples (n = 6 specimen per group) were indented using mapping mode at room temperature; the scan size was 10 spots spaced 25 μm apart along a straight line per sample. The loading and unloading force-displacement curves were acquired during each indentation process and the Oliver-Pharr (Oliver and Pharr, 1992) model was used to calculate tip-substratum area of contact and determine the elastic modulus from the initial linear region of the unloading curve by then assuming Hertzian contact between the indenter and substrate (Casey et al., 2017).

Dry samples (n = 3 per group) for each duration of PPy coating were compressed into films to remove the air between the fibers. The voltage and current of the films were then measured three times using a four probe machine (Silicon Valley Science Labs, Oak Ridge, TN) at room temperature. Conductivity (σ measured in Ω⁻¹. cm⁻¹) was calculated using the equation σ = 1 ( R × T ) where R is the surface resistance (Ω/sq) and T is the thickness (cm) as previously reported (Harjo et al., 2020).

In order to determine incorporation biotin into, and subsequent stability of biotin within the PPy coating; the PPy-coated PVDF fibers were maintained in phosphate-buffered saline (PBS) at 37 °C for two weeks. Samples were extracted from PBS after 0, 7 and 14 days of incubation and treated with Alexa Fluor™ 350 conjugated streptavidin (Thermofisher, Waltham, MA,USA) for 15 min, rinsed three times with PBS, and imaged using a Lionheart FX Automated microscope (BioTek Instruments Inc., Winooski, VT). Biotin incorporated in the PPy coating binds covalently to Streptavidin and the fluorescent intensity of the conjugated Alexa Fluor™ 350 in PPy coating was then used as an indicator of residual biotin in the PPy coating. The fluorescent intensity from the microscopy images was quantified using CTAn software (v1.17, Bruker BioSpin, Billerica, MA) to analyze biotin stability in the PPy coating over 14 days. This analysis was performed for six samples per group (time point).

The electrosensitivity of drug delivery from the PPy-coated PVDF fibers, was determined at various voltage regimes (120, 200, 280 mV/mm, applied at 1 Hz). For this purpose, C-Dish™ (IonOptix, Amsterdam, The Netherlands) carbon electrode-equipped culture plates were used to apply electrical stimulation as programmed by C-Pace EM (IonOptix, Amsterdam, The Netherlands). The PPy coated PVDF fibers were first maintained in streptavidin horseradish peroxide (HRP) (0.2 mg/ml; Thermofisher, MA, United States) solution at room temperature for 15 min and rinsed three times with PBS before being placed in the C-Dish™ plates containing fresh PBS (1 ml) and then exposed to electric pulses of 120, 200, 280 mV/mm (at 1 Hz frequency) applied to each sample for 30 min. The supernatant from these samples was collected every 10 min; and replaced with fresh PBS. The absorbance of the released streptavidin HRP in the aliquot was later measured using a Multi-Mode plate reader (Synergy 2, Biomek Instruments Inc., Winooski, VT). This analysis was performed for six samples per electrical stimulation level and time point.

The sustainability of the PPy-coated PVDF fibers as a drug delivery platform over time was determined by maintaining the PPy-coated PVDF fibers in streptavidin Alexa Fluor™ 350 conjugate for 15 min and then rinsing with PBS three times. The intensity of fluorescence of the residual streptavidin Alexa Fluor™ 350 conjugate in the PPy-coating was examined before and after exposure to electrical stimulation (200mV/mm; 1 Hz) using the C-Dish™ electrodes. The PPy-coated PVDF fibers were imaged using the Lionheart FX Automated microscope and the fluorescent intensity of the collected images were quantified using CTan software as detailed in Tensile Elastic Properties.

PPy-coated PVDF fibers were immersed in Streptavidin HRP solution (0.2 mg/ml) at room temperature for 15 min, then rinsed three times with PBS (to remove unconjugated streptavidin), and successively treated with biotinylated basic Fibroblast Growth Factor (bFGF; ACROBiosystems Inc., Newark, DE, USA) solution at 40 ng/ml in deionized water for 30 min, and again rinsed three times with PBS. A similar procedure was used for loading a solution of biotinylated Nerve Growth Factor (NGF, Alomone Labs, Jerusalem, Israel) at a concentration of 120 ng/ml in deionized water into a different group of PPy-coated PVDF fibers. Figure 1 illustrates the final schematic of the chemical structure of drug carrier platform containing streptavidin conjugated to biotinylated growth factor and anchored to the biotin incorporated within the PPy coating on the PVDF fibers.

The release profiles of 1) bFGF and 2) NGF were investigated under the applied electrical stimulation (200 mV/mm; 1 Hz) in the C-Dish™ plates in 1 ml of PBS. The PBS containing the released growth factors (either bFGF or NGF) was collected after 5 min of exposure to electrical stimulation and samples were equilibrated in fresh PBS for 5 min prior to the next electrical stimulation at the same settings. This sequence was repeated three times to estimate reproducibility and efficacy of the drug delivery platform, this was performed for four technical replicates per growth factor. The released growth factors were quantified using Enzyme Linked Immunosorbent Assays (ELISAs). Since the drug complex contains streptavidin, which can interfere with conjugation of the biotinylated antibodies to the growth factors, a modified ELISA technique was used to determine the released growth factors (Lakshmipriya et al., 2016). The reagents used for conducting the ELISAs in the present study were purchased from Thermofisher (Waltham, MA, United States).

The biological activity of the released bFGF was determined using in vitro cell viability and proliferation assays. For this purpose, BALB/3T3 cells (ATCC #CCL163, Manassas, VA, USA), were cultured in Dulbecco's Modified Eagle's Medium, containing 10% of Fetal Bovine Serum (FBS), and 1% antibiotics (Ryu et al., 2007). Cells were initially cultured in tissue culture flasks, and were passaged using trypsin once they reached 80% confluence (Ryu et al., 2007). For the viability and proliferation assay, 5,000 cells were seeded in individual wells of a 96-well tissue culture treated wellplate. The proliferation of the BALB/3T3 cells was determined after exposure to recombinant human bFGF (positive control), biotinylated bFGF, and the bFGF released via electrical stimulation from the PPy-coated PVDF fibers using the CellTiter 96® AQueous Non-Radioactive Cell Proliferation Assay, for six biological replicates per group. The recombinant human bFGF and biotinylated bFGF were matched in concentration to the measured level of bFGF released by electrical stimulation.

The bioactivity of the released NGF was determined via an in vitro assay using PC12 (ATCC #CRL-1721, Manassas, VA, United States) cells. These cells were cultured using established protocols (Robinson and Stammers, 1994) in RPMI-1640 Medium, containing 10% heat-inactivated horse serum, 5% of FBS, and 1% antibiotics were cultured in tissue-culture flasks coated with Poly-l-lysine solution (0.1% (w/v)). Cells were initially cultured in tissue culture flasks, and were passaged using trypsin once they reached 80% confluence (Robinson and Stammers, 1994). 3.6 × 10⁴ PC12 cells were seeded onto individual wells of a poly-l-lysine-coated 96-well plate. The cells were then exposed to NGF (positive control), biotinylated NGF, and NGF released from PPy coated PVDF fibers by electrical stimulation for 20 h before their proliferation was determined using the CellTiter 96® Aqueous Non-Radioactive Cell Proliferation Assay (Zhu et al., 2015), for six biological replicates per group. The NGF and biotinylated NGF were matched in concentration to the measured level of bFGF released by electrical stimulation.

The proliferation behavior of the BALB/3T3 exposed to the bFGF and PC12 cells exposed to the NGF were reported as a measure of the bioactivity of released growth factors. Cell culture related supplies were purchased from Thermofisher (MA, USA) otherwise mentioned.

All numerical data are reported as the average ± the standard error of the mean. Material characterization data was acquired for four technical replicate samples per group (PVDF and the five durations of PPy polymerization); for six technical replicates per voltage in terms of drug complex release and biotin stability; for four technical replicate samples per growth factor (bFGF and NGF) in terms of voltage stimulated release and for six biological replicates per cell type for the bioactivity assays. The orientation of the fibers was determined using z-scores as the statistical metric in MedCalc (v19.4.1, MedCalc Software Limited, Oostende, Belgium). Significant differences in the numerical data for the material characterization analyses as well as for the drug release and bioactivity assays were identified using one-way Analysis of Variance (ANOVA), followed by Tukey’s test for post-hoc determination. Statistical analysis was conducted using SigmaPlot (v13, Systat Software Inc., San Jose, CA) and Prism (v9, GraphPad Software, San Diego, CA). A value of p < 0.05 was considered statistically significant.

Scanning Tunneling Electron Microscopy (STEM) demonstrated that PPy polymerization resulted in PPy fibers and debris deposited haphazardly around the PVDF fibers in the initial hours (after 1, 6, and 12 h in Figure 2) of polymerization until it formed a uniform coating around the PVDF fibers with minimal randomly oriented debris after 18 h of polymerization (Figure 2). However, random clusters and debris were again observed after 24 h of polymerization. While some PPy particles agglomerate during the polymerization, with increased polymerization time, a uniform coating is formed around the electrospun PVDF fibers. The diameter of the electrospun PVDF fibers was measured at 3.52 ± 0.15 µm and the diameter of PPy coated PVDF fibers was 4.08 ± 0.76 µm after 18 h of polymerization. High resolution cross-sectional images of the group coated with PPy for 18 h are shown in Figure 2 (B-D). The cross-sectional images did not permit discrimination of core and shell materials of the fibers; however, the images of coated fibers showed distinct regions of base fibers (core) and distinguishable coating thickness (shell).

The PPy polymerization process did not impact the alignment of the PVDF fiber system, with the median orientation of the PVDF fibers found to be 4.5^o (z-score 3.19) and PPy coated PVDF fibers had a median orientation of −1.5^o (z-score 3.58), as measured by OrientationJ (Figure 3). The alignment of the PPy-coated PVDF fibers was undisturbed during the PPy polymerization process, with the debris and other PPy clusters observed in the microscopy images resulting in a minor increase in perpendicularly oriented fibers (at −90^o and 90^o respectively).

Hydrophilicity of PVDF fibers, and PPy coated PVDF fibers (after 1, 6, 12, 18 and 24 h of PPy polymerization) was characterized before and after sonication via sessile drop contact angle measurement. All PPy coated groups demonstrated significantly higher contact angle before sonication, compared to the contact angle on samples after sonication (p < 0.001, Figure 4). The highest contact angle before the sonication was observed for the sample with 1 h PPy coating (140° ± 4°) which decreased to 91° ± 4° after sonication (p < 0.001). All other PPy-coated groups demonstrated increased hydrophilicity after sonication and the lowest contact angle (81° ± 4°) was observed in samples coated by 18 h of PPy polymerization. The group coated by 18 h PPy polymerization had a significantly lower contact angle before sonication compared to the 1 h and 6 h PPy polymerization coatings (p < 0.005) and after sonication compared to the 6 h PPy polymerization coating (p = 0.014, Figure 4). There was no change in contact angle measured on the PVDF fibers before and after sonication (120° ± 3°, p = 0.811). After sonication, all PPy coated groups had significantly more hydrophilic surfaces compared to the uncoated PVDF (p < 0.015). These results are in line with the PPy debris observed on the PVDF fiber surfaces during the PPy polymerization process (Figure 2). The uniform coating produced after 18 h of PPy polymerization led to a reduction in contact angle, and sonication which removed PPy debris from the surface also led to a net reduction in contact angle. Since the most uniform coating, with the lowest contact angle (most hydrophilic and hence conducive for biomedical applications) was observed after 18 h of PPy polymerization on the PVDF surface, all further electrosensitive drug delivery studies were conducted using this optimized synthesis parameter. X-ray Photoelectron Spectroscopy analysis confirmed the change in chemical composition from the PVDF substrate to the PPy coated samples (Supplementary Table). The impact of polymerization time on PPy coating characteristics was best represented by the changing ratio between the percentage of F1s and N1s atoms observed from the survey spectra (Supplementary Table).

Attenuated Total Reflection Fourier Transform IR (ATR-FTIR) was used to characterize the piezoelectric characteristics of the electrospun PVDF, demonstrated the coating of PPy on the PVDF and established that biotin was incorporated within the PPy coating (Figure 5). Characteristic peaks observed at 531 cm⁻¹ and 874 cm⁻¹ correspond to the C–F bond stretching vibration of PVDF (Yee et al., 2007) while the peak at 1,403 cm⁻¹ is characteristic of the CH₂ wagging vibration (Bormashenko et al., 2004). The ratio of the β phase (more polar and hence representative of the extent of piezoelectricity) compared to the α phase was measured to be 75% using the magnitudes of the absorbance at 530 and 840 cm⁻¹. The peaks at 1,535 cm⁻¹ and 1700 cm⁻¹ represents characteristic bonds of pyrrole which were observed in samples with PPy coating (Dams et al., 2013). Out-of-plane bending absorption was observed near 800 cm⁻¹ due to the N-H bonds of PPy. Also, the characteristic wide stretch of the C=C bond was observed at 1,535 cm⁻¹ and characteristics of the N-C=O bond were observed at 1700 cm⁻¹, all of which confirmed the presence of the PPy coating. In addition, absorbance spectra of PPy coated samples with and without biotin added as a co-dopant clearly confirmed incorporation of biotin into the polymerized PPy coating (Figure 5). The peak observed at 1708 cm⁻¹ was likely due to an overlap of the C=O bond (Bunaciu et al., 2009; Balan et al., 2012) of the carboxylic acid peak and the carbamide groups characteristic of biotin.

Mechanical characterization of the fibers indicated that the tensile stiffness increased in direct proportion to the duration of PPy coating, increasing significantly from 0.8 ± 0.09 MPa for PVDF to 6.6 ± 0.65 MPa for the PVDF after 24 h of PPy coating (p < 0.0001, Figure 6A). This is intuitively explained by the tensile stiffness of fibers being directly proportional to their diameter and the diameter increasing with coating duration. In the case of local micromechanical properties, coating the PVDF with PPy caused a significant reduction in indentation modulus (p < 0.0001 after 1, 6 and 12 h of coating), as seen in Figure 6B. An increase in indentation modulus was observed with increased duration of PPy coating, such that no significant difference in stiffness was observed between the bare PVDF (5.3 ± 0.14 kPa) and after 24 h of PPy coating (4.3 ± 0.76 kPa).

Incorporation of biotin in the PPy coating as a dopant in the polymerization process, is essential for loading conjugated streptavidin carriers and further controlled drug delivery applications. The successful incorporation of biotin and its continued presence over 14 days as an available site for streptavidin conjugation is shown in Figure 7 by conjugation of fluorescently tagged streptavidin. Samples were stained with fluorescent-tagged streptavidin immediately upon synthesis and then after 7 days and 14 days of incubation in PBS and quantitative measurements of relative fluorescently stained area fraction of the samples indicated that there was no significant difference between the samples at different times (p = 0.428). Lack of significant difference between groups indicates that substantial passive release of biotin did not occur over the first 14 days.

To evaluate the impact of coating duration on the conductivity of the sample, surface resistivity was measured using the four point probe technique and subsequently used to calculate surface conductivity (Figure 8A). The conductivity of the group coated with PPy for only 1h was relatively higher (0.19 ± 0.13Ω⁻¹ cm⁻¹)than groups coated with PPy for 6, 12, and 18h (0.01 ± 0.00Ω⁻¹cm⁻¹). No significant differences were observed in conductivity between the 6, 12 and 18h coating duration. On the other hand, the highest conductivity (0.24 ± 0.016Ω⁻¹ cm⁻¹) was measured in group coated with PPy for 24 h which was significantly higher that the groups coated with PPy for 6, 12, and 18 h. This result revealed the possibility of bare PVDF being present in coated groups even after 18 h of PPy coating while the coating for 24 h caused the highest conductivity potentially due to more congruent coatings, despite a significant amount of PPy debris observed in the system.

Streptavidin conjugated drug release from PPy coated PVDF fibers in response to electrical stimulation was measured by quantifying streptavidin HRP release under different stimulatory electrical regimes (120, 200, and 280 mV/mm) over 60 min (measured every 6 min), as seen in Figure 8B. The least drug quantity released was at 120 mV/mm while the drug released under both 200 and 280 mV/mm stimulation (not significantly different from each other, p = 0.136), each demonstrated significantly greater cumulative release (p < 0.001, main effect between groups). Over the first 12 min, no significant difference was observed in the total drug released between the three stimulation levels (p > 0.068), but from 18 min to 60 min of stimulation, significantly greater drug was released at 200 and 280 mV/mm applied electrical stimulation compared to 120 mV/mm stimulation (p < 0.005). The cumulative release indicated electrosensitive behavior underlying the drug release observed from the PPy coatings, with 14.3 ± 2.1, 96.1 ± 5.0, and 118.2 ± 11.7 ng of streptavidin HRP was released after 60 min of electrical stimulation at 120, 200, and 280 mV/mm respectively (Figure 8B). At 120 mV/mm there was no statistically significant difference between cumulative release after any time point (p > 0.08), indicating that the 200 mV/mm was the minimum stimulatory level to achieve substantial stimulus responsive release.

The sustainability of the PPy-coated PVDF fibers as a drug depot (Figure 9) was demonstrated by measurement of the streptavidin Alexa Fluor™ 350 conjugate remaining in the PPy coating during 15 min of electrical stimulation (at 200 mV/mm and 1 Hz). The fluorescent intensity did not decrease during the first 5 min (p = 0.998) or 10 min (p = 0.993) of electrical stimulation and a subtle decrease was observed by 15 min of electrical stimulation (p = 0.09). The streptavidin remaining in the PPy coated PVDF fibers decreased slightly after 15 min of stimulation, however, the reduction in the streptavidin conjugated fluorescence in the PPy-coated PVDF fibers after 15 min of electrical stimulation was not significantly different from time as synthesized (p = 0.09) or 5 min of stimulation (p = 0.064, Figure 9). This is in accordance with the observations after continuous stimulation at 200 mV/mm and 1 Hz, no significant difference was found in cumulative drug release until after 18 min of continuous stimulation (Figure 8B).

PPy coated PVDF fibers were loaded with streptavidin conjugated with biotinylated growth factors which were then electrically stimulated at 200 mV/mm (and 1 Hz frequency). Biotinylated NGF and bFGF were separately conjugated to streptavidin, released over three intervals and the released growth factor complex was tested to ensure maintenance of bioactivity. As seen in Figure 10, the instantaneous concentration of NGF increased after each stimulation period and returned to the baseline level during incubation periods between electrical stimulation. Figure 10C shows the cumulative release of NGF, which indicates that nearly 45% of the original NGF loaded into the samples was released after electrical stimulation (for a total of 15 min). PC12 cells exposed to recombinant human NGF, biotinylated NGF, and NGF-streptavidin complex released from PPy-coated PVDF fibers at equivalent concentrations of growth factor showed that cell proliferation was significantly higher when exposed to the recombinant human NGF compared to all other treatments (p < 0.01), while exposure to the biotinylated NGF and released NGF resulted in significantly greater cell count than the control group with no exposure (p < 0.003). There was no significant difference in cell number when exposed to the biotinylated NGF compared to the released NGF (p = 0.891), indicating that electrical stimulation and conjugation to streptavidin did not affect activity of the NGF (Figure 10). Figure 10D illustrates the schematic picture of NGF receptor interaction with the streptavidin-biotinylated NGF complex.

Figure 11 quantifies bFGF release from the PPy coated PVDF fibers after electrical stimulation and incubation periods identical to the profile observed for NGF. bFGF release (6 ng/ml under electrical stimulation) was observed consistently during every sequence of electrical stimulation. 49% of the bFGF loaded in the PPy coating was released during the three alternating sets of 5 min stimulation followed by 5 min incubation period sequences. Bioactivity of the released bFGF based on BALB cell proliferation showed that there was no significant difference in the activity of the released bFGF and biotinylated bFGF on cell proliferation (p = 0.934) while both these treatments resulted in significantly higher cell proliferation compared to the control group with no growth factor exposure (p < 0.001). The recombinant human bFGF treatment resulted in significantly higher cell proliferation compared to all other treatments (p < 0.001). Figure 11D shows the schematic of the drug carrier released from PPy coated PVDF fibers and its interaction with the cell surface fibroblast growth factor receptor. As previously mentioned, the receptor interaction with (one of up to three) available growth factors bound to streptavidin.