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Biomaterials (v.29, #9)

Editorial board (pp. ifc).

The effect of electronegativity and angiotensin-converting enzyme inhibition on the kinin-forming capacity of polyacrylonitrile dialysis membranes by Anik Désormeaux; Marie Eve Moreau; Yves Lepage; Jacques Chanard; Albert Adam (pp. 1139-1146).
The combination of negatively-charged membranes and angiotensin I-converting enzyme inhibitors (ACEi) evokes hypersensitivity reactions (HSR) during hemodialysis and bradykinin (BK)-related peptides have been hypothesized as being responsible for these complications. In this study, we tested the effects of neutralizing the membrane electronegativity (zeta potential) of polyacrylonitrile AN69 membranes by coating a polyethyleneimine layer (AN69-ST membranes) over the generation of kinins induced by blood contact with synthetic membranes. We used minidialyzers with AN69 or AN69-ST membranes in an ex vivo model of plasma and we showed that plasma dialysis with AN69 membranes led to significant BK and des-Arg9-BK release, which was potentiated by ACEi. This kinin formation was dramatically decreased by AN69-ST membranes, even in the presence of an ACEi, and kinin recovery in the dialysates was also significantly lower with these membranes. High molecular weight kininogen and factor XII detection by immunoblotting of the protein layer coating both membranes corroborated the results: binding of these proteins and contact system activation on AN69-ST membranes were reduced. This ex vivo experimental model applied to the plasma, dialysate and dialysis membrane could be used for the characterization of the kinin-forming capacity of any biomaterial potentially used in vivo in combination with drugs which modulate the pharmacological activity of kinins.

Keywords: Haemodialysis membrane; Biocompatibility; Electroactive polymer; Hypersensitivity

Recombinant human collagen for tissue engineered corneal substitutes by Wenguang Liu; Kimberley Merrett; May Griffith; Per Fagerholm; Subhadra Dravida; Belinda Heyne; Juan C. Scaiano; Mitchell A. Watsky; Naoshi Shinozaki; Neil Lagali; Rejean Munger; Fengfu Li (pp. 1147-1158).
We successfully fabricated transparent, robust hydrogels as corneal substitutes from concentrated recombinant human type I and type III collagen solutions crosslinked with 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). White light transmission through these gels is comparable or superior to that of human corneas. Hydrogels from both type I and type III collagens supported in vitro epithelium and nerve over-growth. While both these biocompatible hydrogels have adequate tensile strength and elasticity for surgical manipulation, type III collagen hydrogels tended to be mechanically superior. Twelve-month post-implantation results of type I recombinant collagen-based corneal substitutes into mini-pigs showed retention of optical clarity, along with regeneration of corneal cells, nerves and tear film. For clinical use, implants based on fully characterized, recombinant human collagen eliminate the risk of pathogen transfer or xenogeneic immuno-responses posed by animal collagens.

Keywords: Recombinant human collagen; Hydrogel; Cornea regeneration; Cornea transplantation; Corneal substitute

The incorporation of poly(lactic- co-glycolic) acid nanoparticles into porcine small intestinal submucosa biomaterials by Fadee G. Mondalek; Benjamin J. Lawrence; Bradley P. Kropp; Brian P. Grady; Kar-Ming Fung; Sundar V. Madihally; Hsueh-Kung Lin (pp. 1159-1166).
Small intestinal submucosa (SIS) derived from porcine small intestine has been intensively studied for its capacity in repairing and regenerating wounded and dysfunctional tissues. However, SIS suffers from a large spectrum of heterogeneity in microarchitecture leading to inconsistent results. In this study, we introduced nanoparticles (NPs) to SIS with an intention of decreasing the heterogeneity and improving the consistency of this biomaterial. As determined by scanning electron microscopy and urea permeability, the optimum NP size was estimated to be between 200nm and 500nm using commercial monodisperse latex spheres. The concentration of NPs that is required to alter pore sizes of SIS as determined by urea permeability was estimated to be 1mg/ml 260nm poly(lactic- co-glycolic) acid (PLGA) NPs. The 1mg/ml PLGA NPs loaded in the SIS did not change the tensile properties of the unmodified SIS or even alter pH values in a cell culture environment. More importantly, PLGA NP modified SIS did not affect human mammary endothelial cells (HMEC-1) morphology or adhesion, but actually enhanced HEMC-1 cell growth.

Keywords: Scaffold; Small intestinal submucosa; Porosity; Nanoparticle; BiocompatibilityAbbreviations; SIS; small intestinal submucosa; PLGA; poly(lactic-; co; -glycolic) acid; NPs; nanoparticles; EC; endothelial cells; ECM; extracellular matrix; PVA; poly(vinyl alcohol); PEI; poly(ethyleneimine); PBS; phosphate buffer saline; TNE; Tris–HCl/NaCl/EDTA; HMEC; human mammary endothelial cells

The development of alternative vitrification solutions for microencapsulated islets by Carlos A. Agudelo; Hiroo Iwata (pp. 1167-1176).
Bioartificial pancreas in which islets of Langerhans (islets) are enclosed in a semipermeable membrane is one of the approaches to treat insulin-dependent diabetic patients. Although there are advantages in this method, one of the issues that still remains is the long-term storage of tissue engineering devices before transplantation. One of the possible routes to address this is through cryopreservation. In this study, a freezing solution, 2m DMSO in RPMI-1640, a conventional vitrification solution, VS55, and the newly developed vitrification solution KYO-1 were examined to cryopreserve microencapsulated islets in agarose hydrogel. The insulin release ability, morphology of islets, and physico-chemical properties of the agarose gel membrane were examined after a cryopreservation and thawing process. Frozen and vitrified (by KYO-1) groups showed a similar insulin secretion. Frozen groups by 2m DMSO, however, showed destruction of agarose capsules and some islets were out of the capsule. When KYO-1 was used, islets still maintained the ability to release insulin in response to glucose stimulation, and agarose capsule showed morphological integrity, and mechanical properties. In conclusion, vitrification using KYO-1 which is composed of 5.38m ethylene glycol, 2m DMSO, 0.1m PEG 1000 and 0.00175m PVP K10 in EuroCollins, is a suitable method for cryopreservation of microencapsulated islets.

Keywords: Bioartificial pancreas; Islets; Agarose; Cryopreservation; Vitrification

Osteogenicity of biphasic calcium phosphate ceramics and bone autograft in a goat model by Borhane H. Fellah; Olivier Gauthier; Pierre Weiss; Daniel Chappard; Pierre Layrolle (pp. 1177-1188).
The aim of this work was to compare the osteogenicity of calcium phosphate ceramic granules with autologous bone graft in ectopic and orthotopic sites. Biphasic calcium phosphate (BCP) granules composed of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) in a 60/40 ratio were sintered at 1050, 1125 and 1200°C, producing different microporosities. Either BCP ceramic granules or autologous bone chips ( n=7) were implanted into paraspinal muscles. Osteoinduction was not observed in either the BCP implants or autologous bone chips after 6 or 12 weeks in the ectopic sites. Hollow and bored polytetrafluoroethylene (PTFE) cylinders were filled with autologous bone, BCP granules or left empty, then implanted into critical-sized defects in femoral epiphyses. The PTFE cylinders left empty contained marrow and blood vessels but not mineralized bone, indicating that this model prevented bone ingrowth (0.56±0.43% at 12 weeks). Bone formation was observed in contact with the BCP1050 and BCP1125 granules in the femoral sites after 6 weeks. The amount of bone after 12 weeks was 5.6±7.3 and 9.6±6.6% for BCP1050 and BCP1125, respectively. Very little bone formation was observed with the BCP1200 implants (1.5±1.3% at 12 weeks). In both the ectopic and orthotopic sites, autologous bone chips were drastically resorbed (from 19.4±3.7% initially to 1.7±1.2% at 12 weeks). This study shows that synthetic bone substitutes may have superior stability and osteogenic properties than autologous bone grafts in critical-sized bone defects.

Keywords: Osteogenesis; Autologous bone; Calcium phosphate ceramics; Bone ingrowth; Osteoconduction; Osteoinduction

The effect of crosslinking heparin to demineralized bone matrix on mechanical strength and specific binding to human bone morphogenetic protein-2 by Hang Lin; Yannan Zhao; Wenjie Sun; Bing Chen; Jing Zhang; Wenxue Zhao; Zhifeng Xiao; Jianwu Dai (pp. 1189-1197).
Demineralized bone matrix (DBM) is a collagen-based scaffold, but its low mechanical strength and limited BMP-2 binding ability restrict its application in bone repair. It is known that heparin could be immobilized onto scaffolds to enhance their binding of growth factors with the heparin-binding domain. Here, we crosslinked heparin to DBM to increase its BMP-2 binding ability. To our surprise, the mechanical strength of DBM was also dramatically increased. The compression modulus of heparin crosslinked DBM (HC-DBM) have improved (seven-fold increased) under wet condition, which would allow the scaffolds to keep specific shapes in vivo. As expected, HC-DBM showed specific binding ability to BMP-2. Additional studies showed the bound BMP-2 exerted its function to induce cell differentiation on the scaffold. Subcutaneous implantation of HC-DBM carrying BMP-2 showed higher alkaline phosphatase (ALP) activity (2 weeks), more calcium deposition (4 and 8 weeks) and more bone formation than that of control groups. It is concluded that HC-DBM has increased mechanical intensity as well as specific BMP-2 binding ability; HC-DBM/BMP-2 enhances the osteogenesis and therefore could be an effective medical device for bone repair.

Keywords: Heparin; Demineralized bone matrix; Bone morphogenetic protein-2; Ectopic bone formation

Biodegradable poly-β-hydroxybutyrate scaffold seeded with Schwann cells to promote spinal cord repair by Liudmila N. Novikova; Jonas Pettersson; Maria Brohlin; Mikael Wiberg; Lev N. Novikov (pp. 1198-1206).
Cavity formation is an important obstacle impeding regeneration after spinal cord injury and bridging strategies are essential to provide physical substrate allowing axons to grow across the lesion site. In this study we evaluated effects of biodegradable tubular conduit made from poly-β-hydroxybutyrate (PHB) scaffold with predominantly unidirectional fiber orientation and supplemented with cultured adult Schwann cells on axonal regeneration after cervical spinal cord injury in adult rats. After transplantation into the injured spinal cord, plain PHB conduit was well-integrated into posttraumatic cavity and induced modest astroglial reaction. Regenerating axons were found mainly outside the PHB with only single fibers crossing the host–graft interface. No host Schwann cells migrated into the graft. In contrast, when suspension of adult Schwann cells was added to the PHB during transplantation, neurofilament-positive axons filled the conduit and became associated with the implanted cells. Although rubrospinal fibers did not enter the PHB, numerous raphaespinal and CGRP-positive axons were found within the conduit. Modification of PHB surface with fibronectin, laminin or collagen significantly increased Schwann cell attachment and proliferation in vitro. However, transplantation of PHB conduit pre-coated with fibronectin and seeded with Schwann cells did not alter axonal growth response. The results demonstrate that a PHB scaffold promotes attachment, proliferation and survival of adult Schwann cells and supports marked axonal regeneration within the graft.

Keywords: Animal model; Spinal cord injury; Nerve tissue engineering; Neural prosthesis; Transplantation

Protein release kinetics for core–shell hybrid nanoparticles based on the layer-by-layer assembly of alginate and chitosan on liposomes by Ziyad S. Haidar; Reggie C. Hamdy; Maryam Tabrizian (pp. 1207-1215).
The present work is focused on the formulation of core–shell nanoparticles via the layer-by-layer (L-b-L) self-assembly technique for delivery of biomacromolecules such as bone growth factors. The drug encapsulation efficiency of liposomes is enhanced with the increased stability of polyelectrolyte systems achieved through the alternate adsorption of several layers of natural polymers: anionic alginate and cationic chitosan on cationic nanosized phospholipid vesicles. The resulting particles were characterized for their size, surface charge, morphology, encapsulation efficiency, loading capacity and release kinetics over an extended period of 30 days. The L-b-L deposition technique succeeded in building a spherical, monodisperse and stable hybrid nanoparticulate protein delivery system with a cumulative size of 383±11.5nm and zeta potential surface charge of 44.61±3.31mV for five bilayered liposomes. The system offers numerous compartments for encapsulation including the aqueous core and within the polyelectrolyte shell demonstrating good entrapment and sustained linear release of a model protein, bovine serum albumin, in vitro. Our results demonstrate that this delivery system features an extended shelf life and can be loaded immediately prior to administration, thus preventing any loss of the protein.

Keywords: Controlled drug release; Liposome; Polysaccharide; Self-assembly

The use of charge-coupled polymeric microparticles and micromagnets for modulating the bioavailability of orally delivered macromolecules by Benjamin A. Teply; Rong Tong; Seok Y. Jeong; Gaurav Luther; Ines Sherifi; Christopher H. Yim; Ali Khademhosseini; Omid C. Farokhzad; Robert S. Langer; Jianjun Cheng (pp. 1216-1223).
Protein drugs have low bioavailability after oral administration, which is due in part to fast transit of the drugs or drug delivery vehicles through the gastrointestinal tract. Increasing the time that the drugs spend in the intestine after dosing would allow for greater absorption and increased bioavailability. We developed a formulation strategy that can be used to prolong intestinal retention of drug delivery vehicles without substantial alterations to current polymeric encapsulation strategies. A model drug, insulin, was encapsulated in negatively charged poly(lactic-co-glycolic acid) (PLGA) microparticles, and the microparticles were subsequently mixed with positively charged micromagnets, whose size will prevent them from being absorbed. Stable complexes formed through electrostatic interaction. The complexes were effectively immobilized in vitro in a model of the mouse small intestine by application of an external magnetic field. Mice that were gavaged with radio-labeled complexes and fitted with a magnetic belt retained 32.5% of the125I-insulin in the small intestine compared with 5.4% for the control group 6h after administration ( p=0.005). Furthermore, mice similarly gavaged with complexes encapsulating insulin (120Units/kg) exhibited long-term glucose reduction in the groups with magnetic belts. The corresponding bioavailability of insulin was 5.11% compared with 0.87% for the control group ( p=0.007).

Keywords: Drug delivery; Drug release; Microsphere; Magnetism; Protein

Efficient intracellular delivery of functional proteins using cationic polymer core/shell nanoparticles by Ashlynn L.Z. Lee; Yong Wang; Wen-Hui Ye; Ho Sup Yoon; Sui Yung Chan; Yi-Yan Yang (pp. 1224-1232).
Cationic core/shell nanoparticles self-assembled from biodegradable, cationic and amphiphilic copolymer poly{ N-methyldietheneamine sebacate)- co-[(cholesteryl oxocarbonylamido ethyl) methyl bis(ethylene) ammonium bromide] sebacate}, P(MDS- co-CES), were fabricated and employed to deliver lectin A-chain, an anticancer glycoprotein. Lectin A-chain was efficiently bound onto the surfaces of the nanoparticles at high mass ratios of nanoparticles to lectin A-chain. The nanoparticle/lectin A-chain complexes had an average size of approximately 150nm with zeta potential of about +30mV at the mass ratio of 50 or above while the BioPorter/lectin A-chain complexes had a larger particle size and relatively lower zeta potential (150nm vs. 455nm; +30mV vs. +20mV). Therefore, the cellular uptake of nanoparticle/lectin A-chain complexes was much greater than that of BioPorter/lectin A-chain complexes. The results obtained from cytotoxicity tests show that lectin A-chain delivered by the nanoparticles was significantly more toxic against MDA-MB-231, HeLa, HepG2 and 4T1 cell lines when compared to BioPorter, and IC50 of lectin A-chain delivered by the nanoparticles was 0.2, 0.5, 10 and 50mg/l, respectively, while that of lectin A-chain delivered by BioPorter was higher than 100mg/l in all cell lines tested. These nano-sized particles may provide an efficient approach for intracellular delivery of biologically active proteins.

Keywords: Cationic core/shell nanoparticles; Intracellular delivery; Proteins; Lectin

Pharmacokinetics, biodistribution, efficacy and safety of N-octyl- O-sulfate chitosan micelles loaded with paclitaxel by Can Zhang; Guowei Qu; Yingji Sun; Xiaoli Wu; Zhong Yao; Qinglong Guo; Qilong Ding; Shengtao Yuan; Zilong Shen; Qineng Ping; Huiping Zhou (pp. 1233-1241).
Paclitaxel (Taxol®, PTX) is a promising anti-cancer drug and has been successfully used to treat a wide variety of cancers. Unfortunately, serious clinical side effects are associated with it, which are caused by PTX itself and non-aqueous vehicle containing Cremophor EL. Development of new formulation of PTX with better efficacy and fewer side effects is extremely urgent. In the present study, a N-octyl- O-sulfate chitosan (NOSC) micelle was developed and used as the delivery system for PTX. The pharmacokinetics, biodistribution, efficacy and safety of PTX-loaded NOSC micelles (PTX-M) were evaluated. The results showed that NOSC micelles had high drug loading capacity (69.9%) and entrapment efficiency (97.26%). The plasma AUC of PTX-M was 3.6-fold lower than that of Taxol®; but theV d and CL of PTX-M were increased by 5.7 and 3.5-fold, respectively. Biodistribution study indicated that most of the PTX were distributed in liver, kidney, spleen, and lung and the longest retention effect was observed in the lung. Drug safety assessment studies including acute toxicity, hemolysis test, intravenous stimulation and injection anaphylaxis revealed that the PTX-M was safe for intravenous injection. Furthermore, the comparable antitumor efficacy of PTX-M and Taxol® was observed at the same dose of 10mg/kg in in vivo antitumor mice models inoculated with sarcoma180, enrich solid carcinoma (EC), hepatoma solidity (Heps), Lewis lung cancer cells and A-549 human lung cancer cells. These results clearly showed that PTX-M had the similar antitumor efficacy as Taxol®, but significantly reduced the toxicity and improved the bioavailability of PTX.

Keywords: Chitosan derivative micelle; Paclitaxel; Pharmacokinetics; Biodistribution; Antitumor efficacy; Safety study

Efficient and stable gene transfer of growth factors into chondrogenic cells and primary articular chondrocytes using a VSV.G pseudotyped retroviral vector by Stephan Vogt; Peter Ueblacker; Christopher Geis; Bettina Wagner; Gabriele Wexel; Thomas Tischer; Achim Krüger; Christian Plank; Martina Anton; Vladimir Martinek; Andreas B. Imhoff; Bernd Gansbacher (pp. 1242-1249).
Since efficient transfer of foreign genes into primary articular chondrocytes (CC) is difficult, a VSV.G pseudotyped retroviral vector (Bullet) was developed for marker and growth factor gene transfer. Transduction efficiency was analysed by FACS. BMP2 production was determined by specific hBMP2-ELISA. BMP2 effect on cells regarding proteoglycan production was measured by alcian blue staining and dye quantification. Alkaline phosphatase activity was determined by enzymatic reaction with p-nitrophenyl phosphate at OD 405nm and proliferation rate was analysed by MTT-assay. ATDC5 cells (98.3±0.6%SD) were transduced to express the reporter gene eGFP. After 52 weeks 94.7±0.6%SD of cells were positive. Retroviral transduction efficiency for nlslacZ exceeded 92.3±6.1%SD in rabbit CC and expression remained high after 15 weeks (75.7±14.2%SD). ATDC5 cells and CC expressed the growth factor gene hBMP2 after retroviral transduction at different time-points. BMP2 led to an increase in proteoglycan and alkaline phosphatase production. Initially, the proliferation rate detected by MTT-assay increased in both the cell types; afterwards the proliferation rate was similar to controls. The described retroviral vector system achieved high initial transduction rates in ATDC5 cells and CC. Gene transfer was very stable over the time period analysed, rendering it a useful tool for future in vitro and in vivo studies on cartilage remodelling.

Keywords: Chondrocytes; Gene transfer; Retroviral vector; Transduction; ATDC5

Expansion of Foxp3-expressing regulatory T cells in vitro by dendritic cells modified with polymeric particles carrying a plasmid encoding interleukin-10 by Liang Jia; Jeffrey R. Kovacs; Ying Zheng; Hongmei Shen; Ellen S. Gawalt; Wilson S. Meng (pp. 1250-1261).
An emerging focus in experimental gene therapy is to employ non-viral vectors to deliver immunosuppressive cytokines aimed at attenuating damaging immune responses toward auto and alloantigens. In the current study, we present data showing that poly(lactic-co-glycolic acid) (PLGA) particles modified with the cationic peptide O10H6 (PLGAO10H6) were effective in delivering a mouse IL-10 encoding plasmid (pIL10) to skew bone marrow-derived dendritic cells (DCs) to downregulate T cell responses. T cells stimulated by the IL-10 gene-modified DCs exhibited characteristics of regulatory T (Treg) cells, as evident by upregulation of Foxp3 transcription factor concomitant with an increase in TGFβ production. Thus PLGAO10H6 complexed with pIL10 delivers an overriding suppressive signal to T cells. Physical characterization of PLGAO10H6 complexed with pIL10 revealed a stable colloidal dispersion. DNA molecules carried by PLGAO10H6 were protected from serum digestion. Collectively, the results raise the prospects of using PLGAO10H6 as a vector for delivering anti-inflammatory cytokine genes to modulate T cell responses in vivo.

Keywords: Non-viral gene delivery; Interleukin-10; Dendritic cells; Regulatory T cells; Foxp3; PLGAAbbreviations; PS; Polystyrene; PLGA; Poly(; d; ,; l; -lactide-co-glycolide); O10H6; Ornithine(10)-Histidine(6); pIL10; Plasmid encoding interleukin-10; pLuc; Plasmid encoding luciferase; pGFP; Plasmid encoding green fluorescent protein; ODN; Oligodeoxynucleotide; DCs; Dendritic cells; Foxp3; Forkhead box P3

Generation of highly potent nonviral gene vectors by complexation of lipoplexes and transferrin-bearing fusogenic polymer-modified liposomes in aqueous glucose solution by Naoki Sakaguchi; Chie Kojima; Atsushi Harada; Kazunori Koiwai; Kazuhiro Shimizu; Nobuhiko Emi; Kenji Kono (pp. 1262-1272).
We reported previously that complexation of lipoplexes containing 3,5-dipentadecyloxybenzamidine (TRX-20) and transferrin-bearing succinylated poly(glycidol) (SucPG)-modified liposome, which becomes fusogenic under weakly acidic conditions, might produce gene carriers with high transfection activity. For the present study, we prepared the lipoplex–SucPG-modified liposome complexes by mixing them either in phosphate-buffered saline or in an aqueous 5% glucose solution. The complexes prepared in phosphate-buffered saline have large particles of more than 800nm, whereas the complexes prepared in the glucose solution were remarkably small: 200–300nm. The small complexes were taken up more effectively by HeLa cells, and their transfection was induced more efficiently than the large complexes'. In addition, the small complexes achieved cellular transfection more efficiently in the presence of serum than in the absence of serum, without marked cytotoxicity. Considering that their affinity to the cell is based on ligand–receptor interaction, the small complexes are highly promising as a safe vector with high transfection activity and high target cell specificity.

Keywords: Nonviral vector; Liposome; Lipoplex; Gene therapy; Transferrin; Fusion

Experimental and theoretical characterization of implantable neural microelectrodes modified with conducting polymer nanotubes by Mohammad Reza Abidian; David C. Martin (pp. 1273-1283).
Neural prostheses transduce bioelectric signals to electronic signals at the interface between neural tissue and neural microelectrodes. A low impedance electrode–tissue interface is important for the quality of signal during recording as well as quantity of applied charge density during stimulation. However, neural microelectrode sites exhibit high impedance because of their small geometric surface area. Here we analyze nanostructured-conducting polymers that can be used to significantly decrease the impedance of microelectrode typically by about two orders of magnitude and increase the charge transfer capacity of microelectrodes by three orders of magnitude. In this study poly(pyrrole) (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes were electrochemically polymerized on the surface of neural microelectrode sites (1250μm2). An equivalent circuit model comprising a coating capacitance in parallel with a pore resistance and interface impedance in series was developed and fitted to experimental results to characterize the physical and electrical properties of the interface. To confirm that the fitting parameters correlate with physical quantities of interface, theoretical equations were used to calculate the parameter values thereby validating the proposed model. Finally, an apparent diffusion coefficient was calculated for PPy film (29.2±1.1×10−6cm2/s), PPy nanotubes (PPy NTs) (72.4±3.3×10−6cm2/s), PEDOT film (7.4±2.1×10−6cm2/s), and PEDOT nanotubes (PEDOT NTs) (13.0±1.8×10−6cm2/s). The apparent diffusion coefficient of conducting polymer nanotubes was larger than the corresponding conducting polymer films.

Keywords: Conducting polymers; Impedance spectroscopy; Equivalent circuit; Modeling; Nanotubes; Neural electrode

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