Elsevier

Biomaterials

Volume 24, Issue 5, February 2003, Pages 777-787
Biomaterials

In vivo studies of polypyrrole/peptide coated neural probes

https://doi.org/10.1016/S0142-9612(02)00415-5Get rights and content

Abstract

Neural probes are micromachined multichannel electrode arrays that facilitate the functional stimulation and recording of neurons in the peripheral and central nervous system. For long-term implantations, surface modification is necessary for maintaining the stable connection between electrodes and neurons. The conductive polymer polypyrrole (PPy) and synthetic peptide DCDPGYIGSR were co-deposited on the electrode surface by electrochemical polymerization. The stability of PPy/DCDPGYIGSR coatings was tested in soaking experiments. It was found that the peptide was entrapped in the PPy film and did not diffuse away within 7 weeks of soaking in DI water. Coated probes were implanted in guinea pig brain for periods of 1, 2 and 3 weeks. Recording tests were performed and the impedance was monitored. The explanted probes and tissue were examined by immunocytochemical studies. Significantly more neurofilament positive staining was found on the coated electrode which indicated that the coatings had established strong connections with the neuronal structure in vivo. Good recordings were obtained from the coated sites that had neurons attached. First week tissue sections had no significant gliosis. In week 2, a layer of non-neuronal tissue consisting of mostly meningeal fibroblasts and ECM protein including at least fibronectin was formed around the probe tracks of both coated and uncoated probes. Astrocytes started to form a loosely organized layer by the end of the third week.

Introduction

Neural probes are micromachined multichannel electrode arrays that facilitate the functional stimulation and recording of neurons in peripheral and central nervous system. One of the many challenges these devices are facing is their long-term performance in vivo. A current problem for chronic recording in the central nervous system (CNS) is that the devices lose the ability to record neural activity a few days to weeks after implantation. The causes of this problem may be one or more of the following: migration from the intended location after surgery, protein fouling, increase of electrode impedance, decrease of neuron density at the vicinity of the probe and isolation of the electrode sites by scar tissue formation [1], [2], [3], [4], [5], [6], [7], [8]. Presumably, the performance of these devices could be improved by promoting better integration within the nervous system and minimizing the host response.

The CNS tissue reacts to implants through a series of events. The early response to materials is primarily inflammation. The reported local degree of the reaction varies among different materials and implantation locations [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Chronically, the response is characterized by a hypertrophic reaction from surrounding astrocytes [9], [14] with elevated expression of intermediate filament proteins such as GFAP and vimentin [20], the presence of a variable number of microglia and foreign body giant cells [9], [10], [14], [16], [17] and a general thickening of the surrounding tissue wrapping around the implant [2], [4], [14], [19]. As a result, a glial-fibroblastic scar is formed and is characteristically inhibitory to axon growth [21], [22]. This scar tissue surrounding the implant functions as a diffusive barrier [23] that is thought to reduce the ability of implanted devices to communicate with neurons by insulating the device from the surrounding brain tissue [2], [3], [4], [19], [24], [25].

Surface modification of neural probes should provide improvements in their long-term functionality. On non-functional areas of the implant, coatings should minimize chronic inflammatory response and promote some cell adhesion (glia, fibroblasts) to integrate the implant within the tissue. On the electrode sites, there should be a more intimate interfacial contact between the electrode and surrounding tissue, which would facilitate the charge transport from ionically conductive tissue to the electronically conductive electrode and induce selected neurons to attach onto the microelectrode. Presumably, if the neuron–electrode connection can be established before the scar tissue formation, the long-term recording will be stabilized.

We have reported previously an electrochemical deposition method that can be used to deposit conductive polymers together with bioactive molecules onto the electrode sites of neural probes [26], [27]. The conductive polymer facilitated the neural signal transport from the ionically conductive brain tissue to the electronically conductive electrode array. More importantly, this approach provided us with a means of patterning bioactive molecules on the electrode sites. The nona-peptide CDPGYIGSR, a fragment of laminin, has been shown to promote cell adhesion and neuron extension [28], [29]. This nona-peptide was co-deposited with polypyrrole (PPy) onto the microelectrode sites. The coated sites have been shown to attract neurons in vitro [27], [30]. In this paper, we continued our work on PPy/peptide coating of the neural probes and studied the aqueous stability of the coating and its effect on the brain tissue response and neural recording.

Section snippets

Electrochemical deposition

The peptide DCDPGYIGSR was synthesized by the peptide synthesis facility at the University of Michigan. The PPy/DCDPGYIGSR film was grown galvanostatically on gold electrode sites (1250 μm2 in area) of a neural probe under an anodic current density of 0.5 mA/cm2. The electrochemical cell was a 3-ml vial soldered with a platinum wire as the counter electrode. The monomer solution (1 ml) which contained 0.4 m pyrrole and 5 mg/ml DCDPGYIGSR was purged with N2 for approximately 5 min before use. The

Results and discussion

PPy/DCDPGYIGSR was successfully deposited onto the gold electrode sites of the neural probes. The morphology of the film is shown in the SEM image (Fig. 1). The surface started with a nodular base layer with significant amount of finger-like protrusions growing on the top. The chemical composition of this coating was characterized by microfocused FTIR as shown in Fig. 2. From 4000 to 1800 cm−1, all the spectra showed a featureless decrease in absorption. This is the tail of the ∼1 eV (∼8066 cm−1)

Conclusions

The stability of PPy/DCDPGYIGSR coatings has been tested in soaking experiments. It was found that the peptide was entrapped in the polypyrrole film and did not diffuse away within 7 weeks of soaking in DI water. Polypyrrole showed some subtle changes in chemical structure which caused the reduced conductivity by soaking. Impedances of PPy/DCDPGYIGSR coated sites are relatively stable for the first week of implantation. There was an increase in impedance modulus after the first week but the

Acknowledgements

This research was supported in part by the National Science Foundation (DMR-0084304), the National Institutes of Health (NINDS-N01-NS-1-2338), and the Center for Neural Communication Technology (CNCT) at the University of Michigan. The authors acknowledge useful discussions with Prof. Patrick Tresco and Dr. Roy Biran at the University of Utah.

References (37)

  • S. Schmidt et al.

    Biocompatibility of silicon-based electrode arrays implanted in feline cortical tissue

    J Biomed Mater Res

    (1993)
  • A.C. Hoogerwerf et al.

    A three dimensional microelectrode array for chronic neural recording

    IEEE Trans Biomed Eng

    (1994)
  • Oweiss K, Wise M, Lopez C, Wiler JA, Anderson DJ. Chronic electrode–brain interface modeled with FEM. Proceedings of...
  • B.G. Reuben et al.

    Phospholipid coatings for the prevention of membrane fouling

    J Chem Tech Biotech

    (1995)
  • B. Guo et al.

    Modification of a glassy carbon electrode with diols for the suppression of electrode fouling in biological fluids

    Chem Pharm Bull

    (1996)
  • M. Kyrolainen et al.

    Biocompatibility hemocompatibility—implications and outcomes for sensors

    Acta Anaesth Scand

    (1995)
  • S.S. Stensaas et al.

    Histopathological evaluation of materials implanted in the cerebral cortex

    Acta Neuropathol (Berlin)

    (1978)
  • S.R. Winn et al.

    Brain tissue reaction to permselective polymer capsules

    J Biomed Mater Res

    (1989)
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