Elsevier

Biomaterials

Volume 32, Issue 27, September 2011, Pages 6374-6380
Biomaterials

A biofunctionalization scheme for neural interfaces using polydopamine polymer

https://doi.org/10.1016/j.biomaterials.2011.05.028Get rights and content

Abstract

Chemical surface modification of neuron-surface interfaces is essential for the development of biologically active and functional neural interfaces. Different types of surface modification schemes are required to derivatize either electrode or insulator surfaces, which limits the surface chemistry based neural interface design. Herein, we report a novel and powerful approach for modifying neuron-surface interfaces using mussel-inspired polymer (‘polydopamine film(polyDA)’) for generating effective chemical platforms on both electrode and insulator surfaces simultaneously. We applied polyDAs to common neural interface surfaces (gold, glass, platinum, indium tin oxide, liquid crystal polymer) and subsequently functionalized them by covalently linking biomolecules. The surfaces coated with polyDAs exhibited uniform and reproducible surface properties and they all became neuron-adhesive after linking with poly-d-lysine. In addition, polydopamine-coated microelectrode arrays were readily functional such that spontaneous and evoked neural activities could be recorded from cultured neuronal networks. We have successfully showed that a novel polyDA can be effectively used for the neural interface design.

Introduction

Neuron-surface interfaces have become indispensable in engineering problems including neural prostheses [1], [2], neuron-based cell chips [3], [4], and tissue engineering [5]. For neural prostheses, there were concentrated efforts for reducing the effects of inflammatory responses from the biosystems by modifying the surfaces of the devices, which construct neuron-material interfaces at the ends [6]. For example, bioactive molecules such as fibronectin [7], cell adhesion peptide [8], and neurotrophins [9] have been immobilized to the surfaces inspired by the idea that proximity of neurons to the electrodes could minimize the effects of the immune responses. There were also approaches using polyethylene glycol (PEG)-based coatings utilizing distinguishing non-biofouling effect of PEG brushes [10], [11]. In neuron-based cell chip designs, surfaces are functionalized with various biomolecules to convert surfaces into cell-adhesive or neurite guiding surfaces. For example, ECM proteins were immobilized to gold surfaces to induce the attachment and differentiation of DRG neurons [12]; Polyaminoacids or polycations were used to position the neurons and guide the outgrowth of the neurites on gold [13] or polymer microparticles [14]; In addition, PEG or serine were used to obtain long-term stable patterning of hippocampal neurons on silicon-based surfaces (e.g. glass or silicon oxide) [15], [16].

Surface modification schemes are required to functionalize neuron-surface interfaces. In neural probe or array designs, surfaces are composed of metal electrode and inorganic or organic insulators [17]. For insulators, silicon dioxide, silicon nitride or polyimide were derivatized by using organosilane with various functional groups. Mercapto groups were used to link biomolecules through heterobifunctional linker (sulfo-GMBS) [18], while epoxide groups were used for direct linking with an additional linker layer [16]. An organic insulator such polyimide could also be derivatized by organosilane chemistry. For metal electrodes, bioactive peptides were immobilized on electrodes by electroplating with conductive polymer coatings [19]. Despite these developments, there is still a need for some chemical modification schemes that can derivatize a variety of neural materials used for insulators and electrodes.

In this report, we demonstrate an approach for modifying neuron-surface interfaces using polydopamine films, mussel-inspired polymers, for the effective modification of the neuron-material interfaces. Polydopamine films (polyDAs) have recently been introduced by Messersmith et al., who reported their versatility for various substrates and feasibility for facile functionalization [20]. PolyDAs have exhibited uniform chemical characteristics independent of substrate materials, and facile covalent conjugation with nucleophiles (amines and thiols) taking advantage of catechol/quinone equilibrium. Here we applied polyDAs to investigate the application of polyDAs to neural interface designs. PolyDAs were applied to common neural interface materials (gold, glass, platinum, indium tin oxide, liquid crystal polymer) and converted them into biologically functional surfaces by covalently linking polylysines (Fig. 1). Primary neuronal cultures were used to test biocompatibility of polyDAs and cell viability was quantified. To demonstrate the applicability on actual neural devices, a planar microelectrode array surface was biofunctionalized using polyDAs and neural recording and stimulation were successfully obtained from cultured neuronal networks at 3 weeks in vitro.

Section snippets

Materials

2-Propanol (Merck, Germany), acetone (extra pure, Dae Jung Chemical & Metal Co., Ltd., Siheung, Korea), and Hank’s Buffered Salt Solution (HBSS) (WelGENE Inc., Korea), were used as received. Sprague-Dawley rat embryonic stage 18 was purchased from Koatech, Co., Korea. Trypsin, 2.5% (10X), liquid, neurobasal, B-27 serum-free supplement, l-glutamine, penicillin-streptomycin, live/dead viability/cytotoxicity kit (L-3224), and phosphate buffered saline (PBS) 7.4 (1X), liquid were bought from

Characterization of polyDAs and polylysine-linked PolyDAs

The formation of polyDAs were confirmed by measuring film thickness and water contact angle. Samples were simply dipped into a buffered aqueous solution of dopamine (2 mg of dopamine per milliliter of 10 mm tris, pH 8.5) for a desired reaction time (1 h-overnight). We could verify the formation of polyDAs by characteristic dark color of treated substrates. The film thickness measured by ellipsometry confirmed that more polyDAs were deposited as immersion time increased which was consistent with

Conclusion

In summary, we report here an approach using mussel-inspired surface chemistry for generating effective platforms for dissociated neuronal cultures, and demonstrate their performances at the recording of neural signals on MEAs. By exploring chemical properties of the polyDAs with varying the deposition time, we found that 1–3 h of deposition was enough for generating reproducible chemical properties on the surfaces. Applying this facile protocol, we could successfully culture neuronal networks

Acknowledgements

This work was financially supported by the grant from the Industrial Source Technology Development Program (10033657-2010-12) of the Ministry of Knowledge Economy(MKE), and the Basic Science Research Program (2010-0001953) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Korea.

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