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Description
INTRODUCTION
Bionic implants have been widely used in the clinic to replace or restore impaired neurological function [1]. However, conventional devices rely on stiff metallic electrodes, which often trigger inflammatory responses that hinder longevity and performance [2]. Although different biomaterials have been investigated to dampen the mechanics of stiff electrodes, the establishment of a chronically stable device-tissue interface remains challenging.
It has been hypothesised that tissue-engineered interfaces containing live neural cells could promote the biological integration of implantable devices. Furthermore, the establishment of synaptic communication between bionic implants and the nervous system would enable safer and more natural modes of tissue activation. However, the development of these “living electrodes” has been hindered by the lack of biomaterial-carriers that support the development of the encapsulated cells into functional neural networks [3].
Herein, a biosynthetic hydrogel system based on gelatin (GEL) and norbornene-functionalised poly(vinyl alcohol) (PVA) was developed. The synthetic PVA matrix was essential to tailor the mechanical properties and provide stability to the construct. Conversely, the RGD and MMP-2 sensitive motifs from gelatin provided sites for cell adhesion and enzymatic degradation, respectively. Furthermore, recent studies have shown that glial populations are needed to support neuronal development and function in vitro [4]. Hence, the ability of PVA-GEL hydrogels to support the growth of encapsulated primary astrocytes and co-cultures of primary neural cells was evaluated.
METHODS & RESULTS
Primary rat astrocytes were encapsulated in PVA-GEL hydrogels. Cytoskeletal development and cell-material interactions were evaluated via immunofluorescent staining (IFS) of GFAP and paxillin, as well as MMP-2 production in 3D. These results showed that encapsulated cells were able to adhere and develop focal adhesions, while also engaging actively in matrix remodelling to mediate cell migration. Subsequently, primary rat neural stem cells were encapsulated in combination with mature primary astrocytes. The morphology and functionality of the resulting neural constructs was assessed via IFS and calcium imaging. These results showed that PVA-GEL hydrogels were able to support the formation of functional complex co-culture systems in a 3D environment. In addition, the ability of these neural constructs to interface with neural tissues was evaluated using organotypic cultures. Rat brain slices were cultured on top of cell-laden PVA-GEL hydrogels and the scaffold-tissue interface was analysed using IFS of neuronal, astrocytic, and pre- and post-synaptic markers. The results of these organotypic cultures showed seamless integration between cell-laden hydrogels and rat brain slices and IFS suggested the establishment of functional synaptic connections between the two layers.
CONCLUSION
A biosynthetic hydrogel with tailorable biological and physical properties was developed. The incorporation of gelatin enhanced cell growth and adhesion by providing biological and topological cues to the encapsulated cells. The incorporation of a glial component was essential to mediate neural growth and development and the formation of functional neural networks. The engineered neural constructs were shown to interface with the brain slices and support tissue ingrowth in vitro. Therefore, this hydrogel system holds great potential for the development of seamless neural interfaces to mediate the communication between implantable bionic devices and physiological tissues.
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