2, Leibniz Institute of Polymer Research Dresden (IPF), Dresden, , Germany
3, Max Bergmann Center of Biomaterials Dresden (MBC), Technische Universität Dresden, Dresden, , Germany
Currently, implantable neuroprosthetic devices are made of materials with high elastic moduli. The mechanical mismatch between the device and soft neural tissue leads to a foreign body response, which results in encapsulation and can ultimately cause a loss of function. Recent approaches to make neuroprosthetic implants softer and more stretchable employ silicones, metallic thin-films or conductive composites as functional materials. Even though this approach allows for a drastic reduction in device stiffness, the overall mechanical behavior of devices resembles that of connective tissue. Hydrogels are a promising class of materials as they can have elastic moduli similar to that of neural tissues. However, at the same time they are typically brittle, do not support electronic conduction and are challenging to process with standard microfabrication technology.
In this work we design a hydrogel based material that is mechanically tough, electrically conductive and contains a network of interconnected cell-sized pores. To induce toughness, we employ a covalent polyacrylamide and a non-covalent agar-alginate network. Compared to the single polyacrylamide network the interpenetrating network shows superior toughness but only a minor increase in elastic modulus (about 40 time lower than the modulus of PDMS and only 2 times higher than the modulus of neural tissue). By cryogelation of the tough hydrogel with the addition of gelatin A at different temperatures between -10 to -20°C we induced pores with defined sizes ranging from about 100 µm down to 10 µm depending on the processing temperature. The porosity should allow for the growth of cells directly into the gels thus improving hydrogel-tissue integration. The pre-gel solution can be 3D printed using extrusion nozzles with a diameter down to 250 µm. We employed post-gelation polymerization of poly(3,4-ethylenedioxythiophene) around the hydrogel struts to electrically functionalize the material for potential neural interfacing applications. By the use of benzophenone or 3-(trimethoxysilyl)propyl methacrylate we polymerized gels directly to PDMS, glass and titanium surfaces.
Taken together our material shows promise for the fabrication of tailor made neuroprosthetic devices. The combination of toughness and softness may allow for minimally invasive implantation combined with reduced mechanical mismatch between device and host tissue.