Microfabrication of Biomimetic Structures for Neural Interfaces
Interfacing with the brain is a challenging problem. While many innovative methods for providing input and output to neural systems have been developed and demonstrated successfully in human patients, these invasive systems use less biologically compatible means than are realizable. Materials and mechanisms which are closer mimics to biological systems in their behaviors can lead to more stable and effective medical prosthetic and research devices.
Retinal prosthetics, as well as Cochlear implants, are neural implants which provide stimulation via electrical impulses. Currents passing across neurons trigger neurons to begin firing or generating action potentials down their axons, stimulating neurons with dendrites connected to those axons terminals. This scheme transduces the desired simulation into neural firing spikes, but the majority of applied current is shunted around neurons rather than contributing to stimulation. Excess current contributes to the power and thermal budgets of neural simulation devices, which are implanted in tissue, limiting their functionality. Additionally, the electrical contacts which provide a source and return for the stimulation currents are subject to degradation over time, as currents are repeatedly applied during stimulation events. Stimulation of neurons via local delivery of potassium in excess of available intercellular potassium can be used in place of direct electrical stimulation and promises to be a more biologically compatible method.
Several forms of neural recording devices have been developed and the widest know and highest density is the Utah Array, a 3D array of silicon spires, which can record from their tips when inserted into neural tissue. While this 3D array topology can access a field of neural activity, the stiffness of these silicon spires is very different than that of neural tissue, which can lead to an unwanted inflammatory response. Conductive polymer pillars made of softer materials that are a closer match to neural tissues, while mirroring the same density and insertion length as the Utah Array, may provide a better recording mechanism due to their improved mechanical compatibility with neural tissues.
This thesis investigates microfabrication schemes to produce biomimetic structures that can enable neural simulation and recording devices which feature greater biological stability and improve utility.