Research
Optoelectronics to Neuroelectronics
Neuronal interface with materials and devices
Engineering neuronal circuits on artificial substrates using pre-determined external cues and surface parameters provides insights into designing regenerative scaffolds to interface with the nervous system. Advanced fabrication techniques in materials science are increasingly being used to create micro- and nano- scale surface topographies (like grooves and pillars) for directing the growth of neuronal processes. We have been growing neuronal cell cultures on vertically aligned Indium Phosphide (InP) nanowires. Our results show that nanowires act as nanoscale topographical cues for neuronal growth, resulting in a directional growth of the processes and highly interconnected neuronal network. Our first publication from this research work can be found here (Nano letters, 2017).Our group has the expertise of culturing neuronal cells (primary, cell lines) on various surfaces like polymers, nanoparticles, optoelectronic materials and devices. We also work with tissue explants and slices. We use immunocytochemistry, cell-viability assays, live cell imaging and electrophysiology to characterize neuronal growth and function. These techniques form the basis of neuroelectronics in our group.
Semiconductor/Electrolyte Interface
Our group has been studying the optoelectronic properties of semiconducting polymers in contact with aqueous electrolytes. Such interfaces are found in devices such as electrochemical and electrolyte-gated field effect transistors, photoelectrochemical cells, and tandem photovoltaic and dye-sensitized solar cells. The interface between semiconducting polymer and physiological media are also important for their potential use in bioelectronics.
We studied the characteristic features arising from bulk and interfacial properties of semiconducting polymer/aqueous electrolyte devices. The semiconducting polymer layer was chosen as a p:n mixture (Bulk Heterojunction (BHJ)) of P3HT:N2200. We further characterized the photocurrent, photovoltage and photo-capacitance across these BHJ/El devices as a function of various parameters (like thickness, wavelength, frequency), and modeled the device response features using microscopic parameters. Our results highlighted the role of bulk carrier concentration, diffusion length scales and transport mechanisms prevalent in these devices.
More about this interface can be read about in our paper here (J. Phys. Chem. Lett., 2010).
Color Sensing and Biomimicking
We have shown that the optoelectronic signals arising from BHJ/electrolyte devices, upon pulsed optical stimulation, could be used for color sensing. We have utilized the characteristic transient profiles of the photocurrents across these device structures for fabrication of a single- layer, multi-color detector. The detection is based on appropriate composition and thickness of the BHJ layer in contact with aqueous electrolyte, which results in characteristic polarity and temporal profile of the photocurrent signals in response to various incident colors (wavelengths). These results demonstrated the use of a single active BHJ layer of appropriate thickness in contact with an aqueous medium as a color discriminator device.
In particular, the optoelectronic device characteristics of semiconducting polymers interfaced with physiological media resemble various features observed in natural visual systems. The pulse profiles and time scales of these electrical signals are similar to those found in natural photoreceptors like such as bacteriorhodopsin, photosynthetic membranes, and the rod and cone cells in the mammalian retina as measured by our group.
More about our work on color sensing and visual biomimicking can be read here (J. Am. Chem. Soc., 2012). These studies provide design rules for optoelectronic polymer-based sensors that can be utilized as stimulation elements for retinal neurons.
Retinal Prosthesis
The development of bioelectronic retinal implants is aimed towards eliciting visual perception in the retina by electrically stimulating the retina either subretinally from the pigment epithelial side or epiretinally from the ganglion cells side. Conjugated semiconducting materials have a unique combination of mechanical and optoelectronic properties, which can be utilized for advanced bioelectronic and neuroprosthetic interfaces, in particular for artificial retina solutions.
The stability of the semiconducting polymers in contact with aqueous media as well as the interesting similarities of photoresponse of such structures with that of the natural vision systems point to the biointerfacing of these polymers with visual system and to use them as artificial photoreceptors to elicit neuronal activity in a blind retina. Our research work highlighted that semiconducting polymers can be used as an active photosensitive platform to stimulate blind retinas.
As part of this research work, an early stage (embryonic day 12-14) chick retina – whose photoreceptors are non-functional – was interfaced with the BHJ polymer layers based on P3HT:N2200, and the retinal activity was recorded using an MEA set-up. Upon photoillumination of this polymer-blind retina interface in physiological conditions, the optoelectronics signals from the polymer resulted in a neuronal activity in the retina! The features of the elicited neuronal response in the retina (latency, spike rate and spike number) were observed to depend on the incident light parameters like intensity, pulse width and rate, spatial profile, and wavelength, which control the optoelectronic response of the polymer film. Interestingly, the evoked activity resembles the natural response of the retina to light stimulation.
Unlike conventional epi- and sub-retinal approaches using inorganic elements and metal electrodes in contact with the tissue components, our approach utilized a simple, wiring free epiretinal interface. The possibility of patterning the BHJ layer on flexible and conformable substrates having an array of transparent electrodes could further result in utilization of this polymer interface as an optoelectronic epiretinal implant for artificial retina applications.
Our research in this area has been published in a top journal and can be read here (Adv. Mater., 2014).