Fabrication of Injectable Micro-Scale Opto-Electronically Transduced Electrodes (MOTEs) for Physiological Monitoring

Abstract

In vivo, chronic neural recording is critical to understand the nervous system, while a tetherless, miniaturized recording unit can render such recording minimally invasive. We present a tetherless, injectable micro-scale opto-electronically transduced electrode (MOTE) that is ~60μm × 30μm × 330μm, the smallest neural recording unit to date. The MOTE consists of an AlGaAs micro-scale light emitting diode (μLED) heterogeneously integrated on top of conventional 180nm complementary metal-oxide-semiconductor (CMOS) circuit. The MOTE combines the merits of optics (AlGaAs μLED for power and data uplink), and of electronics (CMOS for signal amplification and encoding). The optical powering and communication enable the extreme scaling while the electrical circuits provide a high temporal resolution (<100μs). This paper elaborates on the heterogeneous integration in MOTEs, a topic that has been touted without much demonstration on feasibility or scalability. Based on photolithography, we demonstrate how to build heterogenous systems that are scalable as well as biologically stable - the MOTEs can function in saline water for more than six months, and in a mouse brain for two months (and counting). We also present handling/insertion techniques for users (i.e. biologists) to deploy MOTEs with little or no extra training.

Keywords: CMOS post processing; electrophysiology; heterogenous integration; physiological monitoring; tetherless neural recording.

Fig. 1.

A simplified schematic of MOTEs-based neural recording [12], [14]. *DPSS CW Laser: Diode-Pumped Solid-State Continuous Wavelength Laser. **Si APD: Silicon Avalanche Photodiode.

Fig. 2.

System Block Diagram of a MOTE [12], [14].

Fig. 3.

A MOTE decoding example [12].

Fig. 4.

AlGaAs μLEDs transferred on top of the MOTE CMOS die, before annealing (left) and after annealing (right).

Fig. 5.

MOTEs integration fabrication flow.

Fig. 6.

A scanning electron microscope (SEM) image of a fully released MOTE.

Fig. 7.

Successful encapsulation (top) vs. defective encapsulation (middle). The bottom is a SEM image of the middle.

Fig. 8.

A MOTE insertion using a pulled μ-pipette. The left shows a simplified insertion mechanism while the right shows an in vivo insertion.

Fig. 9.

A mouse with a head bar window, ~2 months after the insertion surgery (left). The right shows the mouse (awake) under measurement using a head-bar fix apparatus.

Fig. 10.

A MOTE measurement in vivo. While the decoded signal (bottom) did not show a neural response, the observed optical pulses (top) indicate that the MoTE is functioning as designed.