POEMS (POLYMERIC OPTO-ELECTRO-MECHANICAL SYSTEMS) FOR ADVANCED NEURAL INTERFACES

Abstract

There has been a growing interest in optical neural interfaces which is driven by the need for improvements in spatial precision, real-time monitoring, and reduced invasiveness. Here, we present unique microfabrication and packaging techniques to build implantable optoelectronics with high precision and spatial complexity. Material characterization of our hybrid polymers shows minimal in vitro degradation, greater flexibility, and lowest optical loss (4.04-4.4 dB/cm at 670 nm) among other polymers reported in prior studies. We use the developed methods to build Lawrence Livermore National Laboratory's (LLNL's) first ultra-compact, lightweight (0.38 g), scalable and minimally invasive thin-film optoelectronic neural implant that can be used for chronic studies of brain activities. The paper concludes by summarizing the progress to date and discussing future opportunities for flexible optoelectronic interfaces in next generation clinical applications.

Keywords: Neural interfaces; flexible optoelectronics; optical interconnects; polymer waveguides.

Figure 1:

Material characterization of fully polymerized, 180 μm-thick, hybrid and non-hybrid polymer films. a-b) Optical transmissivity plots show that hybrid polymers allow >90% transmissivity in 300-800 nm wavelength range while non-hybrid polymers absorb light in near-UV region, c) Change in polymer transmissivity post accelerated aging soak tests conducted for a period of one month in 1X PBS at 75 °C, equivalent to 12-month time at 37 °C in vivo. All polymers withstood soak tests with 2.92% (OrmoClearFX), 8.01% (OrmoComp), 14.07% (EpoCore), and 9.78% (IP-Dip) average drop in transmissivity over 300-800 nm wavelength range, d) Change in transmissivity post photo-exposure with a dose of 150 J/cm² from a 360 nm UV lamp at 17 mW/cm² light intensity. OrmoClearFX, OrmoComp and IP-Dip show less than 0.18% decrease in transmissivity as compared to 17.62% decrease for EpoCore, particularly in the 400-550 nm region.

Figure 2:

Modular POEMS for flexible optoelectronics. a) A conceptual schematic of a POEMS interface with integrated optoelectronic transmitter and receiver subassemblies. Individual modules can be independently fabricated using microfabrication and additive manufacturing techniques and then assembled using printable interconnects or optical wirebonds. SEM images show examples of independent planar and 3D structures, b-c) Microfabricated EpoCore and OrmoClearFX planar multimode waveguides on Cytop, d) 2PP-printed IP-Dip hollow waveguide on Cytop, e) 2PP-printed optical jumper on Cytop, f) 2PP-printed cylindrical vertical waveguide on top of a 670 nm VCSEL, g) 2PP-printed cylindrical horizontal interconnect between two flat-cleaved ends of an optical fiber, h) Process flow to fabricate structures shown in b) through g).

Figure 3:

Measurement cut-back method for optical loss calculation in microfabricated and additively manufactured waveguides. The measured output power for each waveguide length is plotted in a) and b) where the observed slope and y-intercept of the linear-fit corresponds to transmission loss and coupling loss, respectively. a) Optical transmission loss in 10 μm x10 μm microfabricated OrmoClearFX and EpoCore waveguides measured 4.9-7.8 dB/cm and 7.6-11.2 dB/cm, respectively, at 406 nm and 638 nm wavelengths. Fibered bench-top laser source (LDC202C, Thorlabs) was used as the light source at 1 mW optical power, b) Optical transmission loss in 50 μm-diameter 2PP-printed waveguide on a 1 mW-optical power VCSEL measured 4.4 dB/cm at 670 nm wavelength. Zoomed-in view of the printed waveguide shows precise placement of waveguide at the coupling joint and smooth waveguide walls.

Figure 4:

POEMS implantable neural optoelectrode. a) Device schematic with design components, b) Fully assembled prototype, compared to a US dime in size, c) Optoelectrode’s flexible probe shank with waveguides and recording electrodes, d) Eutectic bond pad for EEL diode placement. Zoomed-in view shows eutectic bond pad positioned next to the distal waveguide tapered end to allow easy butt-coupling during assembly (right), e) Probe shank’s tip showing waveguide emission site and Pt/Ir recording electrodes. Magnified view of waveguide front end (right), f) EEL diode flip-chipped at the eutectic pad of the thin film optoelectrode. Inset shows active diode-waveguide interface at 635nm, g) Optical scattering along the waveguide length, without and with patterned cytop cladding around the core.