(L-442) Light-activated on-demand drug delivery from neural implants

Mesoporous silicon nanoparticles are an important tool for drug delivery because of their biocompatibility and high drug loading capacity. The NTE lab at the University of Pittsburgh has pioneered the use of nanoparticle surface modifications on penetrating neural microelectrodes for biomolecule immobilization to improve neural tissue integration and electrically controlled drug release for local manipulation of neural activity [1,2]. This work explores the use of a light-activated gating material to control drug release from mesoporous silicon nanoparticles. Optogenetics employs focused light for minimally invasive neural manipulation with high spatial and temporal sensitivity. Using photo-gated nanoparticles, here we demonstrate the possibility of using an optogenetics light source for non-electrically triggered on-demand drug release in the brain. We use glutamate as our drug-of-choice because of its important and well-characterized role as an excitatory neurotransmitter.

Thiolated mesoporous nanoparticles (TNPs) were synthesized from tetraethyl orthosilicate (TEOS) and (3-Mercaptopropyl)trimethoxysilane (MPTMS) under basic conditions with hexadecyl trimethylammonium bromide (CTAB) as the surfactant template [1]. Photocaged mesoporous nanoparticles were synthesized by the attachment of an isocyanatopropyltriethoxysilane (ICPES) linker and subsequent coupling of phloroglucin tribenzyl ether (azobenzene molecule) to the TNPS. Blue light triggers a conformational change in the azobenzene molecule which opens the pores [3]. Glutamate was loaded into the nanoparticles via sonication.

To test the feasibility of light-activated drug release, the nanoparticles were covalently immobilized on silicon wafers and submerged in 1 mL DI water. To determine if glutamate release was successful, an electrochemical glutamate biosensor was inserted into the solution along with Ag/AgCl reference and Pt counter electrodes. The glutamate oxidase coating on the sensor oxidizes glutamate in the solution with hydrogen peroxide as a byproduct. Using a potentiostat, chronoamperometry was performed at 0.7 V vs. Ag/AgCl and the current from the oxidation of hydrogen peroxide was recorded. To confirm light-triggered glutamate release, a large 20W UV LED lamp was held above the wafer in darkness and switched on after 20 min of baseline recording. To demonstrate that an optogenetics light source, which is much smaller and lower power, can also be used to successfully trigger the drug release for in-vivo experiments, a 30 mW blue LED light was focused through the objective of a 2-photon microscope and positioned above the wafer in darkness. The LED was pulsed at regular intervals after 20 min of baseline recording.

Results

In this work, we use an electrochemical glutamate sensor to detect the release of glutamate from photocaged nanoparticles upon exposure to blue light. In Fig. 1A, a large UV-lamp is held above a vial containing the nanoparticle-coated wafer and electrodes. The light is switched on at 20 minutes and an immediate increase in current is observed. The gradual decay is attributed both to depletion of glutamate from the nanoparticles as well as diffusion of the glutamate away from the sensor tip. In Fig. 1B, the lamp is switched on at 20 minutes above a vial containing no nanoparticles. A minor increase in current due to the photoelectric effect is observed. In Fig. 1C, a vial containing the nanoparticle-coated wafer and electrodes is held under a blue-light focused under a 2-photon microscope objective. The light source is switched on after 1200 seconds (x-axis is shortened for presentation). The light is held constant for 160 seconds, shut off for 40 seconds, and switched on again. This pulsation cycle is done 5 times in a row. The effect of these pulses is evident in the stair-step appearance to the current recorded. The overall decay is due to residual glutamate in the vial diffusing away from the sensor tip. In Fig. 1D, the focused light is pulsed (constant for 160 seconds, shut off for 140 seconds) twice after 1200 seconds above a vial containing no nanoparticles (x-axis is shortened for presentation). A minor photoelectric effect is observed.

Conclusions

The exact correlation between the light onset and current increase as well as the negligible impact of the photoelectric effect on the current recorded from both light sources indicates that we successfully released glutamate on-demand from the immobilized photocaged nanoparticles. The next step for this work will be to coat neural electrodes with the photocaged, glutamate-loaded nanoparticles and release the drug in-vivo in Thy1-GCaMP labeled mice via a transparent cranial window. 2-Photon microscopy will be employed to observe real-time neural activity in response to drug delivery. This developing technology has potential applications for minimally invasive optically triggered drug delivery.

[1] Woeppel, K. M., Zheng, X. S., Schulte, Z. M., Rosi, N. L., & Cui, X. T. Nanoparticle doped PEDOT for enhanced electrode coatings and drug delivery. Advanced Healthcare Materials. 2019;8(21):1900622.

[2] Woeppel, K. M., Cui, X. T. Nanoparticle and Biomolecule Surface Modification Synergistically Increases Neural Electrode Recording Yield and Minimizes Inflammatory Host Response. Advanced Healthcare Materials. 2021;10(16): 2002150

[3] Angelos, S., Choi, E., Vögtle, F., De Cola, L., Zink, J. I. Photo-driven expulsion of molecules from mesostructured silica nanoparticles. The Journal of Physical Chemistry C. 2007 May;111(18):6589–6592.