Gold Nanorod-Embedded PDMS Micro-Pillar Array for Localized Photothermal Stimulation

Abstract

Gold nanorods (GNRs) are one of the most promising biomaterial choices for the photothermal activation of neurons due to their relative biocompatibility, unique photothermal properties, and broad optical tunability through their synthetic shape control. While photothermal stimulation using randomly accumulated GNRs successfully demonstrates the potential treatment of functional neural disorders by modulating the neuronal activities using localized heating, there are limited demonstrations to translate this new concept into large-arrayed neural stimulations. In this paper, we report an arrayed PDMS micropillar platform in which GNRs are embedded as pixel-like, arrayed photothermal stimulators at the tips of the pillars. The proposed platform will be able to localize GNRs at predetermined pillar positions and create thermal stimulations using near-infrared (NIR) light. This will address the limitations of randomly distributed GNR-based approaches. Furthermore, a flexible PDMS pillar structure will create intimate interfaces on target cells. By characterizing the spatiotemporal temperature change in the platform with rhodamine B dye, we have shown that the localized temperature can be optically modulated within 4°C, which is in the range of temperature variation required for neuromodulation using NIR light. We envision that our proposed platform has the potential to be applied as a photothermal, neuronal stimulation interface with high spatiotemporal resolution.

Keywords: PDMS micropillar array; Photothermal stimulation; gold nanorod; near-infrared actuation; soft lithography.

Fig. 1.

Normalized UV-Vis transmission spectra of GNR and silica–coated GNR.

Fig. 2.

Schematic diagram of fabrication steps of PDMS micropillar array.

Fig. 3.

Schematic diagram of fabrication steps of GNR-embedded tips of PDMS micropillar array.

Fig. 4.

Scanning electron microscope images of (a) PDMS micropillar (b) PVA inverted pillar and (c) PDMS micropillar made from the PVA inverted pillar structure. The scale bar is 100 μm.

Fig. 5.

Optical microscope image of PDMS micropillar tip. Bright-field microscope image (a) without GNR and (b) with GNR. Dark-field microscope image (c) without GNR and (d) with GNR. The scale bar is 20 μm.

Fig. 6.

UV-Vis Spectrum of GNR embedded on the PDMS micropillar array. (a) Experimental setup. (b) Region of the sample where the spectrum is measured. (c) Corresponding UV-Vis spectra measured of the region in (b). The scale bar is 50 μm.

Fig. 7.

(a) Optical microscope image of the PDMS micropillar array under white light. (b) Optical microscope image (emission) of the sample soaked in rhodamine B dye under green light (excitation). (c) rhodamine B dye emission intensity with varying temperatures. (d) Normalized function of change of emission intensity with change of temperature. The scale bar is 50 μm.

Fig. 8.

(a) Optical microscope image (emission) of the GNR-embedded PDMS micropillar array soaked in rhodamine B dye under green light (excitation). (b) The image of the sample after setting a threshold value for dye emission intensity to observe the micropillar tip more clearly (NIR OFF). (c) NIR ON. The scale bar is 25 μm.

Fig. 9.

Temperature response of the tip region and base region of the GNR-embedded PDMS micropillar sample when actuated with (93 mW/mm²). laser pulse.

Fig. 10.

Temperature response of the GNR-embedded micropillar array sample with (a) varying laser actuation power (b) varying On and Off cycle.