Su 8 2035

The microfluidic chip consisted of a bilayer of polydimethylsiloxane (PDMS) and glass substrate. All chips were designed using an SU8-2035 negative photoresistor (Micro Chem Corp., Newton, MA) molded from a master PDMS layer. Negative chip molds were spun onto a glass wafer and patterned photolithographically in duplicate (100 mm thickness for lower layer and 190 μm thickness for upper layer), producing unique microstructures with different heights. PDMS base and curing agent (Sylgard Silicone elastomer 184, Dow Corning Corp., Washington, D.C.) were mixed thoroughly 10:1 by mass, degassed under vacuum and poured onto the master. The mold curing process was conducted for 1 h at 80°C. After cooling, the PDMS layer was gently peeled off of the master and trimmed to size. Inlet and outlet holes were created by punching through the PDMS with a razor-sharp punch. The PDMS mold was decontaminated with oxygen plasma for 15 s, bonded to a glass slide and sterilized with UV light for 30 m. Rat Tail High-Concentration Type-I Collagen solution (BD Biosciences, Franklin Lakes, NJ) was compounded according to an alternate gelation procedure, aseptically added into collagen chamber of the chip and allowed to gel at 37°C for 30 m.

PA imaging is sensitive in detecting photoabsorbers, such as metallic particles. Based on this principle, we developed a prototype of PA-sensitive magnetic, micro-scale robots with Nickel (Ni, Alfa Aesar, Haverhill, MA, USA) particles. The average diameter of the Ni particles is approximately 3–7 µm that fits in the microrobot’s envelope dimension. The Ni particles were mixed in a negative photoresist (SU-8-2035, MicroChem Corp. Westborough, MA, USA) at a density of 1.25 g/mL (volume ratio ≈ 1:7). The sample was then patterned through photolithography to produce microrobot prototypes with different dimensions (400 µm, 200 µm, 100 µm, 50 µm square-shaped, as shown in Figure 1a,b). This photolithography process produces an undissolvable polymer structure, with temperature stability as high as 150 degrees Celsius, which will prevent the leaching of Ni particles and will avoid contamination from Ni particles. While we used polymer-based constructs in this study, other strategies such as silica coating [27 ,28 (link)] can be utilized to ensure the thermal stability and reduce microrobot cytotoxicity concerns. The thickness of the developed microrobots is approximately 40 µm. The 99.9% basic metal particles and dark grey to black color provide strong optical absorption, which generates a large PA imaging contrast.

For the lithography procedures, a set of 4-inch diameter and 500–550 $\mathsf{\mu }$ m thick silicon (Si) wafers was obtained from Wafer World Incorporation (West Palm Beach, FL, USA). SU8 developer and the negative photoresist SU8-2035 were procured from MicroChem Corporation (Newton, MA, USA). Polydimethylsiloxane (PDMS) was ordered from Dow corning Corporation (Auburn, MI, USA).
All other chemicals were ordered from Sigma-Aldrich (St. Louis, MO, USA). Typically, C. elegans M9 buffer was prepared by autoclaving a 1 L solution of 3 g of KH₂PO₄, 6 g Na₂HPO₄, and 5 g NaCl in distilled H₂O, followed by the addition of 1 ml of 1 M MgSO₄. C. elegans’ food source of Escherichia coli (E. coli) strain OP50 was prepared in L-broth, a bacterial food source. L-broth was obtained by autoclaving a 1 L mixture of 10 g of Bacto-tryptone, 5 g of Bacto-yeast, and 5 g of NaCl in distilled H₂O. For neurodegeneration, 6-OHDA (636-00-0, Sigma-Aldrich), a known neurotoxin for degenerating the dopaminergic neurons, was used by obtaining a 10 mM stock solution using 5 mg of 6-OHDA in 2 mL of autoclaved M9. 6-OHDA solution was prepared in a dark room and stored at −20 ${}^{\circ }$ C.

The resist is coated at certain revolutions per minute (rpm) to obtain a uniform film thickness. It is known that the higher the viscosity the higher film thickness. SU-8 2035 used here has a viscosity of 7000 cSt (source: Microchem Corp.; note: 1 cSt =10⁻⁶ m²/s.) The spin-speeds for certain film thicknesses are taken from Ghosh et al., (2012)13 . After spin-casting, the coated sample is baked prior to exposure to the laser. The pre-baking time should be long enough to evaporate the solvent of SU-8 completely. For 100 μm film thickness, at least 12 hours of baking is required. Sudden ramp-up of temperature creates a shrinking effect on the film; therefore the rise of the temperature should be gradual starting from room temperature and reaching 65 °C and 95 °C. The supplementary information (Table S1) describes more details about the duration of pre-baking for respective film thickness.
After pre-exposure baking, the sample is allowed to cool down and is placed inside the LaserWriter for patterning. Suitable parameters are to be set for the LaserWriter to achieve the intended micro-/nanostructures. The values of these parameters are achieved by conducting several parametric experiments.

Quartz/Si wafers were cleaned using de-ionized water, acetone, and isopropanol, and then dried using nitrogen. Every substrate was a 2 cm². The side-to-side (M-I-M) and top-gate bottom-contact configurations were selected for this study. Gold electrodes were fabricated by traditional photolithography and liftoff. The IGZO solution was spin-coated onto the electrode-deposited substrates at 3500 rpm for 30 s, and the coated substrate was thermally annealed on a hot plate at 350 °C for 1 h. To reduce parasitic and additional leakage currents, the IGZO thin films were patterned by photoresist photolithography and wet etching (LCE-12; Cyantek, USA). For electrical isolation, epoxy-based SU-8 (SU-8 2035, MicroChem) was used for passivation of the electrodes, except for the active semiconducting-layer regions. SU-8 photoresist was spin-coated at 5000 rpm for 40 s and prebaked at 65 °C for 2 min and then at 95 °C for 6 min to remove the solvent and anneal the SU-8 film, which was then exposed to ultraviolet light for 35 s under hard contact. We used an SU-8 developer (MicroChem) for 2 min as a photoresist developer. The thickness of the patterned SU-8 film was approximately 4 μm.