Pegda

A 27‐gauge expansion needle (Air‐Tite, Tochigi, Japan) was used to fabricate a hollow microneedle, as previously described.^[19] Briefly, after shortening the needle to ≈1.0 mm in length using a conventional cutter, the needle was ground with a cordless rotary tool (Dremel 800, Robert Bosch, Gerlingen, Germany). The length of the needle was measured to be ≈650–750 µm under a stereomicroscope, and the needle was then sterilized using ethylene oxide (Anprolene AN74j sterilizer, Andersen Products, Haw River, NC). An in situ‐forming crosslinked hydrogel (HA‐XL) was prepared using 3% (w/v) thiol‐modified hyaluronic acid (HA‐SH) (Glycosil, ESI Bio, Alameda, CA) and 5% or 9% (w/v) poly(ethylene glycol) diacrylate (PEGDA, MW 3500 Da, JenKem Technology, Beijing, China) dissolved in HBSS. While both the 5% and 9% PEGDA formulations were studied in vitro, only the 9% PEGDA formulation was used in vivo. For the in vivo study, aliquots of HA‐SH and PEGDA were prepared in a conventional biosafety cabinet, and sterile HBSS was used to dissolve all compounds. After 15 min at room temperature (20–25 °C), the HA‐XL hydrogel was loaded in a syringe (1 mL Luer‐lock plastic syringe; BD Bioscience, San Jose, CA) following aseptic technique, and the microneedle was attached.

UV‐photocrosslinkable PEGDA (Jenkem Technology) of molecular weight 3500 Da were mixed with photoinitiator Irgacure 2959, HHEMP (Sigma‐Aldrich) and diluted with 1xPBS to form a prepolymer solution comprising of the photoinitiator. The patterned PDMS stamp was placed on an evenly distributed film of prepolymer solution on a TMSPMA (Sigma‐Aldrich)‐treated cover slip, with two coverslips placed on both sides as spacers. Photopolymerization was achieved by irradiating the set‐up with UV light of 320‐500 nm and at an intensity of 4.96 W/cm² for 30 seconds using the OmniCure^®Series 2000 curing station (Lumen Dynamics) as previously optimized. After photopolymerization, the PDMS stamp was peeled from the fabricated hydrogel microwell arrays, which were submerged in 70% ethanol for 2 hours to remove excess prepolymer solution. Hydrogel microwell arrays were subsequently washed thrice with PBS and stored in sterile PBS under aseptic conditions prior to cell seeding.

UV-photocrosslinkable GelMA synthesized earlier or PEGDA (Jenkem Technology, USA) of molecular weight 3500 Da was mixed with photoinitiator Irgacure 2959, HHEMP (Sigma-Aldrich, USA) and diluted with 1× PBS to form a prepolymer solution containing the photoinitiator. The patterned PDMS stamp was placed on an evenly distributed film of prepolymer solution on a TMSPMA (Sigma-Aldrich, New York, NY, USA)-treated cover slip, with 2 coverslips set on both sides as spacers. Photopolymerization was attained by irradiating the set-up with UV light of 320–500 nm and at an intensity of 4.96 W/cm² for 30 s using the OmniCure®Series 2000 curing station (Lumen Dynamics, Canada) as previously optimized. After photopolymerization, the PDMS stamp was removed from the fabricated hydrogel microwell arrays, which were submerged in 70% ethanol for 2 h to remove excess prepolymer solution. Hydrogel microwell arrays were then washed thrice with PBS and stored in sterile PBS under aseptic conditions prior to cell seeding.