Pegda

The photopolymer used for the majority of experiments was 0.5% v/v photoinitiator 2-hydroxy-2methyl-1-phenyl-1-propanone (Darocur 1173, Sigma Aldrich) in poly(ethylene glycol) (400) diacrylate (PEG-DA, Sigma Aldrich) in 80% aqueous solution. NGM plates were prepared using standard methods outlined in Brenner et al [13 (link)]. The chemical structure for PEG-DA is shown in S2 Fig. Other photoinitiators used include Phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide (Sigma Aldrich) and a mixture of 0.01 mM Eosin Y (Sigma Aldrich) and 0.1% TEA (Sigma Aldrich) (See Table B in S1 File) [14 ].

The PEGDA (455008, Sigma Aldrich, Saint Louis, MO, USA) hydrogel was performed by preparing two types of solution. The first was an Eosin Y solution was composed of Eosin Y disodium salt (E4382, Sigma Aldrich) 0.069% (w/v) and MiliQ water. The second solution, called the buffer solution, contained 100 mM NaCl, 10 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), 1.5% (v/v) triethanolamine (TEA) (90279, Sigma Aldrich), and MiliQ water. After mixing 5% (w/v) PEGDA with buffer solution, added as a cross-linker 3.5 µL N-vinyl-2-pyrrolidinone (V3409, Sigma Aldrich) and then mixed by putting 10 µL Eosin Y solution. For the gelation process, PEGDA hydrogel mixture was photo-polymerized by exposing it to visible white light at room temperature for 10 min.

Inert fluid streams (sheath fluid) consisted of PEG-DA, M_n=250 (Sigma-Aldrich), whereas photosensitive fluid streams (core fluid) consisted of PEG-DA with 5 wt% DMPA (Sigma-Aldrich). Fifteen micrometre fluorescent polystyrene tracers (Thermo Fisher Scientific) were added to visualize fluid flow. A programmable pressure controller was used to control and pump the fluid flow.

Poly(vinyl alcohol) (PVA) hydrogels were prepared by adding 30 µL of 25% glutaraldehyde solution (Sigma) and 10 µL of 37% hydrochloric acid (HCl) solution per 1 mL of aqueous 10 w/w% PVA (Mw 130,000 with 99+% hydrolyzed; Sigma) solution. Alginate hydrogels were prepared by adding 2 w/w% sodium alginate, 5 mM adipic acid dihydrazide (Sigma), 5 mM EDC, and 5 mM Sulfo-NHS in MES buffer. Poly(ethylene glycol) diacrylate-alginate (PEGDA-Alg) hydrogels were prepared by adding 20 w/w% PEGDA (Mw 20,000; Sigma), 2 w/w% sodium alginate, 0.2 w/w% of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone, and 50 mM calcium sulfate in deionized water. PVA and alginate hydrogels were crosslinked in room temperature for 6 h. PEGDA-Alg hydrogels were crosslinked by incubating in a humid UV chamber (CL-1000, UVP) filled with nitrogen for 30 min. All hydrogels were fully swollen in DPBS before mechanical characterizations.

Gallium–Indium eutectic (EGaIn, Ga 75.5% and In 24.5%, ≥99.99%), tert-butyl acrylate (TBAm, 98%), 2-hydroxyethyl acrylate (HEAm, 96%), N-hydroxyethyl acrylamide (HEAAm, 98%), N, N-dimethyl acrylamide (DMAm, 99%), poly(ethylene glycol) diacrylate (PEGDA, M_n = 250, PEGDA, >92%), and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide (TPO, >97%) were purchased from Sigma-Aldrich. All other reagents were used as received unless otherwise specified. Human embryonic kidney cells (HEK-293) and mice macrophage cells (Raw 264.7) were purchased from ATCC (USA).

Gelatine hydrogel (G gel) sheets were prepared in a glass mould. Glutaraldehyde (3% v/w) was added to the solution of gelatine (6% w/w). The solution was placed in the glass mould and frozen in a Julabo cooling chamber at −12 °C for 20 h. It was then defrosted and washed with an excess of water. 2-hydroxyethyl methacrylate gels were prepared by dissolving 2-hydroxyethyl methacrylate (HEMA, Acros Organic, 98%) and poly(ethylene glycol) diacrylate (PEGDA, Aldrich, Mn ~258) in water (6 w/v% solution, HEMA:PEGDA molar ratio 8:1). The reaction mixture was degassed at low pressure for 25 minutes to eliminate dissolved oxygen before gelation. In the conventional method the mixture was cooled to 0 °C for 15 min and then N,N,N′,N′-tetramethyl-ethylenediamine (TEMED, Fisher Scientific, 99%), and ammonium persulfate (APS, 98%) were added and the mixture allowed to freeze completely. In the pre-freezing method the mixture was cooled with constant mixing in an ethanol cooling bath at −18 °C. After ice crystals formed, the mixture was pre-cooled to −2 °C with constant mixing. N,N,N′,N′-tetramethyl-ethylenediamine (TEMED, Fisher Scientific, 99%), and ammonium persulfate (APS, 98%), were added and the mixture allowed to freeze completely. The frozen mixture was kept at −18 °C for 20 hours and then defrosted at room temperature.