NanoTech), glutaraldehyde grade II (Glu, 25% in H2O), HS-PEG5K-NH2 (HCl salt), urease (from Canavalia ensiformis, Type IX, powder, 50 000–100 000 units g/g solid),
urease activity kit, urea (99.9%), and urease (from Canavalia ensiformis, Type IX, powder, 50 000–100
000 units/g solid) were obtained from Sigma-Aldrich. Red fluorescent
amine-modified beads 0.2 μm (2% solids), PBS, Pierce BCA protein
assay kit, and Dulbecco’s modified Eagle’s medium (DMEM)
were purchased from ThermoFischer Scientific. Standard gray V4 resin
(FLGPGR04) for the 3D printing of rigid molds was purchased at FormLabs.