Zirconium propoxide

The photoresist employed for 2PP was the SZ2080,²⁴ (link) a negative organic–inorganic biocompatible resin, extensively validated for cell culture. It is made of two components: methacryloxypropil trimethoxysylane (MAPTMS, 97%, Sigma-Aldrich) and zirconium propoxide (ZPO, 70% in propanol, Sigma-Aldrich). ZPO enhances the material's mechanical stability. 1% wt. Irgacure 369 (IRG, Sigma-Aldrich) is added as a photo-initiator. The mass density as measured on a bulk polymerized specimen⁸² (link) is 1200 kg/m³. The molar concentration of Irg369 is about 33  $\mu \mathrm{M}$ .
About 35 μl of SZ2080 photoresist were deposited by drop casting on a 12 mm diameter circular glass coverslip (#1.5, Bio-Optica, Italy). The operation was always performed leaving a free external annulus on the glass substrate. This annulus has the role of assuring the correct holding inside the support. The solvent was removed through an evaporation phase (i.e., baking procedure) which occurred under chemical hood for at least 48 h at room temperature. Then, the resist reached a sol-gel state and allowed creating the initial chemical bonds between monomers and oligomers of the photoresist, preparing a starting substrate for the following laser-induced cross-linking.

Yttrium nitrate hexahydrate (99%) and acetylacetone (AcAc, 99.5%) were purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA); zirconium propoxide (ZIP, 70% sol. in 1-propanol), lithium methoxide (LiOMe, 2.2M sol. in methanol), and tetramethyl orthosilicate (TMOS 99%) were purchased from Acros Organics (Acros Organics™, Thermo Fisher Scientific, Waltham, MA, USA). All reagents were used as received. Methanol, ethanol, and isopropanol were kept on 3A molecular sieves for 48 h and further distilled under dry nitrogen.