3D printed elastomers with Sylgard-184-like mechanical properties and tuneable degradability

The 3D printing of biodegradable elastomers with high mechanical strength is of great interest for personalized medicine, but rather challenging. In this study, we propose a dual-polymer resin formulation for digital light processing of biodegradable elastomers with tailorable mechanical properties comparable to those of Sylgard-184.

the synthesized poly(DLLA-co-CL)s were functionalized with methacryloyl chloride in THF with triethylamine as proton scavenger (Fig. S1). 1 Synthesis of P1: 2,2-diethyl-1,3-propanediol (0.344 g, 2.6 mmol), CL (25.22 g, 221 mmol) and DLLA (13.55 g, 94 mmol) were added to a Schlenk flask and placed under vacuum for ca. 1.5 h in order to remove water and oxygen. Then, the flask was purged with argon to create an inert atmosphere and inserted in an oil bath at 140 °C to let lactide melt. Thereafter, the flask was removed from the oil bath, three vacuum-argon cycles were performed and Sn(Oct) 2 S9) and polydispersity index based on size exclusion chromatography (SEC) was 1.37 (Table   S1). This copolymer was further dissolved in extra dry THF (250 mL) and after the addition of triethylamine (2.9 mL, 20.8 mmol), the mixture was purged with argon for 15 min.
Thereafter, methacryloyl chloride (2.0 mL, 20.8 mmol) in extra dry THF (ca. 4 mL) was added dropwise, while stirring, and the reaction was left at RT for 24 h. Afterwards, the salts were removed by centrifugation (4000 g, 4 °C, 30 min) and vitamin E (0.20 g) was added to the supernatant to prevent premature crosslinking. The polymer solution was then concentrated under vacuum and later precipitated in methanol. The resulted viscous polymer was dried under high vacuum for 5-7 days. Based on 1 H NMR spectroscopy, the conversion of hydroxyl end groups was ca. 54% (Fig. S9).
The synthesis then followed the same protocol as for unfunctionalized P1, with the only exception of adding higher amount of the catalyst -Sn(Oct) 2 (150 µL, 0.46 mmol) without toluene. Based on 1 H NMR spectroscopy, the conversion of DLLA and CL were 40 and ca.
100%, respectively (Fig. S10). This copolymer was then dissolved in extra dry THF (100 mL) and triethylamine (28.9 mL, 207 mmol) was added. The solution was further purged with argon for 15 min. Afterwards, methacryloyl chloride (20 mL, 207 mmol) in extra dry THF (ca. 20 mL) was added dropwise and the reaction was performed for 24 h at RT. The suspension was centrifuged (4000 g, 4 °C, 30 min) and vitamin E (0.12 g) was added to the supernatant. The polymer solution was then concentrated under vacuum and precipitated in hexane. After drying, the polymer was dissolved in DCM (200 mL), washed with saturated NaHCO 3 and NaCl aqueous solutions, and then dried with anhydrous MgSO 4 . The solution was again concentrated after the addition of vitamin E (0.10 g), precipitated in hexane for the second time, and then dried under high vacuum for 5-7 days. 1 H NMR spectroscopy ( Fig.   S10) could not be used for calculating the conversion of hydroxyl end groups, as after the precipitation in hexane, molecular weight distribution shifted to higher values, which was confirmed by MALDI-TOF (Fig. S11).

Polymer characterization
1 H NMR spectra were recorded in DMSO-d6 on Bruker AV400 spectrometer at 400 Hz. SEC was performed on a Viscotek TDAmax system with two Viscotek columns [D3000, poly(styrene-co-divinylbenzene)] and differential refractive index detector (TDA 302, Viscotek). The sample was dissolved in DMF, filtered using the 0.

Gel fraction and swelling ratio
Three cuboids (5 x 5 x 1 mm) were 3D printed with each of three tested resins (P1/P2 100/0, 85/15 and 70/30, w/w). They were placed separately in closed glass vials with 5 mL THF and incubated at room temperature. After 24 h, the samples were taken out and quickly blotted with a paper tissue and then left for drying under vacuum at RT to constant mass. The gel fraction (wt%) and swelling ratio (wt%) were calculated using Eqs. 1 and 2, respectively 2 (1) gel fraction= wt dry wt 0 x100 where wt 0 is the initial weight of a 3D printed cuboid after the printing, cleaning and curing, wt wet is the mass of a cuboid in a wet state after the quick blotting and wt dry is the mass of a completely dried object.

Mechanical characterization of 3D printed objects
Tensile tests were performed on 3D printed dog bone-shaped specimens using an AGS-X (Shimadzu) universal testing machine with 100-N load cell. The gauge length was set to 13 mm and a testing rate was 20 mm min −1 . Engineering stress (σ) and strain (ε) were calculated using Eqs. 3 and 4, respectively 3 where F is the force, A 0 is the initial cross-sectional area, L is the elongation, and L 0 is the initial gauge length. Young's modulus was calculated as a slope of the initial linear region (first 10 points) of tensile stress-strain curves.             where A is the peak integral at a specific chemical shift, and N is the number of equivalents of a monomer used in the synthesis.