Process optimization and optimal tolerancing to improve dimensional accuracy of vat-photopolymerized functionally graded hydrogels

Functionally graded hydrogels with variation in material composition are useful for a variety of applications in domains like biomedical engineering and soft robotics. Recent developments in 3D printing have enabled the production of such graded hydrogels. However, the variation in the composition of these hydrogels layer-wise can be challenging in terms of selecting optimal 3D printing process parameters and maintaining geometrical accuracy. Unless corrective measures are taken, the variations in material composition can cause a loss in print fidelity and lead to dimensional deviations in the printed samples. In-depth studies have not been reported to address these challenges in the 3D printing of functionally graded hydrogels, and guidelines are not available for correcting the geometric deviations so produced. In this work, we study the deviations in lateral dimensions in functionally graded polyethylene glycol diacrylate (PEGDA) hydrogels produced using digital light processing (DLP) 3D printer and present a method to correct these deviations. The methodology involves specification of optimal printer parameters for each material composition and giving appropriate tolerances on the CAD model. The lateral dimensions of the printed part depend on PEGDA concentration, exposure time, layer thickness and print time. For obtaining accurate samples after printing and swelling, the printer parameters must be changed whenever a change in material composition occurs. Further, the printer parameters have to be chosen such that swelling induced cracking is reduced while maintaining good print fidelity. The tolerance on the CAD model has to consider the lateral deviation that occurs at the edges due to printing and the bulk increase that occurs due to swelling. We demonstrate this method by printing functionally graded PEGDA samples with good print fidelity and dimensional accuracy.