A practical guide to hydrogel working curves for bioprinting

• Working curves are an essential measurement for printability and quality control. • The first reproducible and interpretable hydrogel working curves are presented. • Hydrogel working curves can exhibit Jacobs-like behavior. • Optical thickness measurements are preferred over mechanical methods. • Hydrogel critical exposure is well predicted by functional group concentration. The working curve is a widely implemented, but presently not standardized, method of assessing resin printability for photopolymer additive manufacturing technologies. While the working curve has been studied and refined for plastic resins, application to hydrogel materials used in bioprinting has been limited. Hydrogels present measurement challenges due to their decreased solids content, compliant nature, and significant swelling. Here, adapting lessons learned from interlaboratory studies on plastic working curves, we assess various techniques for hydrogel working curve measurements. Notably, across several formulations with various molecular weights and solids content, hydrogels exhibit near ideal log-linear behavior consistent with the Jacobs model when measured appropriately. However, certain measurement modalities (such as contact-based and rheological) can indicate Jacobs-like behavior, but with systematic errors in the cure depth compared to non-contact optical methods. Overall, this work highlights the challenges when conducting hydrogel working curve measurements and provides several considerations to help further develop and standardize measurements across 3D bioprinting applications.