Impact of intermediate UV curing and yield stress of 3D printed poly(ethylene glycol) diacrylate hydrogels on interlayer connectivity and maximum build height

Extrusion-based 3D printing of photo-curable hydrogel materials can be used for the generation of complex objects layer by layer without the need for molds. Photo-curing often is the final step of the 3D printing process, fixing the shape of the generated object. However, the fabricated objects have to support themselves before curing, limiting the size of the objects. In this contribution, intermediate curing after completing each individual layer with poly(ethylene glycol) diacrylate as a radically curing hydrogel system was investigated compared with single curing of the whole structure after complete layered deposition, and its effect on the mechanical properties and achievable object size was assessed. Defect-free hydrogel samples for mechanical testing were obtained with an optimized washing/swelling protocol. It was found that hydrogel objects cured after completion without intermediate curing steps had the highest fracture stresses and compression at break with 32.5 N cm−2 and 44%, respectively. With increasing intermediate curing time, both the fracture stress and the compression at break decreased down to 7.8 N cm−2 and 26%, respectively, for 5 s intermediate curing. Long intermediate curing times between the layers lead to preferred crack formation parallel to the layers due to decreased chemical bonding. However, the formation of higher hydrogel objects than enabled by the yield stress of the hydrogel was only possible with intermediate curing due to the better self-support of partially cured objects.