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Hydrogels with embedded functional particulates are widely used to create soft materials with innovative functionalities. In order to advance these soft materials to functional devices and machines, critical technical challenges are the precise positioning of particulates within the hydrogels and the construction of the hydrogels into a complex geometry. Inkjet printing is a promising method for addressing these challenges and ultimately achieving hydrogels with voxelized functionalities, so-called digital hydrogels. However, the development of the inkjet printing process primarily relies on empirical optimization of its printing and curing protocol. In this study, a general scaling law is proposed to predict the transport of particulates within the hydrogel during inkjet printing. This scaling law is based on a hypothesis that water-matrix interaction during the curing of inkjet-printed particle-laden polymeric drops determines the intra-drop particle distribution. Based on the hypothesis, a dimensionless similarity parameter of the water-matrix interaction is proposed, determined by the hydrogel's water evaporation coefficient, particle size, and mechanical properties. The hypothesis was tested by correlating the intra-drop particle distribution to the similarity parameter. The results confirmed the scaling law capable of guiding ink formulation and printing and curing protocol.

This paper reports a novel hot embossing technique using rapid heating and uniform pressure for replication of microstructures on polymeric substrates. The rapid heating is made possible by a graphene–polymer composite heater that makes use of the exceptional thermal and electrical properties of graphene. The uniform embossing pressure is achieved by using gas as the pressing medium. A heating rate of 14 °C s−1 can be achieved by applying a voltage of 44 V to a heating area of 30 mm × 30 mm. A facility that integrates graphene-based heating and gas-assisted embossing is designed and implemented. Three microstructures, namely, microlens array, V-groove, and microcylinder array, are successfully replicated on three polymeric substrates. The accuracy and uniformity of replication over the area of 50 mm × 50 mm is also confirmed. Furthermore, the thermal cycle time is reduced to less than 30 s. This study proves the great potential of using graphene–polymer composite heaters in gas-assisted micro hot embossing for replication of microstructures for various applications.

Hydrogel-based soft materials have been used in numerous applications in healthcare, food, pharmaceutical, and cosmetic industries. Manufacturing hydrogels whose functional properties and compositions are voxelized at superior spatial resolutions can significantly improve current applications as well as will enable a new generation of soft materials. However, it remains challenging to control the structure and composition of soft materials reliably. In this context, the drop-on-demand (DOD) printing of hydrogels shows excellent potential to address this manufacturing challenge. Despite this potential, a lack of mechanistic understanding of the behavior of printed hydrogel drops makes it challenging to design and optimize DOD printing protocols for a wide variety of hydrogels. In particular, the curing of hydrogel drops, which requires dehydration of printed hydrogel drops, is poorly understood. In this study, thus, a hypothesis was postulated and tested that water-matrix interaction at drop-drop interfaces during curing processes determine the quality of hydrogels printed. Both computational and experimental studies were performed to establish a mechanism of the water-matrix interaction within printed hydrogel drops. The results were further discussed to establish a dimensionless similarity parameter that can characterize water transports during the hydrogel dehydration process.