Emergent Soft Lithographic Tools for the Fabrication of Functional Polymeric Microstructures

Polymeric microstructures (PMs) are useful to a broad range of technologies applicable to, for example, sensing, energy storage, and soft robotics. Due to the diverse application space of PMs, many techniques (e. g., photolithography, 3D printing, micromilling, etc.) have been developed to fabricate these structures. Stemming from their generality and unique capabilities, the tools encompassed by soft lithography (e. g., replica molding, microcontact printing, etc.), which use soft elastomeric materials as masters in the fabrication of PMs, are particularly relevant. By taking advantage of the characteristics of elastomeric masters, particularly their mechanical and chemical properties, soft lithography has enabled the use of non-planar substrates and relatively inexpensive equipment in the generation of many types of PMs, redefining existing communities and creating new ones. Traditionally, these elastomeric masters have been produced from relief patterns fabricated using photolithography; however, recent efforts have led to the emergence of new methods that make use of masters that are self-forming, dynamic in their geometric and chemical properties, 3D in architecture, and/or sacrificial (i. e., easily removed/released using phase changes). These "next generation" soft lithographic masters include self-assembled liquid droplets, microscale balloons, templates derived from natural materials, and hierarchically microstructured surfaces. The new methods of fabrication supported by these unique masters enable access to numerous varieties of PMs (e. g., those with hierarchical microstructures, overhanging features, and 3D architectures) that would not be possible following established methods of soft lithography. This review explores these emergent soft lithographic methods, addressing their operational principles and the application space they can impact.