Previous Research Highlights

I. Deformation and assembly of stimuli-responsive copolymer networks

• Stimuli-responsive assembly and deformation of shape-programmed soft hydrogel particles at fluid interfaces

The ability to program inter-particle interactions with well defined selectivity, directionality, strength, and range remains a central challenge in efforts to realize complex materials by self-assembly of simple building blocks. Two-dimensional assembly of objects adsorbed at fluid interfaces is of particular interest in this regard, as surface-tension gives rise to long-range interactions that can dominate behavior across a wide range of scales, from mm to sub-μm. In this study, we demonstrate the capillary assembly of stimulus-responsive hydrogel particles with programmed multipolar interactions defined by their prescribed three-dimensional shapes. Low-energy bending deformations of the soft particles, driven by surface tension, modifies the interactions between particles, while their temperature-dependent swelling enables switchable assembly.

• Programming reversibly self-folding origami with micropatterned photo-crosslinkable polymer trilayer

Developments in origami mathematics over the past few decades have enabled the systematic design of folded structures with arbitrary complexity, extending the capabilities of the form well beyond the diversity of shapes achieved with traditional paper art, and highlighting its power as a tool for the fabrication of 3D objects from 2D flat sheets. We demonstrate a simple approach to fabricate reversibly self-folding origami based on trilayer films of photo-crosslinkable copolymers. A thermally responsive hydrogel layer is sandwiched by patterned thin rigid polymer layers, such that the stresses developed during swelling drive bending of micrometer-scale hinges. The placement of open stripes of defined width in either of the rigid layers drives bending to an angle that can be controlled with excellent fidelity, and with control of mountain and valley assignments. We show that this method allows for the preparation of reversibly self-folding origami with substantially greater complexity of achievable fold patterns, and a reduction by two orders of magnitude in length scale compared with existing approaches, thus providing a powerful platform for the design of small-scale actuating and reconfigurable structures.

• Edge-defined metric buckling of temperature responsive hydrogel ribbons

Nominally homogeneous layers are always surrounded by a more highly swelling border with a lateral size scale similar to the film thickness due to the finite resolution of the photo-lithographic patterning method. We consider the importance of edge induced ‘metric imperfections’ in driving the buckling of narrow, photo-crosslinkable hydrogel strips and rings. This provides a very simple, single mask lithographic route to prepare temperature-responsive coiled strips, over-curved rings, and structures that tighten themselves around other object due to changes in temperature, which we dub ‘self-cinching unknots’.

• Elastocapillary bending of thin hydrogel sheets

Bending of thin elastic sheets by liquid/liquid or liquid/air interfaces is a simple and broadly applicable technique for measuring their elastic modulus. The balance between bending and surface energies allows for the characterization of a wide range of materials with moduli ranging from kPa to GPa in both vapor and liquid environments. Compared to existing approaches, this method is especially useful for characterizing soft materials (<MPa in modulus), and samples immersed in a variety of liquid media. Using this method, we can characterize photo-cross-linkable polyelectrolyte hydrogel sheets swelled to equilibrium in an aqueous medium and demonstrate good agreement with the predicted scaling of the modulus and swelling ratio with cross-link density.

II. Assembly of amphiphilic block copolymers at fluid interfaces

• Multi-compartment emulsions and capsules

Multiple emulsions are valuable for the formation of multi-compartmental structures. A new, single-step process is introduced for preparation of water-in-oil-in-water double emulsions by a previously unexplained process of self-emulsification. The origin of this process is the osmotic stress resulting from the presence of salt impurities within the amphiphilic block copolymers used for emulsion stabilization. Further, osmotically driven emulsification enables us to tailor the structures of multiple emulsions, which upon solvent evaporation can yield multi-compartmental capsules or hierarchically structured porous films.

• Nanoparticle-loaded multifunctional micelles

Hybrid spherical and wormlike amphiphilic block copolymer micelles are formed through evaporation-induced hydrodynamic instabilities of emulsion droplets, allowing the spontaneous encapsulation of pre-synthesized hydrophobic inorganic nanoparticles within the micelle cores, as well as co-encapsulation of different nanoparticles. This encapsulation behavior is largely insensitive to particle surface chemistry, shape, and size, thus providing a versatile route to fabricate multifunctional micelles.

III. Assembly of dissimilar soft materials for stretchable devices and sensors

• 3D printing hydrogel-elastomer system for transparent and stretchable ionic devices

An ionically conductive hydrogel and a dielectric elastomer system is adapted for extrusion-based 3D printing for integrated device fabrication. A lithium-chloride-containing hydrogel printing ink is developed and printed onto a treated dielectric elastomer with no visible signs of delamination and geometrically scaling resistance under moderate uniaxial tension and fatigue. A variety of designs are demonstrated, including a resistive strain gauge and an ionic cable.

• Liquid metal-dielectric elastomer system for stretchable electronics

IV. Characterization of thermodynamics of soft active matter

• Microcalorimetric sensors for cellular bioenergetics

Calorimeters have great potential as powerful biosensors, but their relatively low sensitivity and difficulty in fluid handling present critical challenges. This project focuses on breaking through the current limitations to quantitatively measure cellular bioenergetics. We have developed a highly sensitive micromachined calorimetric sensor with pico-Watt resolution for the analysis of very small volume of samples in real time.