Ultrathin thermoresponsive self-folding 3D graphene

Graphene and other two-dimensional materials have unique physical and chemical properties of broad relevance. It has been suggested that the transformation of these atomically planar materials to three-dimensional (3D) geometries by bending, wrinkling, or folding could significantly alter their properties and lead to novel structures and devices with compact form factors, but strategies to enable this shape change remain limited. We report a benign thermally responsive method to fold and unfold monolayer graphene into predesigned, ordered 3D structures. The methodology involves the surface functionalization of monolayer graphene using ultrathin noncovalently bonded mussel-inspired polydopamine and thermoresponsive poly(N-isopropylacrylamide) brushes. The functionalized graphene is micropatterned and self-folds into ordered 3D structures with reversible deformation under a full control by temperature. The structures are characterized using spectroscopy and microscopy, and self-folding is rationalized using a multiscale molecular dynamics model. Our work demonstrates the potential to design and fabricate ordered 3D graphene structures with predictable shape and dynamics. We highlight applicability by encapsulating live cells and creating nonlinear resistor and creased transistor devices.United States. Office of Naval Research. Multidisciplinary University Research Initiative (FA9550-16-1-0031)United States. Office of Naval Research. Multidisciplinary University Research Initiative ( FA9550-15-1-0514)National Science Foundation (U.S.) (CMMI-1635443)United States. Office of Naval Research (N00014-16-1-2333

Photo- and Nanoimprint Lithographically Patterned Self-Folding Devices and Their Applications

The ability to manufacture complex functional three-dimensional (3D) structures in small scale enables numerous applications in fields including micro- and nano electronics, photonics, tissue engineering, and drug delivery. However, due to inherent two-dimensionality of state-of-the-art micro/nanofabrication techniques, it is extremely difficult to construct 3D integrated devices in a highly parallel and cost-effective manner. To overcome this limitation, a number of 3D manufacturing technologies have been developed including molding, anisotropic etching, focused ion beam lithography, and stereolithography. These technologies have greatly advanced small scale manufacturing but have limited resolution, material versatility and are usually not cost- and time- effective. Thus, there is still a need to improve and find new approaches especially to manufacture and enable large scale integration of small scale 3D structures. The body of this dissertation is focused on developing high-volume and high-throughput methodologies to create 3D structures with material versatility, precise surface patterns, and applications in tissue engineering. Specifically, my work is focused on the development of self-folding structures made with various materials in micro- and nanoscale using lithographic approaches such as photolithography and nanoimprint lithography. Self-folding 3D hydrogel structures were fabricated using photolithography. Depending on the polymers used, the structures could reversibly fold and unfold or stay in the folded state. I have studied reversible actuation of and chemical release profiles from stimuli-responsive structures for applications in drug delivery. I have investigated approaches to fabricate and modify properties of non-cell adherent hydrogel structures to enable long-term cell culture. Here, various 3D structures inspired by human tissue are also demonstrated. In the second half of my dissertation, I demonstrated approaches to pattern nanostructures using home-built nanoimprint system, small weight, and simple thumb pressure. Here, fabrication approaches by electron beam lithography and nanoimprint lithography are compared. By combining photolithography and nanoimprint lithography, parallel patterns that have both micro- and nanoscale dimensions could be fabricated. I have also investigated methods to fabricate polymeric self-folding nanopatterned microstructures. Further, potential application of these nanopatterned, curved structures in tissue engineering is also discussed. Overall, this work provides highly parallel, high-throughput and versatile lithographic methodologies to fabricate small scale self-folding 3D structures with applications in tissue engineering. Other applications in electronics, robotics, optics, nano and biomedical engineering are also anticipated

Water Adsorption Isotherms on CH₃‑, OH‑, and COOH-Terminated Organic Surfaces at Ambient Conditions Measured with PM-RAIRS

The water adsorption isotherms on methyl (CH₃)-, hydroxyl (OH)-, and carboxylic acid (COOH)-terminated alkylthiol self-assembled monolayers (SAMs) on Au were studied at room temperature and ambient pressure with polarization modulation reflection–absorption infrared spectroscopy (PM-RAIRS). PM-RAIRS analysis showed that water does not adsorb at all on the CH₃–SAM/Au at subsaturation humidity conditions. In a dry Ar environment, the OH-SAM/Au holds at least 2 layer thick strongly bound water molecules which exhibit a broad O–H stretch vibration peak centered at ∼3360 cm^–1. The peak position implies that the strongly bound water layer on the OH SAM is more like a liquid than an ice. The additional uptake of water in humid environments is relatively weak, and the peak position changes very little. Unlike the OH-SAM/Au, the COOH-SAM/Au does not have strongly bound water layer. This seems to be due to the strong hydrogen bonding between terminal COOH groups in dry conditions. The weak interactions between water and carboxyl groups at low relative humidity (RH) and the solvation of dissociated carboxylic groups in high RH lead to a type III isotherm behavior, based on the BET categories, for water adsorption on the COOH-SAM/Au. The water spectra on the COOH-SAM at RH > 45% are centered at ∼3430 cm^–1 and very broad, indicating that the hydrogen-bonding network of water on the COOH-SAM is much different from that on the OH-SAM

Self-Folding Thermo-Magnetically Responsive Soft Microgrippers

Hydrogels such as poly­(<i>N</i>-isopropylacrylamide-<i>co</i>-acrylic acid) (pNIPAM-AAc) can be photopatterned to create a wide range of actuatable and self-folding microstructures. Mechanical motion is derived from the large and reversible swelling response of this cross-linked hydrogel in varying thermal or pH environments. This action is facilitated by their network structure and capacity for large strain. However, due to the low modulus of such hydrogels, they have limited gripping ability of relevance to surgical excision or robotic tasks such as pick-and-place. Using experiments and modeling, we design, fabricate, and characterize photopatterned, self-folding functional microgrippers that combine a swellable, photo-cross-linked pNIPAM-AAc soft-hydrogel with a nonswellable and stiff segmented polymer (polypropylene fumarate, PPF). We also show that we can embed iron oxide (Fe₂O₃) nanoparticles into the porous hydrogel layer, allowing the microgrippers to be responsive and remotely guided using magnetic fields. Using finite element models, we investigate the influence of the thickness and the modulus of both the hydrogel and stiff polymer layers on the self-folding characteristics of the microgrippers. Finally, we illustrate operation and functionality of these polymeric microgrippers for soft robotic and surgical applications