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We describe the use of light in a lithographic form of grayscale patterning as a means to program the properties and folding mechanics of flat, thin-film-polymeric materials. In this process, a finely dispersed (phase-separated) mixture of photoresist (SU-8 50) in polydimethylsiloxane (PDMS) is irradiated with ultraviolet light through a photomask. The subsequent photoresist cross-linking in the exposed regions causes changes in the material’s chemo-mechanical properties (notably making it stiffer and more resistant to solvent-induced swelling in the area of exposure). Light scattering due to the dispersed, non-index-matched photoresist domains leads to an intrinsic grayscale profile of the pattern width through the depth of the exposure field, a feature bringing significant and previously unexplored consequences for strain-induced folding dynamics. Solvent induced swelling, where PDMS absorbs a nonpolar solvent, is used to actuate folding mechanics with complex temporal and spatial profiles that explicitly follow the design rules established by the gradient cross-link density, features that elicit a programmable swelling (and therefore, folding) of the two-dimensional sheets. During investigation and optimization of the system, we observed an interesting temporal and biomimetic folding phenomenon that distinguishes the current results from other forms of strain induced folding reported in the literature. Under specific fabrication parameters, an evolution through an intermediate metastable state is observed, one in which the material will fold in one direction, then flatten and fold in the opposite direction. Mechanics modeling and finite element simulations have led to a detailed understanding of the system and the dynamics that allow a temporal evolution of 3-D structure through a double mode of folding. Insights into these mechanisms provide an advanced understanding of strain-induced folding in the field of soft materials.

We describe a composite hydrogel containing an embedding coupled chemistry for light-sensitized catalytic reactions that enables chemo-mechanical actuation of poly(acrylic acid)-based gels. In these materials, a photosensitizer and catalyst—ruthenium trisbipyridine and iridium dioxide nanoparticles, respectively—are incorporated into the hydrogel where together, with visible light irradiation, they undergo a catalytic water-oxidation reaction that lowers the pH and induces a dissipative/chemically-driven strain change in the gel. To demonstrate the capacity for 3D chemo-mechanical actuation, a layer of non-pH responsive poly(2-hydroxyethyl methacrylate) is added to the photo-active composite gel to create a model bimorph actuator. Triggering and terminating the water-oxidation reaction leads to a programmatic expansion and contraction of the active layer, which induces different modes of biomimetic curling motions in the bimorph actuator in light and dark environments. The efficiency of this system is fundamentally limited by the system-level design, which provides no capacity to sustain a local pH gradient against diffusive mixing. Even so, if the initial pH of the background solution is reestablished either actively or passively between each reaction cycle, it is possible to realize multiple cycles of reversible actuation. We describe a thermodynamic analysis of this system which identifies specific features mediating efficiency losses and conceptual requirements for mesoscopic design rules for optimization of this system and for advancing soft actuation systems in general.

Combining compliant electrode arrays in open-mesh constructs with hydrogels yields a class of soft actuator, capable of complex, programmable changes in shape. The results include materials strategies, integration approaches, and mechanical/thermal analysis of heater meshes embedded in thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) hydrogels with forms ranging from 2D sheets to 3D hemispherical shells.

A novel biosensor for hydrogen peroxide was prepared by immobilizing horseradish peroxidase (HPR) on newly synthesized dendritic silver/silicon dioxide nanocomposites, which were coated on a glassy carbon electrode. The modified electrode was characterized with XPS, SEM, and electrochemical methods. This biosensor showed a very fast amperometric response to hydrogen peroxide with a linear range from 0.7 to 120 μM, a limit of detection of 0.05 μM and a sensitivity of 1.02 mA mM−1 cm−2. The Michaelis-Menten constant of the immobilized HRP was estimated to be 0.21 mM, indicating a high affinity of the HRP to H2O2 without loss of enzymatic activity. The preparation of the proposed biosensor was convenient, and it showed high sensitivity and good stability.

We describe and characterize elementary designs for electrochemical micro- and macro-scale chemomechanical hydrogel actuators. The actuation of a pH-sensitive cross-linked polyacrylic acid (PAA) hydrogel is driven in the model devices through the oxygen reduction reaction (ORR) occurring at the electrodes of an embedded Au mesh micro-electrochemical array. Proton consumption by the ORR at the cathode of the embedded electrochemical cell leads to the formation of a localized pH gradient that in turn drives the strain response in the composite actuators. The dynamics result from the ionization of the carboxylic acid moieties of the PAA network in the high pH region, yielding an osmotic pressure that drives a volumetric expansion due to water imbibition. This system actuates both stably and reversibly; when the electrochemically-induced ORR is halted, the localized pH gradient dissipates due to diffusive mixing, which in turn relaxes the induced strains. Two approaches to the fabrication of hydrogel actuators were examined in this work. The first method adopted a design based on small flagella (∼0.2 mm × 1.5 mm × 60 μm, width × length × height) in which the actuating PAA structures are molded atop a set of fixed electrodes mounted on a supporting substrate. These hydrogel actuators show fast, large-amplitude, and largely reversible responses in the ORR mediated chemomechanical dynamics. We also investigated larger hydrogel actuators (∼4.5 mm × 11 mm × 1 mm, width × length × height), based on an autonomous design that embeds an open mesh stretchable micro-electrode array within the hydrogel. The significant and design-dependent impacts of mass transfer on the chemomechanical dynamics are evidenced in each case, a feature examined to elucidate more efficient mesoscopic design rules for actuators of this form.

The preparation, characterization and application of a three-layer magnetic nanoparticle composed of an Fe(3)O(4) magnetic core, a Prussian Blue (PB) interlayer and a gold shell (it can be abbreviated as Au-PB-Fe(3)O(4)) for an ultrasensitive and reproducible electrochemical immunosensing fabrication were described for the first time in this work. With the employment of the Au-PB-Fe(3)O(4) nanoparticle, a new signal amplification strategy was developed based on bienzyme (horseradish peroxidase and glucose oxidase) functionalized Au-PB-Fe(3)O(4) nanoparticles for an electrochemical immunosensing fabrication by using Carcinoembryonic antigen (CEA) and alpha-fetoprotein (AFP) as model systems, respectively. The experiment results show that the multilabeled Au-PB-Fe(3)O(4) nanoparticles exhibit satisfying redox electrochemical activity and high enzyme catalysis activity, which predetermine their utility in high sensitivity antibody detection schemes. Furthermore, this immunosensor could be regenerated by simply using an external magnetic field which ensured a reproducible immunosensor with high sensitivity.

This work describes a two-step conjugate synthesis of a porous organometallic nanostructured materials composed of ferrocenemonocarboxylic (Fc-COOH) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and then a reagentless amperometric immunosensor prepared with positively charged gold nanoparticles (PGN) immobilized in this nanostructure conductive film is developed. This nanostructured material containing ferrocenyl (PTC-Fc) could easily form stable film on the electrode surface with efficient redox-activity and excellent conductivity. Furthermore, with the negatively charged surface, this film can be used as an interface to adsorbing the PGN, which was prepared in organic solvents at relatively high concentrations with improved monodispersity compared to those prepared in aqueous solution. The presence of PGN provided a congenial microenvironment for adsorbed biomolecules and decreased the electron-transfer impedance. Thus, with carcinoembryonic antibody (anti-CEA) as a model antibody, the proposed immunosensor showed rapid and highly sensitive amperometric response to carcinoembryonic antigen (CEA) with acceptable preparation reproducibility and stability.