CURRENT STUDIES

APPLICATIONS OF HYDROGELS AND ORGANOGELS

3D-PRINTED HYDROGEL-BASED COLORIMETRIC CHEMICAL SENSORS: A pH-indicating colorimetric tough hydrogel was synthesized for applications in smart wound dressings.[J73] pH-indicating molecules, such as phenol red, may be trapped in hydrogels to measure the pH value of the surrounding media. However, the indicating molecules leach out over time, limiting sensor lifetime, while the toxicity of these colorimetric molecules raises health and environmental concerns. The uniqueness of our invention is in modifying phenol red to incorporate a methacrylic group so that the pH-indicating molecules are co-polymerized within the backbone chain of polyacrylamide hydrogels. By co-polymerizing the double-network hydrogel of polyacrylamide and alginate with methacryloylphenol-red monomers, the colorimetric modulation range was optimized for open wound applications. The addition of Laponite® enabled further optimization of the rheological properties of the precursor solution, resulting in a 3D-printable colorimetric hydrogel pH sensor. We are developing the co-polymerized dye strategy further as a platform technology for colorimetric chemical sensors that detect various analytes.

ELECTROCHEMICAL ENERGY STORAGE: Alkaline electrolyte-infused hydrogels are used as gel electrolytes for zinc-air battery [J52,J53,J54,J70] and supercapacitor [J29] applications. Most notably, we have studied the efficacy of these hydrogels at low temperatures to understand the roles of liquid-phase electrolytes [J70] and polymer matrix[J39]. As a theoretical foundation for low-temperature operation, a thermodynamic model predicted ice content in additive-doped aqueous KOH electrolytes confined in porous electrodes.[J40] We also incorporated the colloidal solution of amorphous photonic crystals as a colorimetric overpotential indicator [J51]

WEARABLE E-SKIN SENSOR ARRAYS: Optically transparent and electrically conductive properties of the polyampholyte hydrogel (PAH) were utilized to fabricate an electronic skin (e-skin) array that enables simultaneous capacitive tactile sensing and colorimetric temperature sensing.[J46]

ANTIFOULING/DE-ICING SURFACES: We recently invented an organogel-based composite with an auxetic skeletal structure that was 3D printed and embedded in the gel. Experimental results support our hypothesis that the auxetic property will be effective in removing ice contaminants upon application of externally induced stretching/releasing cycles.[in review] We also invented a novel fabrication method to create the size-regulated open pore structure of a millimeters-thick Sylgard 184 film with a nonporous skin layer. Created by a humidity-assisted fabrication method, the porous film is infused with silicone oil to offer a transparent and heavy-duty anti-fouling surface. [J74]

Charge-balanced polyampholyte hydrogels (PAHs), where cross-linking originates from inter- and intra-chain ionic cross-linking between counter-charged functional groups, have unique advantages such as anti-polyelectrolyte effects, self-healing ability, and adhesion onto contacting surfaces. My group’s series of studies used a polyampholyte from sodium 4-vinylbenzenesulfonate (NaSS) and [3-(methacryloylamino)propyl]trimethylammonium chloride (MPTC). First, we studied the structure of the PAH in as-prepared state by using small-angle x-ray scattering (SAXS) [J39] and electron microscope techniques [J41]. The SAXS results indicated a networked globule structure in the charge-balanced polyampholyte hydrogels, whereas the size and clustering of the globules were dependent on synthesis parameters. Second, at low temperatures, an interconnected globular network structure of polymer-rich phase at low temperature (measured down to –54 °C) preserved ion-conducting channels of nonfrozen water molecules at low temperatures, wherein the mobility of water molecules was confirmed by solid-state NMR.[J39] Third, specific ion effects on mechanical properties and ionic conduction were studied. For anions, the trend of ionic interaction follows the Hofmeister series almost exactly, whereas some anomalies was observed among cations.[J45]

This was the first discovery that a unique networked globular nanostructure in quenched polyampholyte hydrogels can strongly suppress ice formation. The absolute molar ratio of ice crystals to amorphous water decreases notably compared to salt water in unconfined or in other polymeric hosts. In addition, the nanostructure of polymer chains disrupts the crystalline growth of ice, resulting in ‘slush-like’ ice formation. As a result, ions in the polymer exhibit relatively high conductivity at very low temperatures. This phenomenon envisions aqueous gel electrolytes for low-temperature energy storage and other anti-freezing applications of tough hydrogels.

Natural tissues possess a so-called J-shaped strain-stiffening behaviour, being soft and compliant at small strains followed by a rapid stiffening at higher strains to prevent tissue damage. In addition, they have significant anisotropy and local variations of mechanical properties that are difficult to reproduce with homogeneous materials. Inspired by many biological organisms, biomimetic and soft robotics research has been focusing on composites of soft components reinforced by structured stiff components, such as fabrics and 3D-printed structures.[J57] These studies provide structural motifs and theoretical/numerical models for the design and characterization of smart actuator gels and elastomers operating under various stimuli, for example, under varying magnetic field.[J61]

MOCK-AORTA MATERIAL DESIGN: The mechanical properties of 1 human and 14 porcine aortas were characterized under uniaxial tensile stress from five different regions of the aorta to quantify their nonlinear, anisotropic, and locally varying behaviour.[J50] These complex mechanical properties were then mimicked by embedding fabrics in a silicone elastomer matrix. The knitted structures of the fabric provide strain-stiffening as well as anisotropic mechanical properties of the resulting composite in a deterministic manner. By optimizing the combination of different elastomers and fabrics, the resulting composites matched the J-shaped and anisotropic stress−strain behaviour of aorta.[J50]

ELASTOMERIC TUBES WITH SELF-REGULATED DISTENSION: Compliant elastomer tubing with a fabric ‘jacket’ has been essential in various applications in soft robotics, including artificial muscles made of McKibben actuators. In our experiment using a custom hydraulic pressure testing rig and our 3D finite element model, we found that the tube’s distension is regulated by the stain-stiffening of the ‘jacket’, and the degree of distension with respect to the material design is quantitatively predicted by our simulation with remarkable precision. Interestingly, highly asymmetric distensions similar to aneurysms, common to elastomeric tubes due to imperfections in fabrication, are prevented by the self-regulation of the ‘jacket’, as confirmed both in static [J68] and dynamic [J71] operation.

CONSTITUTIONAL MODELLING: My group contributed to the collaborative development of novel constitutional models to improve precision in predicting hyperelastic deformation from the widely accepted Holzapfel-Gasser-Ogden (HGO) model by either introducing numerical components of Mooney-Rivlin and Gent models into the HGO model [J50] or by treating fibres as interlocked rods that bend nonlinearly when constructing energy density function.[J58]

Textile-based electronics, known as ‘e-textiles,’ is a platform electronics technology that improves patient comfort. My group invented 2-layer e-textile patches by controlled permeation of Ag-particle/fluoropolymer composite ink into a porous nonwoven textile substrate.[J43,J47] While the permeated composite ink forms a cladding onto the nanofibers in the textile substrate, which is beneficial for the mechanical and electrical properties of the e-textile, the printed e-textiles have conductivity values of 3,000–4,000 S/cm, whereas 4,000 cycles of 20% uniaxial stretching caused the value to degrade only by a factor of ~2.5. With our custom-developed electronic circuits, we demonstrated a surface electromyography (sEMG) and electroencephalography (EEG) system with wireless data emission to provide comparable signal quality to commercial gel electrodes. This project produced two granted patents that were licensed.[P10,P11]