Instant bonding of hydrogels to soft and hard materials

Hydrogels are fascinating soft, stretchy materials that contain a high amount of water, up to 98% , making them not only permeable for liquids and small molecules, but also ionically conductive. Having their origin in nature, many hydrogels are biocompatible or even degradable, rendering them a powerful tool in biomedicine for tissue engineering, drug delivery and wound care, and also in industrial applications ranging from degradable packaging to anti-fouling coatings and from soil fertilizers to water purification. Hampered in the beginning by their soft and brittle structure, a glance to biological hydrogels like cartilage revealed what potential slumbers in those squishy water-rich materials, what toughness can be achieved by their networks. Evolving novel so-called tough hydrogels, exceeding the strength of cartilage while still showing remarkable stretchability, advanced not only regenerative medicine, but also excited engineers in diverse disciplines from soft robotics, stretchable electronics, microfluidics and sensor technologies to optics. The fusion of hydrogels with other material classes is necessary for devices that show high functionality as well as elevated complexity. Integration of hydrogels into sophisticated hybrids is challenged by the weak and brittle bonds between hydrogels and antagonistic materials. Bonding of hydrogels became a key issue for reliable, robust hydrogel systems that demand intimate contact and powerful anchoring with strong bonds whilst having negligible impact on the mechanical, electrical as well as optical properties of the components. Some existing bonding methods partially fulfill the desired requirements, but they all rely on complex surface pre-treatment and are only employable to a limited range of materials. In addition, these methods are only applicable to pre-gel solutions as the bond is formed during fabrication of the hydrogel. Taking bonding of hydrogels one step forward, I developed a frugal instant bonding method applicable to diverse fully polymerized hydrogels. Using a cyanoacrylate-based dispersion we achieved bonding within seconds to materials spanning eight orders of magnitude in Youngs moduli. The novel approach yields thin adhesive layers that are transparent, stretchable and strong, exceeding in any case the intrinsic bulk rupture strength of the hydrogels. Our frugal, diverse and instant method allowed us to demonstrate an entirely soft hydrogel lens with 110% tunable focal length that is based on the principle of a dielectric elastomer actuator (DEA). Working in reverse as dielectric elastomer generator (DEG), we designed an energy harvester with hydrogel-electrodes yielding 11% total conversion efficiency. We advanced the field of stretchable electronics by using our bonding method for enhanced stretchable zinc (Zn)/manganese dioxide (MnO2) batteries. Implementation of an electrolytic hydrogel separator yields a ten times lower internal resistance and drastically boosts battery performance. Whit this, the hydrogel-based battery is able to power an autonomous stretchable circuit adhered on top of the battery. Bonding of imperceptible foils with a patterned metal circuit to a pre-stretched hydrogel enables a 20% stretchable electronic hydrogel skin. The stretchable hydrogel circuit with integrated heater and sensor array in combination with a new approach for heat-triggered drug delivery provides routes for smart wound care patches. The novel instant bonding approach as well as the wide-ranging applications corroborate the strength of the developed method and led to the Science Advances publication Instant tough bonding of hydrogels for soft machines and electronics. During my PhD, I employed and developed other highly stretchable electrodes to show the applicability of soft matter in diverse research fields. An electrostatic energy converter (EEC) is presented in the Extreme Mechanics Letters publication Electrostatic converter with an electret-like elastomer membrane for large scale energy harvesting of low density energy sources. The generator converts low mechanical input energy into electrical output energy with a total conversion efficiency of 63% , without any bias voltage. Additionally, I contributed to the development of carbon fiber-based elastomeric sensor layers. Implementation of such soft sensors into an Epidural Needle Insertion Simulator designed to train medical students enhanced the device by giving feedback on the success of the operational skill without hampering the tissue-like mechanical haptics of the soft simulator. The improved training equipment was so far not only evaluated by anesthesiologist, but also presented as A hybrid, low-cost tissue like epidural needle insertion simulator at the prestigious 2017 IEEE EMBC meeting in South Korea.submitted by DI Daniela Wirthl BScUniversität Linz, Dissertation, 2018OeBB(VLID)258191