Electrochemical Osmotic Hydrogel Actuators—A Fundamentally New Soft Material with Many Possibilites

Intelligent systems combine sensing, actuation, and computation to achieve complex tasks and functions. Soft electrically controlled multifunctional materials, especially hydrogels, are the most promising materials for such systems as they are as adaptable as biological systems yet compatible with advanced systems through electronics. There are however no reports on such soft multifunctional materials and not even hydrogel acuators with direct electric control.<br/><br/>We show an electroactive hydrogels fabricated from two components: a hydrophilic, ionically charged nanofibril (cellulose nanofibrils from trees), and carbon nanotubes. These nanoparticle composite networks are mechanically robust, yet they have an open mesoporous structure that can holds lots of water and be highly permeable to substances in their surroundings. The anisotropy of the network allows high expansion in one direction while maintaining high strength and electric conductivity in the other. These hydrogels are fundamentally different from polymer hydrogels, and their behavior is not described by classical polymer physics: e.g., polymer gels have entropy elastic deforamtion, while nanowire hydrogels deform plastically via a stick-slip-stick behavior. As a result, they show emergent properties not present in any previously known soft material. In particular, they possess two unique properties which are immediately suitable in the area of soft intelligent systems:<br/><br/>The electrochemical charge/discharge of the carbon nanotubes in the hydrogels controls the internal salt concentration and consequently their osmotic swelling. This allows direct electrically controlled actuation where around 700 water molecules expand/contract the structure for each ion/electron pair inserted/de-inserted at only ±1 volt, resulting in up to 300% electroosmotic expansion. This mode of electronic actuation has not been shown before, and is a fundamental advancement for soft materials.<br/><br/>The coupling between expansion and electric resistance allows self-sensing and possible feedback to control and lock a particular expanded state. The relation between expansion and pore size further allows for electrically tunable membranes with a mesoporosity relevant for permeability to larger molecules. These tunable membranes are the first example of electrically controlled mesoporosity suitable for larger molecules such as drugs or proteins. No such active membrane has been presented before, and the best current tunable membranes are dense graphene or conducting polymer membranes for controlling the permeability of gases or water.<br/><br/>Our hydrogels are a new form of soft material and groundbreaking as they monolithically integrate actuation with other functions, not easily achieved with other material systems. They can further be made in any lab using available cheap bulk materials, and shaped into various forms ranging from sheets to fibers, to advanced 3D printed sturctures.