Rise of the aquabots
A research collaboration led by Professor Anderson Shum Ho Cheung (Croucher Senior Research Fellowship 2020) from the University of Hong Kong and Professor Thomas P. Russell from Lawrence Berkeley National Laboratory have created ‘aquabots’, a novel class of water-soluble soft robots. The flexibility of soft robots have allows them to squeeze into places where rigid bodies cannot, which can be useful to perform biological functions within the human body. Soft robots are safe for human interaction and have potential biomedical applications such as surgery, prosthetics, and pain relief.
In the current landscape, bio-inspired soft robots typically use elastomeric materials, such as silica gel, which require introducing bulky components and extensive synthetic steps. They are limited in their capacity to change shape, or “deformability”, compared with their aquabot counterparts.
Aquabots function in a unique two-phase aqueous system. They are composed mostly of water, polyethylene glycol, dextran, and polyelectrolytes. Polyelectrolytes are macromolecules that when dissolved in a polar solvent like water, have many charged groups covalently linked to them. These charged polymers can swell and hydrate in physiological solutions. In the aquabot system, if you add enough additives to water, the additives would rather be surrounded by molecules of their own type rather than the other. When then concentration of two additives is sufficiently high, the aqueous solution will separate into two immiscible phases, each phase still containing two additives but one additive with richer amount. Fluorescence microscopy enabled a clear view of the formation of polyelectrolytes and assembly of the membrane.
“The core of this technology is about the materials, the assembly of the polyelectrolytes in an all-aqueous solution (charged molecules in solution). The assembly of these charged molecules interact through electrostatics at the aqueous-aqueous interface,” Shum explained.
The microscale biocompatible structures had been printed on all-liquid 3D printing technology, which dispensed two polyelectrolyte containing liquids so that assembly can happen at the interface and the shapes of the interface can be controlled by the printer.
The study was published in ASC NANO with a cover feature containing the aquabot with its micrometer-scale multicompartmental structures.
Aquabots change shape in a way that can reach a target site for drug delivery. Drug delivery is challenging, as most oral drugs have constraints in bioavailability, or rate of absorption, while intravenous drugs are usually administered in a clinic or hospital.
“Aquabots can flexibly navigate into tortuous paths to get to target organs or target sites in the human body. If flexible enough they can be temporary shrunken or pushed through a narrow constriction and get to where they need to be. They can swell and perform their functions which could be about delivery, even carrying active molecules to where they need to be for release,” according to Shum.
The membrane of the aquabot structures can be functionalised for pharmaceutical applications, allowing for enzyme binding, catalytic transformations, and magnetic responsiveness. In their published study, the research team demonstrated that aquabots functionalised with glucose oxidase enzyme can perform cascade reactions.
Aquabots are fluid in their movements as shown in this video of a demonstration of aquabots swimming through a glass capillary. Their biocompatible, multicompartmental, and multifunctional nature can lead the way for advancements in biomedical applications, including medical micromanipulations, targeted cargo delivery, tissue engineering, and biomimetics.
Next, the research team is looking to expand the potential of their aquabots technology. They are examining the incorporation of biocompatible and biodegradable hydrogels into the assembly of aquabots. The group is also attempting to make the membrane conductive for electricity by integrating water-soluble conductive polymers. This may have further industrial applications such as building biological electrical circuits and ion-selective channels.
“Conductive polymers or hydrogels can be easily assembled on the water-water interface and make the system electrically conductive to build electronic sensors of the somatosensitive aquabots,” Dr Zhu Shipei, a core member of Shum’s research team. In this case, ion-diode, ion transistors and ultrathin conductive thin films maybe able to be produced by phase-separation induced self-assembly in aqueous two-phase system.” When combined with 3D printing of programming flexible logic patterns, Zhu says, this method may bring in the next advance in novel ion logic circuits, neuromorphic computing, and patterning of functional units devices.