Bio-inspired droplet-based systems herald a new era in biocompatible devices

Oxford University researchers have made a significant step toward realizing a form of “biological electricity” that could be used in a variety of bioengineering and biomedical applications, including communication with living human cells. The work was published on 28 November in the journal Science.

Iontronic devices are one of the most rapidly-growing and exciting areas in biochemical engineering. Instead of using electricity, these mimic the human brain by transmitting information via ions (charged particles), including sodium, potassium, and calcium ions.

Ultimately, iontronic devices could enable biocompatible, energy-efficient, and highly precise signaling systems, including for drug-delivery.

Up to now, however, iontronic devices are typically set within solid scaffolds, which hinders their integration with soft tissues. In this new study, Oxford University researchers succeeded in developing miniature, multifunctional iontronic devices constructed from biocompatible hydrogel droplets.

The hydrogels function as ionic analogs of electronic semiconductors, enabling ion movement to be controlled similar to the control of electron movement in electronics. The tiny microscale droplets are assembled with the aid of surfactants (soap-like molecules) and conduct ions after they have been triggered by light to link together (a technique developed by the group).

The researchers have named their collection of devices dropletronics, a compound of droplet and iontronics. By creating combinations of microscale nanoliter hydrogel droplets, the team produced dropletronic diodes, transistors, logic gates, and memory devices.

The dropletronic devices perform better than any soft iontronic devices developed to date, including a higher efficiency and faster response time. They are even comparable to solid iontronic devices, with the added advantage of not being embedded in a hard matrix.

Dr. Yujia Zhang (Department of Chemistry, Oxford University), the lead researcher for the study, said, “Ions have many advantages over electrons: for instance, the fact they have various sizes and charges means they could be used to achieve various functions in parallel.

“Through the incorporation of large ionic polymers, we demonstrated a dropletronic device with long-term memory storage, which has not been achieved with previous iontronic approaches and offers an unconventional pathway to neuromorphic applications.”

In addition to controlling ion movements, dropletronic devices can also interface with cells and record biological signals from them, since the devices and cells speak the same “ionic language.” In this study, the research team used the devices to produce biocompatible sensors to record electrical signals from beating human heart cells.

“This is the first example of a lab-built biological sensor that can sense and respond to changes in function of human heart cells in a dish,” said Dr. Christopher Toepfer, Associate Professor of Cardiovascular Science at Oxford University’s Radcliffe Department of Medicine.

“This finding is an exciting step towards the fabrication of more complex biological devices that will sense a variety of abnormalities in an organ and react by delivering drugs intelligently inside the body.”

The researchers envisage the integration of dropletronics with living matter, which would provide a biocompatible approach to direct ionic communication, including the possibility of identifying multiple vital ionic and molecular species, which will open up new possibilities in various areas, notably clinical medicine.

Dropletronic circuits may also provide a route to build ionic logic systems that mimic neurons for neuromorphic information processing and computations.

Professor Hagan Bayley (Department of Chemistry, Oxford University), the research group leader for the study, said, “Dr. Zhang has used a creative, highly multidisciplinary approach including aspects of electrochemistry, polymer chemistry, surface physics, and device engineering to produce the first microscale ‘dropletronic’ devices.

“The functional capabilities of these structures demonstrate that they might soon be elaborated into practicable devices with applications in both fundamental science and medicine.”