"Learning to question the science behind the reactions was pivotal"

As a PhD student in biomedical engineering at the Chinese Academy of Sciences in Shanghai, Yujia Zhang was first bothered, then intrigued, by the interface between bioelectronic devices and living tissues.

“On one side, you have devices like sensors and implants that are based on electrons, and on the other side, you have complex interactions between cells and ions in a soft, aqueous environment,” Zhang says. “I really wanted to understand what happens at that interface.”

Born in Beijing, Zhang has always kept an open mind when it comes to his career, never hesitating to explore new fields in pursuit of his scientific curiosity. “I actually studied astrophysics as an undergraduate, but I decided that I preferred hands-on experimentation to pure theory,” he says. He ended up earning his bachelor’s degree in Electronics and Information Science and Technology from the University of Science and Technology of China.

It was during his postdoctoral research at Oxford University that Zhang discovered the field of iontronics, which uses charged atoms and molecules called ions, rather than electrons, as signal carriers. Instead of solid metals, ionic conductors are often liquids or soft gels, making them much more compatible with biological tissues and systems than electronic devices.

With the guidance of his advisor, Hagan Bayley, Zhang forged his own research direction, focusing on miniaturizing iontronic systems for biological applications using microscale droplets. Zhang credits Bayley with encouraging him to always try and understand the fundamental science at the heart of his experimental observations: “Coming from an electrical engineering background, learning to question the science behind the reactions was pivotal for me,” he says.

This approach helped Zhang develop a fabrication process for ‘dropletronic devices’, which begins with submerging positively and negatively charged hydrogel droplets in an oily solution. This forms a lipid coating around each droplet, so that they can be combined to form batteries, transistors, logic circuits, and memory storage devices. The movement of charged ions through these devices mimics the movement of electrons through a semiconductor, allowing information to be transmitted and stored.

Speaking the language of ions

While iontronics is an established field, and droplets have also been previously used in other fields like microfluidics, Zhang is the first to apply droplets to iontronics for therapeutic biointerface applications (known as bio-iontronics).

“Biology speaks the language of ions and small molecules, so we can use these droplets as a platform to control the movement of ions and charged molecules to stimulate and record signals from biological tissues, like the brain and heart, more efficiently,” he explains. Indeed, just before coming to EPFL, Zhang and his Oxford colleagues reported in Science the use of a dropletronic device to record electrical signals from beating human heart cells. They have also generated ionic currents using droplet batteries to stimulate neurons in brain organoids.

My vision is that in the next 10-20 years, these systems will become more functional, more integrated, and more powerful.

At EPFL, Zhang is working hard to build his lab, focusing on assembling an interdisciplinary research team as well as establishing collaborations with researchers in the Institute of Bioengineering and School of Life Sciences. These activities, not to mention adjusting to Swiss life and language, leave Zhang with little free time. Nevertheless, he still manages to indulge his passion for powerlifting – a sport he participated in semi-professionally while living in the UK – at the UNIL-EPFL Sports Center.

Ultimately, Zhang hopes to use 3D-printed networks of droplets to create 3D multifunctional synthetic tissues for soft implants and biointerfaces. He envisions these tissues being used, in parallel with bioelectronics, for a wide range of medical therapeutics, ushering in a new ‘iontronic age’ in biomedicine.

“If you think about the beginning of electronics, the first transistor prototype was on the scale of tens of centimeters. Today, we have already achieved the microscale with dropletronics. My vision is that in the next 10-20 years, these systems will become more functional, more integrated, and more powerful.”