Energy storage devices are crucial in a number of implantable biomedical electronics systems, including neurostimulators, biosensors and pacemakers. The complex internal environment of the body, coupled with the need for being integrated into different-shaped implants, and the need for prolonged operation inside the body (to avoid being changed, as that would require multiple and potentially unnecessary surgeries), has meant that the energy storage devices powering these implants need to be different from those in conventional electronics. 

For a lot of implantable bioelectronics, conventional energy storage devices are not suitable for a number of reasons, including the use of materials that are too toxic and materials that don’t have a good enough mechanical flexibility and stability to be used in the ever-changing biological environments that put stress on devices. These are the common challenges, but there are also some issues with the conventional redox reactions exhibited in conventional battery architectures, as the potential for side reactions and device degradation can compromise the safety of the implant over time, especially when there are many implants nowadays that are designed to be installed for many years. 

Advances in Biocompatible Supercapacitors and Hydrogel Fibre Designs

In recent years, biocompatible supercapacitors have emerged as a promising option for delivering power to bioelectronic devices without needing redox reactions. To meet the longevity demands of many bioelectronic devices, biocompatible supercapacitors need to be non-toxic, be stable for long time periods, and be able to mechanically adapt to biological tissue to avoid mechanical mismatch. 

Fiber shape designs have become an advantageous architecture because they can easily adapt to physiological movements, reducing immune responses in the body. Hydrogel materials have also become a material of interest because they offer fast ion transport, low interfacial resistance and biocompatibility properties. However, developing biocompatible hydrogel electrodes and electrolytes has been a challenge due to the trade-off between mechanical robustness and electrochemical performance. Researchers have now combined the properties of traditional hydrogels with a fibre-based structure to develop new fibre supercapacitors. 

New Fibre-Supercapacitors Can Be Inserted into the Body 

Researchers have developed a robust hydrogel-based supercapacitor fibre (THBS) using a thermal drawing process. This approach enabled all the different components, such as the electrolyte, electrodes, and current collector, to be integrated into a single, unified and mechanically robust fibre. Hydrogels are commonly more amorphous polymeric materials, but the researchers created the fibres using a thermal drawing process, which created fibres with optimised thermal and mechanical properties, as well as self-healing capabilities.  

The fabrication process creates fibres on the order of a few hundred microns, and could theoretically be capable of mass production for fibres ranging from centimetre to micrometre dimensions. The hydrogel fibres were fabricated using only biocompatible materials, such as polycaprolactone (PCL), polyethylene glycol (PEG), polyvinyl alcohol (PVA), poly(ethylene-co-vinyl acetate) (EVA), and sodium chloride (salt). By using only biocompatible materials, the fibres have a much-reduced risk of invoking an immune response.  

The supercapacitor system is composed of a PVA-based dual-network hydrogel, which acts as both the electrolyte and the electrode. The hydrogel possesses hydrogen bonds between the PVA and PEG molecules, while ionic coordination bonds are created between the PVA and sodium borate. These coordination bonds ensured that the hydrogel fibre had a high toughness and the ability to self-heal. Additionally, the formation of a borate-diol complex between the PVA and sodium borate at the electrode-electrolyte interface helped to improve the capacitance of the device. 

While the hydrogel was used for both the electrode and electrolyte, other materials were added to improve the performance of the device. In the electrode, a high surface area activated carbon was added to improve the charge storage capabilities of the electrode, while carbon black was added to improve electrical percolation, to enhance the ion transport and electrochemical efficiency of the device. The sodium chloride was integrated into the fibres to act as the electrolyte ion source and helped to facilitate rapid ion migration to the electrode surface, leading to efficient charge and discharge operations. The current collectors for the fibre were made of a conductive PCL that enabled a longitudinal current to flow along the fibre. The mechanical durability and prevention of electrical leakage were ensured by encapsulating the internal structure of the fibre with an EVA layer.  

In-Body Performance and Future Applications

The THBS fibres exhibited a high durability, good electrochemical performance, even under dynamic and high curvature deformations that mimic in-vivo physiological movements, and a robust mechanical and electrochemical stability. This was due to the fibre-based architecture being much better at managing different types of mechanical deformations, such as knotting, twisting and bending. The fibre supercapacitors fabricated were only a few hundred microns in size and were tested for 5 weeks in an in vivo environment (a mouse), continuing to show a high performance and minimal immune response over this time. While implanted, the THBS maintained a successful LED operation and was able to stimulate both the central and peripheral nervous systems. 

As the devices are being implanted into the body, there is a need to make sure that they are sterile to prevent infections after implantation. Gamma irradiation and ethylene oxide treatment could be used in the future to preserve hydrogel functionality in the body, but the researchers have said they are confident that the mechanical and chemical properties can remain stable in the body due to the encapsulation design. It’s also thought that the small geometry of the fibre will only require minimally invasive implantation and will be more conformable to tissue in larger animals. 

The approach used in this study has provided a potentially scalable way of creating biocompatible supercapacitor fibres that could be suitable for continuous high-throughput production. While there’s still work to be done to reach that stage, it potentially offers a way of fabricating more biocompatible implantable energy storage systems.  

The researchers have stated that they intend to continue developing the capabilities of the THBS supercapacitors by integrating them with wireless power transfer systems. This could help to provide a continuous and non-invasive way of delivering energy for medical devices, but these wireless systems are prone to disruption and power fluctuations. However, integrating them alongside THBS fibres could enable the fibres to act as an on-demand energy buffer that provides an uninterrupted operation for the implanted system and helps to rectify any short-term issues within wireless power transfer systems attached to medical devices. Combining the two approaches could eventually lead to more robust and biocompatible energy storage/wireless power transfer systems for fully autonomous, closed-loop implantable bioelectronic technologies. 

Reference: 

Park S. et al, Fully biocompatible, thermally drawn fiber supercapacitors for long-term bio-implantation, Nature Communications, 16, (2025), 8207.