Injectable tissue prosthesis to aid in muscle regeneration

Researchers from the Institute of Basic Science (IBS) in South Korea have developed a novel approach to healing muscle injury, by employing an injectable tissue prosthesis in the form of conductive hydrogels and combining it with a robot-assisted rehabilitation system. Their work has been published in the journal Nature.

Imagine you are swimming in the ocean when a giant shark approaches and bites a huge chunk of meat out of your thigh, resulting in a complete loss of motor/sensor function in your leg. Traditional rehabilitation methods for these kinds of muscle injuries have long sought an efficient closed-loop gait rehabilitation system that merges lightweight exoskeletons and wearable/implantable devices. Such assistive prosthetic systems are required to aid the patients through the process of recovering sensory and motor functions linked to nerve and muscle damage.

Unfortunately, the mechanical properties and rigid nature of existing electronic materials render them incompatible with soft tissues. This leads to friction and potential inflammation, stalling patient rehabilitation.

To overcome these limitations, the IBS researchers turned to a material commonly used as a wrinkle-smoothing filler, called hyaluronic acid. Using this substance, an injectable hydrogel was developed for tissue prosthesis, which can temporarily fill the gap of the missing muscle/nerve tissues while it regenerates. The injectable nature of this material gives it an advantage over traditional bioelectronic devices, which are unsuitable for narrow, deep or small areas, and necessitate invasive surgeries.

Thanks to its ‘tissue-like’ properties, the hydrogel seamlessly interfaces with biological tissues and can be easily administered to hard-to-reach body areas without surgery. The reversible and irreversible cross-links within the hydrogel adapt to high shear stress during injection, ensuring good mechanical stability. The hydrogel also incorporates gold nanoparticles, which give it decent electrical properties. Its conductive nature allows for the effective transmission of electrophysiological signals between the two ends of injured tissues. In addition, the hydrogel is biodegradable, meaning that the patients do not need to get surgery again.

The researchers put their idea to the test in rodents that had had a large chunk of muscle removed from their hind legs. By injecting the hydrogel and implanting the two kinds of stretchable tissue-interfacing devices for electrical sensing and stimulation, the researchers were able to improve the gait of the rodents. The hydrogel prosthetics were combined with robot assistance, guided by muscle electromyography signals. Together, the two helped to enhance gait without nerve stimulation. Furthermore, muscle tissue regeneration was effectively improved over the long term after the conductive hydrogel was used to fill muscle damage.

The injectable conductive hydrogel was found to excel in electrophysiological signal recording and stimulation performance, offering the potential to expand its applications. It thus presents a fresh approach to the field of bioelectronic devices and holds promise as a soft tissue prosthesis for rehabilitation support.

“We’ve created an injectable, mechanically tough and electrically conductive soft tissue prosthesis ideal for addressing severe muscle damage requiring neuromusculoskeletal rehabilitation,” said Professor Shin Mikyung. “The development of this injectable hydrogel, utilising a novel cross-linking method, is a notable achievement. We believe it will be applicable not only in muscles and peripheral nerves but also in various organs like the brain and heart.”

“In this study, the closed-loop gait rehabilitation system entailing tough injectable hydrogel and stretchable and self-healing sensors could significantly enhance the rehabilitation prospects for patients with neurological and musculoskeletal challenges,” added Professor Son Donghee. “It could also play a vital role in precise diagnosis and treatment across various organs in the human body.”

The research team is currently pursuing further studies to develop new materials for nerve and muscle tissue regeneration that can be implanted in a minimally invasive manner. They are also exploring the potential for recovery in various tissue damages through the injection of the conductive hydrogel, eliminating the need for open surgery.

Image caption: The new hydrogel system (IT-IC) can protect the gel from destruction by external forces. In conventional covalently cross-linked hydrogels, fragmentation occurs due to the destruction of covalent bonds during the injection process, but IT-IC hydrogels maintain covalent bonds due to stress dissipation with multiple bonds containing biphenyl structure and allow injection into narrow areas.