What sort of research are you engaged in?

My laboratory is engaged in research on soft matter, particularly soft and wet matter.

Living organisms cannot be created by machinery built by humans and have extremely complex, amazing functions. Muscles, for example, move strongly and smoothly, while capillaries are capable of smoothly passing red blood cells larger than their own diameter, and the cartilage in our joints is capable of absorbing significant shock resulting from daily movement. At present, developments in robot engineering and medical fields have produced man-made devices that have some of the movement and functions of organisms, but even now we believe that it will be a long time before they can reproduce the potential of a living thing.

Fig. 1 High-strength, high-toughness double network hydrogel is achieved through sacrificial bonding.

 

So why is the tissue of living organisms so high functioning? In our laboratory, we are considering this from the perspective of “soft and wet matter science.” If we think about what living organisms are built from, then with the exception of hard tissue such as teeth and bones, we find that they are almost completely soft and wet matter that contains water, in other words, a type of hydrogel. This is in stark contrast to man-made devices, which are mostly hard and made from substances that do not contain water.

“The amazing functions of living organisms originate from the fact that living organisms are largely made up of hydrogels.” Our laboratory bases its work on this thought, and we are engaged in trying to explain the occurrence mechanisms behind high functioning living organisms through research into gels. Our objective is to create gels that demonstrate some of the wonderful functions of living organisms.

The birth of wondrous “gels” with new mechanisms – the synergy between hard and soft

With the exception of hard tissue such as bones and teeth, living organisms are formed from gels containing water. The types of gel we have always been capable of producing are those such as tofu and jelly, which are much weaker and incapable of becoming living organisms. In our research laboratory, however, we have combined a hard, brittle gel with a soft, stretchy gel to produce a substance known as ultra-tough double network gel (DN gel), which, while containing 90% water, is superior to cartilage and demonstrates strength and toughness that rivals rubber.

 

DN gel is sufficiently strong so that if a sample 1 cm thick is run over by a large truck it does not break. It vibrates like water in response to impact, but returns to its original shape. DN gel is a highly stretchable material, with some kinds of DN gel capable of stretching as much as 30 times their original length. The secret to this strength is in two types of networks, with different properties, which assist one another and inhibit the development of cracking. When force is applied to DN gel, the hard, brittle gel will initially crack. With regular gel, a single crack will immediately spread across the whole area, causing it to break, but with DN gel, the stretchy gel connects the breaks in the damaged gel, preventing the cracking from spreading, with the result that multiple small cracks occur within the gel and it absorbs a large quantity of energy. As a result of these multiple internal cracks being applied to the gel, the force is dispersed and the DN gel demonstrates high levels of strength and toughness. The covalent bonds in the hard component behave as a type of “sacrificial bonding.” The sacrificial bonding principle of DN gel is similar to the toughening model of bone. When bones are subjected to impact, calcium ions, which were sacrificially bonded to collagen molecules, are separated, and the energy is dissipated. When the impact is removed, the calcium ions re-bond to the collagen molecules. It is believed that bone strength is increased by these repairs to broken areas.

Fig. 2 Highly tough, self-repairing gel, achieved through plasticized sacrificial bonding.

When DN gel, in which covalent bonding is created as sacrificial bonding, is broken, it does not return to its former state. It is possible to add a self-repairing function by using physical bonding as sacrificial bonding. Recently, we have demonstrated that gels made with ion bonding used as sacrificial bonding not only self-repair at the molecular level, but even self-repair at the macro-level, so that even if a piece of gel is cut it is able to join back together.

Medical Applications for Gels

Gel is the substance closest to living tissue and may provide a means of inhibiting the proliferation or differentiation of cells, in addition to inducing regeneration of organic tissue. We are engaged in joint research with Professor Kazunori Yasuda’s group at Hokkaido University’s School of Medicine, into the uses of DN gel in regenerative medicine. Joint cartilage (meniscus) plays a cushioning role when we exercise and is an extremely important part of the body. To date, joint cartilage that has been damaged was thought not to regenerate within an organism, and as such, its treatment has always been difficult. The insertion of DN gel with organic compatibility into a rabbit joint with missing cartilage, however, has been shown for the first time to facilitate natural regeneration of cartilage. This discovery is likely to lead to a revolution in methods of treating joint and cartilage problems.

Development of further applications

Gel is not only soft, it also has low surface sliding friction and material permeability, as well as ionic electrical conductivity, among various functions not present in other materials. In the future, it could be developed into artificial tissue that is gentle to the human body, as well as in equipment for use in medical treatment and care, such as cardiac valves, artificial blood vessels, lubricated endoscopes that do not damage organs, artificial touch sensors (e-skin), etc. We are also looking at areas of application such as human interfaces and soft robotics.

These are not things that can be realized within one field of specialization. Cross-disciplinary research is required so that various areas of knowledge can work in synergy to move forward flexibly and without giving up.