We used a series of DN gels consisting of poly(2-acrylamido-2-methylpropanesulfonic acid) sodium salt (PNaAMPS) as the brittle network and poly(acrylamide) (PAAm) as the stretchable network. Each network is covalently cross-linked.
Fig. 1. Conceptual scheme of the self-growth of materials induced by mechanical training. (A) A skeletal muscle becomes bigger and stronger under the application of repetitive mechanical stress and a continuous nutrient supply through metabolic cycles. (B) As a result, such a tissue is initially weakened (purple arrows) but then recovers or even shows enhanced performance (red arrows) due to mechanical stimuli. The green outline indicates one mechanochemical cycle. (C) Our strategy to develop self-growing materials based on mechanical training of DN gels. Mechanical stress leads to breakage of the brittle network (blue), whereas the highly stretchable network (pink) maintains the integrity of the gel. The mechanoradicals generated at the broken ends of the brittle network strands trigger polymerization of monomers supplied from the external environment to form a new network (orange). The chemistry of the newly formed network can be the same or differ from that of the original brittle network, as required.
The process is reported to be repeatable and reproducible:
With proper conditions, our DN gel exhibits persistent growth in strength and size under repetitive mechanical training. Two requirements must be satisfied: Enough monomers should be continuously externally supplied, and the newly formed network should serve the role of a brittle network that then breaks during the next deformation to trigger subsequent network formation.
However, with muscle growth there are limiting factors such as hormones and bones structure, and even supplying enough protein one cannot grow the mass infinitely*. Considering constant feeding with monomer, is there a theoretical/practical limit for strength and growth for these DN-hydrogels?
*not even Tetsuo Shima — sorry, couldn't resist considering the origin of this work and the anime.
- Matsuda, T.; Kawakami, R.; Namba, R.; Nakajima, T.; Gong, J. P. Mechanoresponsive Self-Growing Hydrogels Inspired by Muscle Training. Science 2019, 363 (6426), 504–508. https://doi.org/10.1126/science.aau9533.