When you hear IUPAC, one of the first things that likely comes to mind is the IUPAC nomenclature system for naming chemical compounds, it is afterall where most chemists are exposed to the name. What some of us may not know, is that IUPAC stands for the International Union of Pure and Applied Chemistry, and each year they compile what they deem to be the top 10 emerging technologies in chemistry. This week's article will highlight one of the most interesting of those technologies - artificial muscles.
It’s widely known that muscles function via the contraction and relaxation of muscle fibers, but what isn’t as commonly known, is that expansion and contraction is also a common property to many polymers. The current premise being tested is the use of polymers as “artificial muscles”, to provide movement around joints for those unable to use their muscles in conventional ways.
Like muscles, these polymers also need a stimulus in order to contract, and leading studies are investigating the use of a voltage stimulus. Voltage may seem ideal, as it provides the near-instant response that mimics biological function, but current models require voltages far above human limits. Many health and safety organizations state that the safe upper limit to voltages is 50 V, and these polymers require stimuli of over 4000 V. It’s clear that there is a massive disparity between the required voltages and what the human limitations are, and this is the focal point of the current research being done.
Most electroactive polymers have thicknesses of over 100 µm, and Adeli et al. were the first group to take a step towards decreasing strand thickness as an attempt to decrease the voltage required. The electroactive polymer they use is called a bottlebrush polymer, and they synthesized it using a ring-opening metathesis reaction with norbornene monomers that possess a poly(dimethylsiloxane) side chain. The synthesis scheme can be seen in figure 1. They designed this synthetic scheme specifically for this purpose, and the unique nature of it allows them to terminate the reaction at a desired polymer thickness. Balancing the desire to decrease voltage while maintaining polymer integrity, they found that a thickness around 60 µm achieved the greatest results. This thickness drastically decreased the operating voltage, bringing it down to 1000 V, and it was still very structurally sound, being able to withstand 10000 cycles of expansion and contractions on a circular actuator before degrading.
In a follow up experiment, they introduced a series of polar side chains in place of the poly(dimethylsiloxane) side chain, and found that the polymers responded to voltages as low as 800 V. It’s evident that with more investigation, they will continue to reduce the voltage needed and can one day hopefully break the 50 V barrier making this technology feasible in human-based applications.
While the technology itself is exciting, it still leaves a lot unanswered. How will these artificial muscles be controlled? Can they be innervated similar to biological muscles? These are both questions that can only be answered by further research, so this will be a worthwhile field of study to keep tabs on.
The finding of this work has been published in ACS Applied Materials & Interfaces: Adeli, Y.; Owuso, F.; Nueschthi, F. A.; Opris, D. M. On-Demand Cross-Linkable Bottlebrush Polymers for Voltage-Driven Artificial Muscles. ACS Appl. Mater. Interfaces. 2023, 15 (16), 20410-20420. https://doi.org/10.1021/acsami.2c23026
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