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"Shape memory" - the surprising feature that allows you to control the change of shape of materials

The researchers created two-dimensional networks from polymeric microfibers, which change their shape depending on the temperature

Prof. Amit Sit and Shiran Ziv Shahrabani, photo: Tel Aviv University Spokesperson
Prof. Amit Sit and Shiran Ziv Shahrabani, photo: Tel Aviv University Spokesperson

Researchers from Tel Aviv University have discovered for the first time a series of physical properties that exist in networks of mica and polymer fibers, the first of which is the "shape memory" feature. These discoveries open the door to a variety of technological and biological applications, from tissue engineering to robotics.


"In the research, we produced two-dimensional networks from polymeric microfibers, which change their shape depending on the temperature." Dr. Amit Sit explains from the school of chemistry. "We discovered for the first time that networks have a shape memory feature - a particularly surprising feature that we did not know about. When cooling, the nets swell and can change their original shape, but we recognized that after heating the nets, they are able to reacquire the original shape they had before heating. This principle, demonstrated on different types of networks, offers a new way to control the change of shape of materials, and it is clear that even minor changes in the structure of the fibers translate into a dramatic change in the macroscopic behavior of the networks."

The two-dimensional networks developed and produced in Dr. Sit's laboratory are based on a polymer called PNIPAAm, and are "printed" in a process known as dry spinning. In this process, the fibers are stretched out of the liquid polymer solution, and in the process they quickly harden and become solid, with rapid evaporation of the solvent leaving the polymer as a thin fiber. This method allows the creation of fibers with a diameter of one hundredth the thickness of a hair as well as their organization in an orderly manner in space, similar to XNUMXD printing.

The research was led by Dr. Sit and PhD student Shiran Ziv Shahrabani from the School of Chemistry in the Raymond and Burley Sackler Faculty of Exact Sciences, and it was published in the prestigious newspaper Advanced Functional Materials.

Dr. Sit adds: "One of the main ways in which biological systems create movement and generate forces is through hierarchical networks that consist of thin and tiny fibers, and can change their shape and size depending on external stimulation. Such networks exist at the level of the individual cell, and take part in a variety of cellular and bodily processes. For example, the muscles in the human body are based on networks of protein fibers, which contract and relax following nerve stimulation. We were able to show that the shape change of two-dimensional networks composed of heat-responsive polymer fibers is controlled by the physical properties of each fiber. When these conditions are changed, the networks tend to exhibit one of two behavior paths upon cooling - in one path the fibers remain straight, and the network maintains its orderly form, and in the other path the fibers curve, and the network becomes messy just like spaghetti. The beauty is that the shape memory feature is present in both of these behavior paths, and upon heating, they return to the original network configuration."

Computational model of springs and networks

The interesting results that Dr. Sit and his team saw, they explained using a simple computational model. Doctoral student Shiran Ziv Shahrabani expands on the subject: "Our theoretical model is based on a basic understanding of spring systems, which are classic and well-known systems. We were able to describe the two behavior paths we observed in the laboratory by changing parameters in the spring system, and the model helped us show unequivocally that a variety of geometric factors, mainly the diameter of the fiber but also the density of the entire network, are closely related to the macroscopic properties of the network."

"With the applications of the polymer networks," adds Dr. Sit, "it is possible to sail into the realms of science fiction, but on a practical level and in the near future we plan to use the networks to produce fabrics and three-dimensional structures that will change their shape with a micron resolution, in a way that will be programmed as part of the structure of the material itself . At the same time, we are working on the use of shape-changing networks for the development of tiny artificial muscles that can change the focus of soft lenses, separate nano and micro-particles and move tiny forceps in order to take a biopsy of individual cells."

Ziv Sha'Rabbi summarizes and says: "With the insights from our research it is possible to analyze and conclude what toolbox is required for such developments. The research lasted over three years, and Prof. Eli Flexer from the Afka Academic College of Engineering in Tel Aviv also took part in it, as well as students, research students and even a high school student. There is no doubt that the knowledge we acquired during it is innovative and has great technological potential."

לThe scientific article

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