Biological tissues have complex mechanical properties that are difficult to reproduce with synthetic materials. An international team has succeeded in producing a biocompatible synthetic material that behaves like biological tissues and changes colour as it deforms, like the skin of chameleons. They promise new materials for biomedical devices.
Po produce a medical implant, it is necessary to choose a material with mechanical properties similar to those of biological tissues, at the risk of causing inflammation or necrosis. Although the implant industry has grown rapidly in recent years, it is not yet in a position to offer a system that can perfectly replace or repair skin or bone damaged by trauma or disease. In addition, the human body is an extremely corrosive environment for the implanted material, and there is as yet no biomaterial capable of eliminating the reaction that occurs when in contact with a living organism.
Implants must adapt as well as possible to the patient's body and be quickly assimilated in order to be effective and fulfil their assigned role (maintaining, assisting or restoring the patient's mobility). Many tissues, such as the skin, intestinal wall or heart muscle, have the particularity of being flexible while at the same time hardening when they are stretched. Until now, it has been impossible to reproduce this behaviour with synthetic materials. "The creation of a material that behaves like skin is very difficult." says Barbara Gilchrest, a dermatologist at Massachusetts General Hospital and lead author of an article on XPL. (1) published in the Nature Materials magazine. "Many people have tried to do this, and the materials that are available so far are not as flexible, nourishing and protective as real skin, she adds.
Researchers from the CNRS and the University of Haute-Alsace(2) and ESRF, the European Synchrotron, together with colleagues from the American Universities of North Carolina and Akron, attempted to do so with a single polymer (3). To do this, they have synthesized a particular elastomer composed of a central block on which are grafted side chains (like a pin), and extended at each end by two end blocks (see figure). The researchers found that by choosing the right structural parameters for the polymer, the material followed the same deformation curve as a biological tissue, in this case pig skin. Furthermore, it is biocompatible since it requires no additives and remains stable in the presence of biological fluids as well as in the open air.
Top - left: molecular structure of a plastomer synthesized in this work; right: supramolecular structure formed by the assembly of identical plastomers.
Bottom - left: stress-strain curves for plastomers ("M300-2" and "M300-3") that mimic the mechanical behaviour of pig skin samples ("pig", in cross-section or longitudinal); right: image showing the iridescent colouring of the plastomers. The edges are less blue because they receive light at a different angle.
D.A. Ivanov and S.S. Sheiko
Another property of the material appeared during the experiments: its colour changes according to its state of deformation. As scientists have shown, this is a purely physical phenomenon, which arises from the interaction of light with the structure of the material. Observations in atomic force microscopy and X-ray scattering experiments have shown that the end blocks of these polymers come together in nanometric spheres, distributed in a matrix formed by the pin structures. Light interferes with this architecture, which diffuses a given colour depending on the distance between the spheres; stretching of the material therefore results in a change of colour. It is the same mechanism that explains - to a large extent - how chameleons change colour (variable homochromia (4)).
The researchers were therefore able to encode both mechanical properties (flexibility, deformation profile) and optical properties in a single synthetic polymer, something that had never been done before.
By adjusting the length of the different chains or the density of the side chains of the "brush", it is possible to modulate these properties.
This discovery could lead to more personalised medical implants or prostheses (vascular implants, intraocular implants, intervertebral disc replacement), but also to materials with completely new deformation profiles, with as yet unsuspected applications.
These results, to which researchers from the CNRS, the University of Haute-Alsace, the University of Haute-Alsace and the University of Paris, have contributed (3) and ESRF, the European Synchrotron, together with colleagues from the American Universities of North Carolina and Akron, are published on 30 March 2018 in the journal Science (5).
Source: CNRS - 29/03/2018
(1) XPOL: a "second skin" capable of protecting and firming the epidermis created by a team of scientists from MIT and Harvard Medical School in 2016.
(2) At the Mulhouse Institute of Material Sciences (CNRS/UHA) and the Laboratory of Textile Physics and Mechanics (UHA).
(3) Biological tissues are composed of cells and molecules such as, in the case of skin, collagen, which is rigid, and elastin, which is flexible.
(4) Variable homochromia: the ability of an animal to change colour rapidly and reversibly in seconds to hours, i.e. the ability of an animal to harmonise its colours with those of its environment. Homochromia is to be differentiated from true mimicry which corresponds to the external and generally permanent resemblance between two species belonging to more or less distant clades.
(5) References: Chameleon-like elastomers with molecularly encoded strain-adaptive stiffening and coloration. M. Vatankhah-Varnosfaderani, A.N. Keith, Y. Cong, H. Liang, M. Rosenthal, M. Sztucki, C. Clair, S. Magonov, D.A. Ivanov, A.V. Dobrynin, S.S. Sheiko. Science, March 30, 2018. DOI: 10.1126/science.aar5308