Replicating skin-like deformation-induced stiffening and colouration with synthetic elastomers

29-03-2018

Mimicking mechanical behaviour and colouration of biological tissues is a major challenge for polymer scientists. Novel brush-like elastomers possess skin-like mechanics combining extreme softness and strain-induced stiffening. In addition, the elastomers exhibit skin-like mechanochromism due to their iridescent colouration.

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Living soft tissues possess special mechanical behaviour and active camouflage. Reproducing the mechanics of objects that combines extreme softness in the isotropic state and significant strain-induced stiffening is extremely challenging for scientists working with synthetic materials. For example, rapid stiffening of skin occurs shortly after the beginning of deformation and prevents its premature rupture. The increase of the mechanical modulus of skin during stretching can reach several orders of magnitude. This has been largely out of reach for any synthetic polymer material. Interestingly, synthetic rubbers such as Ecoflex and Dragon Skin, which are frequently employed in cinematography for masks and synthetic skin, reproduce the softness of skin but fail to mimic its significant stiffening in the course of deformation. An alternative way to mimick the extreme softness of biological materials is based on the use of solvents (e.g., water) to decrease the mechanical modulus. However, such solvent-containing systems are neither stable in air nor sufficiently ductile and can easily be swollen by physiological fluids while inside the body. In addition to their special mechanical properties, biological tissues often exhibit colourful patterns resulting from the constructive interference of visible light. This feature is an important part of the natural defence and signalling mechanisms of living organisms. Reproducing the mechanics of living tissues together with their optical features such as mechanochromism with synthetic elastomers were the main objectives of the present work.

To this end, novel brush-like elastomers have been designed that are capable of mimicking the full stress-strain behaviour of biological tissues. The polymers in question contain three chemically-different blocks where the main block has the form of a dense brush (the so-called “bottle-brush”) providing stiffness to the polymer backbone on the one hand and reducing the density of the chain entanglements on the other hand. The structure and ordering of the brush-like polymers have been previously addressed [1,2]. The unusual tissue-like mechanical behaviour of the brushes has been the focus of a recent paper [3]. The reduced concentration of the chain entanglements compared to the conventional linear polymers makes it possible to fabricate ultra-soft solvent-free systems based on these brush-like macromolecules.

In the case of the brush triblock copolymers employed in the current work, the terminal linear blocks undergo phase separation: they segregate from the main brush-containing matrix in the form of nanometre-sized spheres. The supramolecular structure of such micro-phase separated systems allows the mechanical behaviour of the system to be adjusted. This is shown schematically in Figure 1a. Indeed, the spheres built from the terminal blocks serve as physical crosslinks forming a physical network controlling the stiffness of the matrix. The microstructural parameters were explored using the USAXS setup at beamline ID02. The typical USAXS curves of a series of triblock copolymers are given in Figure 1b. The USAXS measurements reveal complex structural organisation in these systems at different spatial levels. The main interference maximum is due to ordering of the terminal-block-containing nanospheres built of poly(methyl methacrylate), PMMA. At shorter distances, the spheres generate a series of ripples, i.e. the so-called form-factor, which proves their sharp polydispersity. At even shorter distances, a peak standing for the diameter of the bottle-brush block containing the polydimethylsiloxane (PDMS) side chains can be observed.

Supramolecular self-assembly of triblock copolymers containing a brush-like central block and two linear end-capping blocks segregating into nanometre-sized spheres

Figure 1. a) Supramolecular self-assembly of triblock copolymers containing a brush-like central block and two linear end-capping blocks segregating into nanometre-sized spheres. b) USAXS curves corresponding to a series of brush copolymers with an increasing volume fraction of the terminal PMMA blocks (from samples 300-1 to 300-4, respectively). The main interference maximum (denoted as d1 in the figure) reflects the average distance between the PMMA spheres, a series of ripples (d2) is due to the form-factor of the PMMA spheres and the characteristic peak d3 reflects the diameter of the central brush-like block with the PDMS side chains.

The mechanical properties of the bottle brush copolymers can be controlled by the density and lengths of the side chains as well as by the lengths of all of the blocks. Using the scaling concepts and a molecular constitutive network deformation model, it was found that the full stress-strain curve can be encoded in the chemical structure of the novel elastomers. Figure 2a shows the examples of elastomers closely reproducing the mechanical behaviour of porcine skin in the directions parallel and perpendicular to the spine line. One can see that the tensile curves perfectly match the initial softness of the skin followed by rapid and intense strain-stiffening.

Apart from mimicking the mechanical behaviour of soft biological tissues, the novel elastomers exhibit iridescent colouration, which is due to the characteristic distance between the PMMA nanospheres lying in the submicron range (cf. Figure 2b). Moreover, the elastic nature of the central block makes the samples mechanochromic: the experiments at beamline ID02 realised with an in situ stretching device allowed observation of the colour variation during tensile deformation (cf. Figure 2, c and d). This phenomenon is similar to the colour variation in living organisms such as chameleons where a change from excited to relaxed state of the animal results in a change in the characteristic distance between the diffracting guanine nanocrystals.

The novel elastomers can exactly replicate the full stress-strain curves of porcine skin measured perpendicular and parallel to the spine line

Figure 2. a) The novel elastomers can exactly replicate the full stress-strain curves of porcine skin measured perpendicular and parallel to the spine line. b) Supramolecular organisation of the bottle-brush-based copolymers leads to the characteristic intense colouration. c) The mechanochromism of the elastomers visualised as the shift of the main interference peak during stretching (transverse direction) and d) the corresponding change of colouration (λ = L/L0).

The findings reported in the present paper can pave the way for new strategies in the fabrication of soft biological implants fully compatible mechanically with the surrounding tissues and possessing skin-like mechanochromism.

 

Principal publication and authors
Chameleon-like elastomers with molecularly encoded strain-adaptive stiffening and coloration, M. Vatankhah-Varnosfaderani (a), A.N. Keith (a), Y. Cong (a), H. Liang (b), M. Rosenthal (c), M. Sztucki (c), C. Clair (d), S. Magonov (e), D.A. Ivanov (f,g), A.V. Dobrynin (b), S.S. Sheiko (a), Science (2018); doi: 10.1126/science.aar5308.
(a) Department of Chemistry, University of North Carolina at Chapel Hill (USA)
(b) Department of Polymer Science, University of Akron (USA)
(c) ESRF
(d) Laboratoire de Physique et Mécanique Textiles, Université de Haute Alsace, Mulhouse (France)
(e) Scanning Probe Microscopy Labs, Tempe (USA)
(f) Institut de Sciences des Matériaux de Mulhouse-IS2M, CNRS UMR 7361, Mulhouse (France)
(g) Faculty of Fundamental Physical and Chemical Engineering, Lomonosov Moscow State University, Moscow (Russian Federation)

 

References
[1] J.R. Boyce et al., Langmuir 20, 6005-6011 (2004).
[2] S.Y. Yu-Su et al., Macromolecules 42, 9008-9017 (2009).
[3] M. Vantankhah-Varnosfaderani et al., Nature 549, 497-501 (2017).