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Title: Shape memory behavior of skin collagen and its materials
Authors: Han, Yanting
Degree: Ph.D.
Issue Date: 2020
Abstract: During the natural evolution process, living organisms have gradually adapted to the environment and been adept in synthesizing high-performance structural materials at mild conditions by using fairly simple building elements. The skin, as the largest organ of animals, is such a representative example. It can make adaptions in response to environmental stimuli including moisture, heat, and light, thereby assisting the body in accomplishing a variety of physiological functions. Moreover, conferred by its intricate organization where collagen fibers with hierarchical structures are arranged in a randomly interwoven network, skin display remarkable performance with the combinations of mechanical properties, chemical-reactivity, and biocompatibility, far surpassing those of synthetic materials working as a skin substitute. Such notable attributes have made skin itself as potential smart materials. By removing epithelial components (keratinocytes, sweat glands, and sebaceous glands) and dermal non-structural components (fibroblasts, vascular endothelium, and smooth muscle, etc.), skin collagen (SC) matrix can be obtained. In this study, SC was prepared from the skin, and highlights would be held on revealing the smart properties of SC. In addition, different treatments, including modification through chemical (inorganic/organic) or compositing strategies will be applied to optimize SC from the molecular level, with particular emphasis on shape memory behavior as smart materials. For the first time, it is discovered that water can induce the shape memory effect of SC. The exploration of corresponding mechanisms in terms of its molecular and morphological structure revealed that SC can completely fix temporary bending shapes and retain a high shape recovery ratio (>90%) under repeatable memory bending cycles. The reversible cleavage and reformation of interpeptide hydrogen bonds can work as "switch" to trigger the shape deformation while peptide chains together with cross-links within collagen work as "netpoint" for shape recovery. This water-responsive adaptable mechanical behavior (switchable modulus) allows SC to be either used as a robotic handle for loading or flexible material for covering. Benefit from the reactivity of collagen, the modification of SC becomes feasible. Inspired from the structure of metalloprotein, an organic-metal biosystem of chromium (III) complexed skin collagen (Cr-SC) was prepared. The obtained Cr- SC retains the inherent conformation of collagen and its fiber morphology in the skin. Compared with pristine SC, Cr-SC showed enhanced waterproofness and breathability. Due to the coexistence of hydrogen bonds and newly introduced chromium (III) complexing linkages, Cr-SC exhibited water-adaptive mechanical property. In this organic-metal biosystem, hydrogen bonds show reversible cleavage-reformation underwater interference while chromium (III) complexing linkages remain stable. Such a combination of "switch" and "netpoint" facilitated water responsive shape memory ability of Cr-SC with stretched shape fixation and recovery reaching over 80%.
Apart from metal ions, SC also shows an affinity to organic agents. In this project, tea-polyphenol has been used to modify SC. Commonly, tea-polyphenol as organic natural component has been studied as skin-care agents from chemical, biological, and physiological perspectives. Herein, it reveals physical-beauty-intelligences of a tea polyphenol-treated SC (TP-treated SC) through an in-vitro study using a material-approach. Compared to untreated one, the TP-treated SC shows coalesced adaptive-hydration, smart-stretch and shape-memory advantages. These effects are attributed to gyro-like-polyphenol-rings as a plasticizer with chains crosslinked to multi-sites on collagen-fibers as netpoints. Materially, collagen is a structural protein and plays an essential role in skin-integrity. Hence, the study indicates that the TP treatments may bring skin with both anti-dehydration and anti-overhydration concurrently, tender and stretch-resistant simultaneously as well as sagging and wrinkling free property. The discovery, mechanism, and approach are independent of existing reported benefits, which not only helps to quantify the effectiveness of skin-care products and dermatological interventions and treatment but also provides insights into the physiological behavior of collagen contained tissue. Learned from the skin collagen-elastin interpenetrating network, a skin collagen/polyurethane (SC/PU) composite with a dual-network was successfully prepared by a facile "paper-making" procedure. The first collagen nonwoven network was built up by spontaneous entangling and sticking of collagen fibers recycled from animal skin. Such a fibrous structure was then interpenetrated by waterborne PU. By virtue of chemo-mechanical adaptability of both collagen fiber and PU elastomeric matrix, a water responsive shape memory with high shape fixation (99%) and shape recovery (>90%) has been achieved. Destruction and reformation of hydrogen bonds within collagen fiber works as "switch" to achieve shape deformation and fixation. This "switch" opens under the stimulation of water, while elastic entropy of PU promotes shape recovery. The obtained SC/PU composite can be degradable in simulated body fluid whilst biological evaluation in vitro (MTT assay) proved that SC/PU composite has better biocompatibility than pure PU. In general, this project provides new opportunities and guidance in emerging research areas such as shape memory biopolymer devices, as well as the physiological function of animal skin. In addition, it offers a novel, simple and eco-friendly route for the fabrication of a bio-smart material, which has potential for biomimetic sensors, regenerative medicine, and artificial skins.
Subjects: Collagen
Hong Kong Polytechnic University -- Dissertations
Pages: xxxi, 226 pages : color illustrations
Appears in Collections:Thesis

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