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Applications of Polymer, Composite, and Coating Materials
Soft and Flexible Bi-layer TPU Foam for Development of Bio-inspired Artificial Skin Huan Li, Tridib Kumar Sinha, Jeong Seok Oh, and Jin Kuk Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018
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ACS Applied Materials & Interfaces
Soft and Flexible Bi-layer TPU Foam for Development of Bio-inspired Artificial Skin Huan Li1†, Tridib K. Sinha1†, Jeongseok Oh2, and Jinkuk Kim1* 1
Elastomer Lab, Department of Materials Engineering and Convergence Technology and
2
School of Materials Science and Engineering, Polymer Science and Engineering, Engineering
Research Institute, Gyeongsang National University, 501 Jinju-daero, Jinju, Korea 52828 E-mail:
[email protected] ABSTRACT Inspired by epidermis-dermis composition of human skin, here we have simply developed a light-weight, robust, flexible and biocompatible single electrode triboelectric nanogenerator (STENG) based prototype of bi-layer artificial skin, by attaching one induction electrode with unfoamed skin layer of microcellular thermoplastic polyurethane (TPU) foam, which shows high performance object manipulation (by responding differently towards different objects, viz., Al-foil, balloon, cotton-glove, human-finger, glass, rubber-glove, artificial leather, polyimide, PTFE, paper and wood), due to electrification and electrostatic induction during contact with the objects having different chemical functionalities. Comparative foaming behavior of ecofriendly supercritical fluids viz., CO2 over N2 under variable temperatures (e.g., 130°C and 150°C) and constant pressure (15 Mpa) have been examined here to pursue the soft and flexible triboelectric TPU foam. The foam derived by CO2-foaming at 150°C has been prioritized for development of S-TENG. Foam derived by CO2-foaming at 130°C was not well responding due to the smaller cell size, higher hardness and thicker skin. Inflexible N2-derived foam was not considered for S-TENG fabrication. The object manipulation performance has been visualized by principal component analysis (PCA) which shows good discrimination among responses of different objects. KEYWORDS
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Artificial skin, super critical foaming, TPU foam, single electrode triboelectric nanogenerator, PCA INTRODUCTION Skin, the largest organ of human body, consisting mainly the outermost epidermis over underlying dermis, protects the interior organs and also transduces various mechanical stimuli from the external environment.1 Mimicking of the human skin and development of bi-layer artificial skin was first accomplished by Yannas and coworkers, which is composed of an upper silicone sheet over a lower sheet of the collagen sponge.2 Following this work, many researchers have developed different types of artificial skin for prosthetic and wound-dressing applications, which mainly consist of silicon upper layer over a modified collagen sponge.3-4 In addition to the environmentally less-stable and inflexible collagen, very recently flexible thermoplastic polyurethane (TPU) foam based fenestrated-type artificial dermis has been successfully used in topical negative pressure wound therapy for the venous leg ulcer.5 Apart from these biological applications, immense interest is being motivated on development of artificial skin having tactile sensing capability for the promise of creating advanced humanoid robots, biomedical prostheses, surgical electronic gloves, and wearable health monitoring devices.6-11 It is very important for an artificial skin having the ability to distinguish the tactile sensation induced by mechanical stimuli of different objects for ensuring the object manipulation. Flexibility is another most important requirement of artificial skins, ensuring conformable covering of curved and moving surfaces, which also withstand repeated and prolonged mechanical deformations (e.g., stretching, bending, twisting, compression, etc.). Also, large-scale production of the artificial skin demands the developed process to be facile and cost-effective. In these regards, flexible and portable piezoelectric nanogenerator (PENG) or triboelectric nanogenerator (TENG) based artificial skins have been started to develop in the last few decades.12-25 Instead of health-hazard and brittle lead-based materials, PENG made of PVDF and its copolymers are being globally accepted because of its adequate flexibility and 2 ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
biocompatibility, but it needs poling to create electroactive phases within these polymers through corona-charging or by self-polarization using different nanomaterials or metalcomplexes, which are again complicated, time-consuming and cost-intensive in some cases.2635
Alternatively, TENG, that converts mechanical stimuli into electrical response simply by
contact electrification and electrostatic induction could serve as an effective technology for the tactile sensing purpose. Although different types of TENG based potential sensors have been demonstrated in recent years, execution of sophisticated texturing or patterning are usually needed for maximum of the TENGs.18-25 Dielectric polymer foams having in situ trapped charged air inside (also known as electret material) has been extensively used for development of PENG35-39, which can be the better choice to pursue TENGs without any texture engineering because there is an immense possibility of electrification simply by contact of polymer foams (having induced surface charge by the trapped charged air inside) with objects of different functionalities.40-42 Foams of different polymers will show a different kind of surface charge, depending on the electron affinity of the polymer functional groups. Accordingly, TPU foam has been recognized as positive triboelectric material as per the triboelectric series provided.4345
It is being a potential triboelectric material and well accepted artificial skin for medical
diagnostics,5 exclusive research opportunities are there on mimicking of human skin and development of biocompatible bi-layer artificial skin for tactile sensing performance. Although, many techniques have been reported to prepare polymer foams such as, solgel techniques,46 micro-emulsion templating,47 molecular imprinting,48 block copolymer-based techniques,49 and foaming,50-51 the facile and large-scale production of flexible polymer foam with adequate cell size and density, still a topic for extensive research and development. Among all of these techniques, foaming technology can be the most promising and industrially-viable, because, it is time-resolved, cost-effective, more facile and eco-friendly. Instead of chlorofluorocarbons (CFC), supercritical fluid (SCF) based foaming technology involving with
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eco-friendly gases, such as CO2 and N2, is being globally pursued as a viable alternative to current foaming processes for large scale production of polymer foams.51-58 In situ production of skin layer covering the foam structure,51-58 is another most important advantage of these foaming techniques, which avoids the tricky operation mode and large working space, needed during development of bi-layer skin through layer-by-layer attachment process.2-4 Many researchers have studied the effect of foaming conditions (such as temperature, pressure, duration, etc.) on the quality of TPU foams.50-58 But, no such discussion has yet been reported to compare the foaming aptitude of different SCFs such as CO2 over N2 for production of TPU foam with adequate flexibility towards development of futuristic bi-layer artificial skin having potential tactile sensing capability. Here we report comparative foaming performance of CO2 over N2 to design TPU foam based S-TENG as prototype bi-layer artificial skin having adequate softness and flexibility, which can potentially recognize different objects due to contact electrification. This simple device is composed one induction electrode attached with the skin layer of TPU foam. When an active object contacts the film surface, triboelectricity generates due to contact electrification between the object’s surface and S-TENG surface. As a result, the S-TENG respond differently towards arbitrarily chosen different objects, such as, Al-foil (AF), rubber-balloon (RB), cottonglove (CG), bare human-finger (HF), glass-slide (GS), latex rubber-glove (RG), artificial leather (i.e., mobile cover (MC)), polyimide-film (PF), PTFE-sheet (PS), tissue-paper (TP), wood (W), and also produce maximum of ~26 volt cm-2 when the objects were PS, HF, GS and RB, and minimum of ~10 volt cm-2 of output voltage when the objects were CG, MC, PF, TP, etc. Further, the object manipulation performance has satisfactorily been visualized by principal component analysis (PCA).59-61 Artificial skin made of TPU foam lacking adequate flexibility has been discarded when N2 was used. During optimization of temperature effect on CO2foaming, it was observed that the TPU foam with effective cell size and cell density (for 4 ACS Paragon Plus Environment
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fabrication of efficient S-TENG), was obtained at higher foaming temperature (i.e., 150°C when compared with 130°C). EXPERIMENTAL SECTION Preparation of TPU foams using SCF (viz., CO2 and N2): TPU sheet (obtained from compression molding) was enclosed in a high-pressure stainless steel vessel equipped with a heating system. After thermal equilibrium, gas (viz., CO2 or N2) was charged into the vessel to obtain the desired pressure (i.e., 15 MPa). At this condition, gases behave like supercritical fluid (SCF). The pressure was kept constant and continued for 2 hours to ensure equilibrium absorption of gas by the matrix, and then it was rapidly (
