Friction Enhancement between Microscopically Patterned

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Friction enhancement between microscopically patterned polydimethylsiloxane and rabbit small intestinal tract based on different lubrication mechanisms Hongyu Zhang, Ying Yan, Zhibin Gu, Yi Wang, and Tao Sun ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.5b00558 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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ACS Biomaterials Science & Engineering

Friction enhancement between microscopically patterned polydimethylsiloxane and rabbit small intestinal tract based on different lubrication mechanisms Short title: PDMS-intestine interfacial friction

Hongyu Zhang1, *, Ying Yan2, Zhibin Gu3, Yi Wang1, Tao Sun1

1

State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua

University, No. 1 Tsinghuayuan, Haidian District, Beijing 100084, China. 2

Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of

Education, Department of Mechanical Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116024, China. 3

Institute of Electronics, Chinese Academy of Sciences, No. 19 of North 4th West Road,

Haidian District, Beijing 100190, China.

*

Correspondence to Dr. Hongyu Zhang

State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, No. 1 Tsinghuayuan, Haidian District, Beijing 100084, China. Tel: +86 010 62773129 Fax: +86 010 62781379 Email: [email protected]

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ABSTRACT A control of friction characteristics between self-propelled robotic system and gastrointestinal tissues plays a key role in achieving active locomotion. Fabrication of micro-patterns on soft polymers has been proposed to enhance frictional traction. In the present study, micro-pillar arrays with different diameters of 60-140 µm were prepared on polydimethylsiloxane (PDMS) by soft lithography, and a series of friction tests were performed between microscopically patterned/non-patterned PDMS and rabbit small intestinal tract (SIT) on a universal material tester, with the record of friction coefficient under various experimental conditions (sliding speed: 0.25 mm/s; sliding distance: 40 mm; applied loading: 0.4-1.0 N). Surface morphology of microscopically patterned PDMS samples was evaluated by scanning electron microscopy (SEM) before and after the friction tests. It was demonstrated that micro-pillar arrays aligned regularly on the microscopically patterned PDMS samples and maintained the shape after friction tests. At 0.4 N, the friction coefficient of PDMS samples with the micro-pillar diameter from 80 µm to 140 µm presented a decreasing trend, which was significantly larger than that of non-patterned PDMS samples. However, the smallest friction coefficient (~0.12) was obtained for the 60-µm micro-pillar diameter PDMS samples. In addition, the friction coefficient of non-patterned PDMS samples decreased as the applied loading varied from 0.4 N to 1.0 N, whereas the 60-µm micro-pillar diameter PDMS samples showed an opposite trend. It is proposed that the enhancement in friction between PDMS and SIT, which is achieved through the introduction of micro-pillars, may be determined based on different lubrication mechanisms. Keywords: friction; surface texture; biotribology; capsule endoscopy; robotic system

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1. INTRODUCTION The introduction of wireless capsule endoscopy (WCE) is regarded as a great invention in the examination of gastrointestinal tract disorders, which enables visual inspection of potential lesion or even cancerous growth.1 These diagnostic capsules were approved for clinical use by Food and Drug Administration, with great advantages in alleviating discomfort or trauma over conventional techniques, such as gastrointestinal endoscopy.2 However, WCE moves passively in gastrointestinal tract by exploiting peristalsis, as a result a prolonged diagnosis at the suspicious site can not be achieved intentionally. Additionally, the contradiction between the improvement in image quality/quantity and the requirement of low energy consumption usually limits its usage to one organ.3,4 As a consequence, a self-propelled robotic system, which integrates microelectromechanical mechanisms for active locomotion and navigation inside gastrointestinal tract, represents a promising methodology and a valuable revolution in minimally invasive therapy.5 However, the presence of a mucus layer covering intestinal mucosa results in a slippery surface, making active locomotion and navigation of the miniaturized robotic system quite challenging.6 In the past few years, various robotic systems have been developed, in which hooks or spikes are fabricated for the actuation parts in order to facilitate efficient movement of the robotic system in the complicated gastrointestinal environment.7,8 Nevertheless, in these designs the sharp grippers contact directly with the intestinal wall, as a result the intestinal tissues are prone to be injured if the robotic system is not properly steered. In order to achieve active locomotion and navigation, a control over the friction behaviors between self-propelled robotic system and intestinal wall is exceedingly important. Due to 3



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the intrinsic slippery nature of the intestinal wall, the robotic system represents a typical scenario where friction is to be maximized to generate an optimal traction force.9,10 Over the years, different strategies have been introduced in an attempt to strengthen the interaction between the robotic system and the intestinal wall, e.g. adhesive films and micro-patterns. Dodou et al. reported that the application of a mucoadhevie film generated a larger friction force due to the formation of molecular bonds with the mucus layer.11,12 With the rapid development of microelectromechanical systems, the friction characteristics of micro-patterns, which can be integrated into the legs of a robot, on soft materials attract more and more attention.13-15 A typical study in this area was the one performed by Lee et al. in 2010, in which the friction properties were greatly enhanced on intestinal tissue by using micro-patterned polyurethane acrylate polymer with controlled shape and geometry.16 Additionally, inspired from biomimetic texture in nature, Varenberg and co-workers investigated in detail the effect of hexagonal surface micro-pattern on dry and wet friction17-19, and they recognized that the hexagonal texture had a friction-oriented feature capable of suppressing both stick-slip and hydroplaning, while enabling friction tuning.20 Based on a systematic review of previous studies, one potential drawback is that the apparatus used to measure friction force is custom-built tribo-tester, as a result the accuracy may not be guaranteed without a proper calibration. Furthermore, few studies have deeply examined the mechanism resulting in the friction enhancement by these micro-patterns. It is envisaged that an insight into this issue would provide valuable guidance for the development of an optimized robotic system. Consequently, in the present study we intend to investigate the friction characteristics between microscopically patterned polydimethylsiloxane (PDMS) and

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rabbit small intestinal tract (SIT) through a standard commercial instrument, which has been commonly used in our previous research21-24 and to propose a potential lubrication mechanism for the friction enhancement. 2. MATERIALS AND METHODS 2.1 Fabrication of micro-pillars on PDMS The flexible and biocompatible polymer, i.e. PDMS (Dow Corning Corp., MI, USA), was used as substrate for micro-fabrication. The micro-patterns were prepared by soft lithography in a class 1000 clean room, which generally included two steps: (1) Preparation of master mould, Figure 1 (a)-(c). Firstly, negative photoresist (SU-8) was spin-coated onto a clean silicon wafer, and soft baking was performed in a drying oven. Then, the photoresist film was irradiated by ultraviolet light at hard-contact mode, using a photomask with microscopical holes. Finally, post-exposure baking was performed in the drying oven, and the photoresist film was developed at room temperature before drying with compressed air. (2) Preparation of micro-pillar arrays, Figure 1 (d)-(f). Basically, a replica of micro-pillar arrays was obtained by spin-coating PDMS onto SU-8 master mould, and then being peeled off from the mould with the use of trimethylchlorosilane (TMCS). The height of the micro-pillars was fixed at 100 µm, which was determined by the thickness of the spin-coated photoresist film. The diameter of the micro-pillars varied from 60 µm to 140 µm by using photomasks with different hole sizes, with edge-to-edge spacing the same value as the micro-pillar diameter.

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Figure 1: Typical procedures of soft lithography to fabricate micro-pillar arrays on PDMS: (a) spin-coating of SU-8 on silicon wafer; (b) ultraviolet exposure; (c) development and drying; (d) evaporation of TMCS on SU-8 master mould; (e) spin-coating of PDMS on SU-8 master mould; (f) micro-pillars on PDMS following peeling-off. 2.2 Friction tests between PDMS and SIT 2.2.1 Preparation of PDMS samples and SIT Two types of PDMS samples (diameter: 6 mm; height: 1.5 mm) were manufactured, namely non-patterned (i.e., flat) surface and micro-patterned surface, with the latter prepared by soft lithography. Additionally, an amount of medical-grade stainless steel cylinders (diameter: 6 mm; height: 8 mm) were fabricated, and the PDMS samples were attached to the stainless steel cylinders with cyanoacrylate adhesive, acting as the upper specimen in the friction tests. Rabbit intestinal tissues were kindly supplied by Center of Biomedical Analysis, Tsinghua University, China. SIT was maintained in 0.9% saline solution immediately after sacrifice of the rabbit, and used as the lower specimen in the friction tests within 3 h. Therefore, surface

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degradation in the mechanical property of SIT can be ignored. Particularly, the mucus layer covering intestinal mucosa was kept intact, and SIT was maintained wet by periodically sprinkling with saline solution during the friction tests. 2.2.2 Friction tests The friction characteristics between PDMS and SIT were measured using a universal material tester (UMT-3, Centre for Tribology Inc., CA, USA). As shown in Figure 2, the PDMS sample was mounted to the stainless steel cylinder, which was further connected with a force sensor (DFM-0.5, a proprietary and patented design by the company, range: 0.05-5 N, resolution: 0.25 mN) of the tester. The SIT was carefully cut into appropriate size and stably positioned on a custom-made stainless steel platform (designed to connect with a linear stage of the tester) using two stainless steel plates and four screws. The temperature was controlled at ~37 ºC by using a heating rod and a magnetic block as the temperature feedback.

Figure 2: The universal material tester and custom-made platform designed for experimental investigation of the friction characteristics between PDMS sample and SIT. 7



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Sliding of PDMS sample (non-patterned and micro-patterned) on SIT was accomplished by actuating the linear stage, and the friction coefficient-dragged distance plot was recorded simultaneously. The sliding distance was 40 mm, and the sliding speed was 0.25 mm/s. Two experimental conditions were investigated: (1) non-patterned PDMS samples and microscopically patterned PDMS samples with different micro-pillar diameters (60-140 µm), constant applied loading (0.4 N); (2) non-patterned PDMS samples and microscopically patterned PDMS samples with the same micro-pillar diameter (60 µm), different applied loadings (0.4-1.0 N). For each friction test, a new PDMS sample was used to slide along a different track on the SIT, and a new SIT was changed when necessary. All friction tests were performed four times, and eventually the average values were calculated and compared under different experimental conditions. 2.3 Characterization of PDMS samples and SIT Prior to the friction test, the microscopically patterned PDMS samples were sputter-coated with a platinum layer by an etching & coating system (Gatan 682, Pleasanton, USA), and the surface morphology of the samples was evaluated using a Quanta 200 FEG scanning electron microscope (SEM, FEI, Eindhoven, Netherlands) associated with an energy dispersive X-ray (EDX) analysis. Additionally, the longitudinal cross section of the microscopically patterned PDMS samples was prepared and the height of the micro-pillars was characterized using a BX60 optical microscope (Olympus, Tokyo, Japan). Following the completion of each friction test, a visual examination of the SIT was initially performed for potential injury of the tissue, and the PDMS sample was collected, sputter-coated, and examined using the SEM to detect the presence of mucus layer on the surface. 8


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2.4 Statistical analysis The results of friction coefficient obtained from two experimental conditions were presented as mean value ± standard deviation, and analyzed using Statistical Product and Service Solutions 19.0 software (SPSS, Chicago, IL, USA). The data were tested for normal distribution using Kolmogorov-Smirnov Test prior to the statistical analysis. The differences among the friction coefficients from experimental condition (1) and (2), as a function of micro-pillar diameter and applied loading respectively, were evaluated through one-way analysis of variance (post hoc multiple comparison: least-significant difference test). The comparison of friction coefficient under different applied loadings between non-patterned and microscopically patterned PDMS samples with the micro-pillar diameter of 60 µm was performed using independent sample t-test. The statistical significance was set at p

Friction Enhancement between Microscopically Patterned

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