Polydopamine Coating for Thermal Insulation of ...

Proceedings of the ASME 2016 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS2016 September 28-30, 2016, Stowe, VT, USA

SMASIS2016-9265

POLYDOPAMINE COATING FOR THERMAL INSULATION OF SHAPE MEMORY ALLOY WIRES Mohammad Sahlabadi, Yao Zhao, Kyle Jezler, David Gardell, Harold H. Lee Fei Ren, Parsaoran Hutapea Department of Mechanical Engineering Temple University Philadelphia, PA 19122, USA ABSTRACT Shape memory alloy (SMA) materials, such as Nickel Titanium (NiTi), can generate stress and strain during phase transformation induced by thermomechanical stimulation. Therefore, they may be used to construct active actuating devices for various biomedical applications such as smart surgical tools. Since temperature rise during the operation of SMA devices may damage the surrounding tissue, it is important to thermally shield such devices. We propose to use polydopamine (PDA) as an insulating coating for NiTi-based smart needles. PDA is a biomolecule (dopamine) derived polymer that can form conformal coating on various materials including NiTi. It is hypothesized that the surface temperature of the PDA coated needle can be reduced by the low thermal conductivity of PDA and the thermal resistance of the PDA/NiTi interface. Our experiments conducted in ambient air at room temperature showed that the coating reduced the surface temperature by as much as 45%. In this paper, we will present the thermal insulating performance of the PDA coating on NiTi wires. An experimental setup where the wire is embedded inside a gel phantom/tissue has been developed to simulate needle-tissue interaction. Effects of the coating thickness (material thermal resistance) and the number of layers (interfacial thermal resistance) will be discussed. 2D finite element analyses (FEA) were performed using ABAQUS to investigate the thermal distribution around the coated NiTi wires and the tissue gel phantom. In addition, using thermal distribution, potential tissue damage was assessed.

Shape memory alloy (SMA) materials, such as Nickel Titanium (NiTi), can generate stress and strain during phase transformation induced by thermomechanical stimulation. Therefore, they may be used to construct an active actuating device for various biomedical applications such as smart needles. Since temperature rises during the operation of SMA devices may damage the surrounding tissue, so it is important to thermally shield such devices. Nitinol alloys (NiTi) show two unique properties. Those two properties are superelasticity and shape memory effect. When Nitinol undergo deformation in one temperature, it can recover its original undeformed state if we increase NiTi temperature above its transformation temperature, this effect is called Shape memory effects (SM). In a temperature slightly above the transformation temperature of Nitinol, it shows a big range of elasticity, which could be 10-30 times larger than ordinary metals. The mentioned effect is called the superelasticity effect. Polydopamine (PDA) is a composition of dopamine, dihydroxy indole, and indole drone units, which are linked to one another through covalent bonds [3, 4]. PDA is created by the oxidation of dopamine. PDA are generally formed and used for coating various types of material. In order to test the effectiveness of PDA coating to reduce the surface temperature of the wire, a transparent PVC gel has been made and used to simulate situation inside human tissue [5]. In this study a current applied to the NiTi wire. The applied current increases the temperature of the wire and induces it to shrink. To estimate the surface temperature of the wire, with and without coating, an infrared (IR) thermal camera and a thermocouple have been used simultaneously during the experiment to record surface temperature of the wire and the thermal distribution around the wire and inside the gel. Finally, results obtained from the experiment were compared with finite element results.

INTRODUCTION Smart materials have been widely used to make smart devices, namely devices with biomedical applications like smart needles [1, 2]. Hutapea and his group [1, 2] utilized NiTi to control needle tip movement, which can be used to guide needle to reach a specific organ by passing obstacles inside the human body.

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Copyright © 2016 by ASME

NOMENCLATURE A = ampere AFM = atomic force microscopy D = diameter of the NiTi wire FEA = finite element analysis NiTi = Nitinol PDA = Polydopamine PVC = Polyvinyl chloride SEM = scanning electron microscopy STDEV= standard deviation T= temperature t = thickness of the coating

cross-section of a PDA-coated NiTi wire, where the PDA coating can be identified.

MATERIALS AND METHODS In this study, PDA films were applied onto NiTi wires using a dip coating method. Coating was conducted in mild base solutions at room temperature. Samples with different PDA layer thickness and total number of layers were fabricated. The thickness of the PDA coating was controlled by varying the coating time. Thermal properties of the PDA were studied using differential scanning calorimetry and thermogravity analysis. Coating morphology and thickness were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). To study the thermal insulation performance, NiTi wires with and without PDA films were inserted into a gel phantom [5]. Electrical current was applied through the wires, and the temperature distribution in the gel surrounding the wires was measured using thermalcouples and infrared (IR) imaging camera. The thermocouple was attached to the wire using a high temperature adhesive. Finite element analysis (FEA) was performed to study the temperature distribution on the gel phantom. Since the thermal properties of the polydopamine such as thermal conductivity and specific heat are currently being measured, the FE model was developed to include only the gel phantom and to use the experimental temperature on the interface of the coating and gel phantom as boundary conditions. When the thermal properties are determined, FEA study will be conducted to numerically predict the effect of coating on the insulation performance of the coated NiTi wire and to estimate the potential amount of damaged inside tissue caused by NiTi wire.

Figure 1: SEM images of NiTi wire with and without PDA coating. (a) NiTi wire without PDA coating. (b) NiTi wire with PDA coating. (c) The cross-section of NiTi wire (brighter) with PDA coating (darker film). (d) The overlay image of secondary/backscattered electrons of PDA coating (orange) on the surface of NiTi alloy (blue).

A preliminary study using NiTi wires with diameter of 0.25 mm was performed. The uncoated and coated wires were tested using the test setup as shown in Fig. 2. The NiTi wire was dipped inside of the PDA solution, which was made by adding dopamine hydrochloride into Tris buffer. The NiTi wires were activated using Joule heating by applying current as a ramp function and wire temperature was measured using k-type thermocouple, which is attached to the wire by a high temperature adhesive. Note that in this experiment, the wire was exposed to the ambient air.

RESULTS The NiTi wires can be coated with PDA, and the thickness varies from tens to hundreds of nanometers. The thickness of the coating on the wire can be observed using SEM (Fig 1). Fig. 1(a) shows a bare NiTi wire surface without PDA coating. The grain boundaries can be clearly identified. Fig. 1(b) shows a PDA-coated NiTi wire surface, where a continuous film was observed and the grain boundaries became blurry to distinguish. Fig. 1(c) is a backscattered electron image of the cross-section of a coated NiTi wire. Fig. 1(d) shows a combined secondary/backscattered electron microscopy image of the

Figure 2: Schematic of test setup to measure the surface temperature on the coated and uncoated wires by using thermocouples.

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In this study, 30 test specimens with NiTi wires with different diameters and coating conditions embedded on the tissue gel phantom have been tested (see Fig. 4a).

In this preliminary study, a series of experiments on coated and uncoated wires were performed. As shown in Fig. 3, the average surface temperature is about 80-82ºC on the uncoated wires and 47-48ºC on the coated wires. This represents a 45% reduction of temperature when the wires were coated by PDA.

Table 1: Number of different wire diameter with different coating time (2, 3, 6, 24, and 48 hours) Wire Diameter (in) 0.008 0.010 0.012

Uncoated 2 2 4

2 hrs 2

3 hrs 2

6 hrs 2 2 4

24 hrs 2 2 4

48 hrs 2

The preliminary result for the surface temperature of the wire inside the gel is shown in Fig. 5. From the experiment using an IR camera, it is shown that the surface temperature of the wire reduced by approximately 30% at the highest current/temperature for both 6-hour and 24-hour-coated wires.

Figure 3: The average surface temperature as a function of time on 24-hour coated and uncoated NiTi wires tested in ambient air.

In order to simulate the influence of the temperature change of the wire on tissues, an experimental setup was developed such that the wires are embedded in a gel phantom. Fig. 4(a) shows the experimental setup, which is used in this study. The gels are made of PVC in combination with a softener, and wire placed inside the gels in a way which all its part is in full contact with the gel. As a result of the good connection, conduction is believed to be the major mechanism of heat transfer. Fig. 4(b,c,d) represent the temperature distributions around the uncoated, the 6-hour-coated and the 24-hour-coated NiTi wire inside the gels, respectively.

Figure 5: Applied Current vs Temperature for uncoated, 6, and 24_hour-coated wires. Polynomial fits were presented.

In order to investigate the consistency of our experiments, the average and standard deviation of the surface temperature for 24-hour-coated NiTi wires in different current amplitude was calculated (Fig. 6). The standard deviation indicates that data obtained from the four experiments is repeatable (Table 2).

Figure 4: Temperature distribution around the Ni-Ti wires and inside the PVC gels measured using an IR camera. (D= 0.01 in, t= 50 nm) (a) Experimental setup. (b) The uncoated wire, (c) the 6hour-coated wire, (d) the 24-hour-coated wire.

Figure. 6: Standard deviation and average of surface temperatures of 24-hour-coated NiTi wires.

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Table 2: STDEV and average data obtained for 24-hour-coated NiTi wires. Current Data set #1 Data set #2 Data set #3 Data set #4 Average STDEV

0.4A 34.6 34.6 34.6 34.6 34.6 0

0.5A 43.4 38.8 39.0 41.5 40.68 2.19

0.6A 47.9 46.7 48.2 47.5 47.58 0.65

0.7A 55.2 55.6 52.4 54.9 54.53 1.44

Finite Element Study A 2D FE model was generated and analyzed using ABAQUS (Fig. 7). In order to simplify the model, the wire temperature obtained from experiments was applied to our FE model as a boundary condition. With this assumption, the surface area of the wire was modeled as a hole. The analyses were then performed using transient heat transfer analysis to obtain the thermal distribution contours inside the gel. The minimum temperature of tissue damage in human is 48°C for 300s of thermal exposure, [6].

Figure 8: Thermal distribution around the 0.01 NiTi wire obtained from transient heat transfer analysis using ABAQUS, (a) for uncoated wire, (b) for 6-hour-coated wire.

DISCUSSION Our results (Figures 3 and 5) indicate the PDA coating can effectively reduce the surface temperature of an activated Ni-Ti wire. Two mechanisms may be responsible for this effect: low thermal conductivity of the PDA film, and the NiTi/PDA interface. Further studies are needed and being performed. In particular, detailed study is being carried out to evaluate the contribution of the two factors to determine the major contributor to the enhanced thermal resistance. In addition, the thermal property measurements of the polydopamine and finite element analyses (FEA) are currently ongoing to study in much more details the thermal distribution on the interface of the wires and tissues. The FEA results will be used to optimize the coating parameters including the thickness and number of NiTi/PDA interfaces.

Figure. 7: An FE model and with initial and boundary conditions.

As shown in Fig. 8, the damage areas are like rings (Fig. 8(a)) with a thickness of 0.71 mm (A=2.26mm2) for uncoated wire (Fig. 8(b)), a thickness of 0.28 mm (A=0.45mm2) for 6hour-coated wire (Fig. 8(c)) and with a thickness of 0.37 mm (A=0.78mm2) for 24-hour-coated wire (Fig. 8(d)). It can be seen that the PDA coating decreases the area of the potential damaged part inside the tissue by 60-85%.

ACKNOWLEDGMENTS The authors thank the Department of Defense (DoD) CDMRP Prostate Cancer Research Program (Grants# W81XWH-11-1-0397/98/99) for the funding support and Temple University OVPR Seed and Bridge Grant Funds. REFERENCES

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