Fundamental Property Assessments of Biocompatible Silk–Pt

Keywords: Electroless plating; Supercritical carbon dioxide; Pt metallization;. Biocompatibility; Corrosion tests. Page 2 of 35. ACS Paragon Plus Envi...
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Fundamental Property Assessments of Biocompatible Silk−Pt Composite Prepared by Supercritical Carbon Dioxide Promoted Electroless Plating Wan-Ting Chiu,*,†,‡ Yuma Tahara,§ Chun-Yi Chen,†,‡ Tso-Fu Mark Chang,†,‡ Tomoko Hashimoto,§ Hiromichi Kurosu,§ and Masato Sone†,‡ †

Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan CREST, Japan Science and Technology Agency, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan § Department of Clothing Environmental Science, Nara Women’s University, Kitauoya, Higashimachi, Nara 630-8506, Japan ‡

S Supporting Information *

ABSTRACT: This study reports preparation and characterization of biocompatible, conductive, and flexible silk−Pt composite materials for applications in wearable and medical devices. The distinct Pt and silk materials were integrated via supercritical carbon dioxide (sc-CO2) promoted electroless plating. Sc-CO2 was introduced into the catalyzation step, which is a critical step in electroless plating, to overcome the common difficulty of inlaying the catalyst into the textile substrate. High surface coverage and thick Pt layer were constructed as the metallization time extended. Metal ions released in a simulated body fluid in an immersion test were negligible when compared to the daily metal input and output of the human body. The composites showed high corrosion resistances in both 3.5 wt % NaCl and simulated body fluid solutions. The electrical conductivity and corrosion resistance persisted after the adhesion tests. The assessments revealed the applicability of this silk−Pt material to wearable and medical devices.

1. INTRODUCTION Wearable devices and implantable medical devices have been greatly developed in this decade, and their use is predicted to keep blossoming in the near future.1 Wearable devices composed of flexible and conductive materials are in high demand because of the requirement of combining electronic devices with clothes or the human body. Thus, the integration of distinct materials such as textiles and metals becomes the greatest challenge. There are many techniques to integrate distinct materials together, such as sputtering, ink printing, and dipping processes.2 Electroless plating,3−5 also known as autocatalytic plating, can firmly deposit metallic materials onto a substrate. In addition, the substrate can be nonconductive, flexible, and irregularly shaped. Electroless plating, owning simple procedures and equipment, can be operated without external electrical power. There are three major procedures in electroless plating: the first one is a pretreatment step to clean and roughen the substrate, catalyzation is the second step to inlay the activate nucleation sites on the substrate, and metallization is the last step to deposit a layer of coating on the substrate. Supercritical carbon dioxide (sc-CO2) is introduced to the catalyzation step to improve the electroless plating characteristics in this study.6,7 CO2 develops into the supercritical fluid state, owning properties midway between a gas and a liquid © 2017 American Chemical Society

when the temperature and pressure are beyond its critical point (31 °C and 7.4 MPa). In the conventional catalyzation step, the catalysts are inlaid on the substrate by treatment with SnCl2 and PdCl2 solutions, which are also corrosive.8 Thus, the substrate is damaged during the catalyzation step, and the catalysts are merely inlaid on the substrate surface. Although extending the immersion time in the catalyzation solutions might be a possible solution to embed the catalyst into the polymer substrate, the substrate is damaged severely in the meanwhile. The difficulties can be overcome by introducing scCO2 to the electroless plating system.8 With the affinity to nonpolar materials,9 high self-diffusivity,10 and low viscosity11 of sc-CO2, it therefore can carry the catalyst into the nonpolar polymer substrate while the substrate structure remains undamaged. This catalyzation technique also allows exclusion of the pretreatment steps. In addition, it can further cut down the usage of chemical solutions in the catalyzation step, which leads to a more environmental friendly process. Therefore, the sc-CO2-promoted Pt electroless plating on silk textile is utilized Received: Revised: Accepted: Published: 8864

April 25, 2017 June 9, 2017 July 14, 2017 July 14, 2017 DOI: 10.1021/acs.iecr.7b01749 Ind. Eng. Chem. Res. 2017, 56, 8864−8871

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Industrial & Engineering Chemistry Research

2.2. Electroless Plating. The silk substrate was hooked on top of the reaction cell. Twenty-five milligrams of Pd(acac)2 was introduced into the cell to maintain the catalyst concentration at the saturation point throughout the catalyzation process. A magnetic stir bar was enclosed into the reaction cell to provide the agitation. The catalyzation conditions were set at 80 ± 1 °C and 15 ± 0.1 MPa in the reaction cell with agitation for 2 h. No post-treatment was carried out after the catalyzation. The Pt metallization was carried out at 70 ± 1 °C under ambient pressure with agitation. Various metallization times were performed to examine the relationship between the metallization time and the silk−Pt characteristics. No post-treatment was conducted after the metallization. In order to confirm the reduction of Pd(acac)2 to Pd metal, a piece of sc-CO2 promoted catalyzation treated silk was immersed in a 2 vol % reduction agent solution (pH = 12) at 70 ± 1 °C under ambient pressure with agitation for 30 min. 2.3. Characterization. 2.3.1. Surface Morphology. The surface morphology of the Pt metallized silk was observed through an optical microscope (OM; VHV-5000, KEYENCE, Japan) and a scanning electron microscope (SEM; JSM-7500, JEOL, Japan). The thicknesses of the Pt layer were estimated by observing the cross section of the silk−Pt composite using the SEM, and the image was processed by Image-J (National Institutes of Health, U.S.A.) to calculate the thickness. The crystal structure and phases were identified by an X-ray diffractometer (XRD; Ultima IV, Rigaku, Japan). The SEM and an energy-dispersive X-ray spectroscope (EDX; EMAX Evolution, HORIBA, Japan) equipped in the SEM were used to evaluate the composition and coverage of the Pt layer on the silk substrate. 2.3.2. Polarization and Immersion Methods. The corrosion resistance was measured by polarization measurements in both 3.5 wt % NaCl and a SBF solution (r-type).22 The 3.5 wt % NaCl solution was used to simulate human sweat.14 These two solutions were thus chosen for the in vitro and in vivo simulation, respectively. The potentiodynamic polarization curves were measured by using a Potentiostat/Galvanostat (1287A, Solartron Analytical, UK). The silk−Pt composite with 120 min of the metallization time, a Pt plate, and a Ag/AgCl reference electrode were used as the working electrode, the counter electrode, and the reference electrode, respectively. The total surface area of the working electrode, which is a piece of the 1 × 1 cm2 silk−Pt composite, was calculated by considering the surface area of every bundle of the Pt metallized silk textile, and the value was 9.5 cm2. The Pt counter electrode was cleaned ultrasonically in ethanol and pure water for 1 min, respectively, before the polarization measurements. The scan range started from −0.4 to +0.6 V vs Ag/AgCl at a scan rate at 1 mV/s. Prior to the potentiodynamic test, the three electrodes were immersed into the solution for 25 min to certify the stability. The metal ion releasing rate was evaluated by an immersion test. In the immersion test, a piece of the sample was immersed in the SBF in a 30 mL container at 37 °C for three months. Then, ∼15 mL of the SBF solution was drawn out and centrifuged for 2 h to remove undesired particles from the solution. The released metal ion concentration was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES; ICPS-8100, Shimadzu Co. Ltd., Japan). The ICP measurement was carried out ten times to ensure reliability of the values reported.

in this study in an effort to overcome the traditional difficulties for the wearable and medical device applications. Materials selection for the wearable device is an important topic among the critical biocompatibility issues. Common wearable devices are made of Ni alloys because of its simple procedures and high corrosion resistance.12−15 However, allergic reaction occurs due to the nonbiocompatible property of Ni.16 Platinum metallization was chosen in this study for being biologically inert17 to avoid the allergic reactions. In addition, Pt also has high corrosion resistance,18 good electrical properties,19 and high thermal stability. Based on the aforementioned advantages, Pd(acac)2 catalyzation followed by Pt metallization was carried out in this study. On the other hand, silk is a common material for clothing, bearing biocompatibility and flexibility. It was thus chosen as the substrate in this study. Although a method for the fabrication of a silk−Pt wearable device was performed in the previous literature, the process efficiency is low, which becomes an issue in practical application.20 A modified catalyzation step was conducted in this study to improve the efficiency. The metallized silk samples further showed higher electrical conductivity, lower metal ion releasing rate, and better biocompatibility than those reported in the literature.20 Corrosion resistance is a critical property for the application due to the exposure to liquids during the employment. Corrosion tests were carried out in two different solutions to evaluate the corrosion resistances of the silk−Pt composite materials. Furthermore, a critical issue concerning biocompatibility assessment mentioned in the previous paragraph must be addressed for the targeting application in wearable and medical devises. According to Wataha,21 allergic reactions would be triggered when there is metal ion released from the environment that enters the human body. The metal ion released from the silk−Pt composite becomes a criterion for evaluating the biocompatibility. For this reason, metal ionreleasing rate was examined by immersing the silk−Pt composite in a simulated body fluid (SBF) for three months to assess the biocompatibility. In addition, electrical conductivity is an essential property for applications in electronic devices. Hence, the electrical resistance of the silk−Pt composite was evaluated by an in-line four-point probe measurement in this study. On the other hand, wearable devices are subjected to an external disturbance frequently while being used; assessments for the reliability thus are also a fundamental prerequisite. An adhesion test to evaluate the adhesion firmness between the Pt layer and the silk substrate was conducted in this study.

2. EXPERIMENTAL PROCEDURES 2.1. Materials. A 50 mL stainless steel cell was used as the catalyzation cell. Silk textile with dimension at 2 × 4 cm2 and Pd(acac)2 (99%, Tokyo Chemical Industry Co. Ltd., Japan) were used as the substrate and the catalyst, respectively. A highpressure apparatus8 (Japan Spectra Company, Japan) was used to inject CO2 (99.99%, Nippon Tansan Gas Co. Ltd., Japan) into the reaction cell. A furnace was used to keep the temperature during the catalyzation step. The Pt metallization electrolyte consisted of 50 vol % of a basic Pt solution (Pt1505281, Matex Co. Ltd., Japan) and 2 vol % of a N2H4 reducing agent solution (Pt1505282, Matex Co. Ltd., Japan). The pH value was tuned to 12 by ammonia solution (01266-00, Kanto Chemical Co. Inc., Japan). 8865

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Industrial & Engineering Chemistry Research 2.3.3. Electrical Resistance and Adhesion Tests. An in-line four-point probe with 1.5 mm distance between each probe (MCP-T370, Mitsubishi Chemical Analytech Co. Ltd., Japan) was used to evaluate the electrical resistance with the samples dimension in 1 cm × 0.5 cm (length × width) at room temperature. An adhesion test was conducted to evaluate the reliability of the silk−Pt composite. For the adhesion test, a piece of 3 M tape (810 Scotch Magic Tape, 3M, USA) was firmly stuck to the sample surface by applying a 1-kg load and peeled off repeatedly. The adhesion tests with one and two cycles of the sticking−peeling procedure are named first adhesion test and second adhesion test. The electrical resistance and corrosion resistance were measured before and after the adhesion test to evaluate the durability and firmness of the Pt layer. The electrical resistance measurement was carried out ten times to ensure the reliability of the values reported and construct the error bar.

3. RESULTS 3.1. Morphology and Structure of the Catalyzed Silk. Figure 1 shows the OM images of the nontreated pure silk, catalyzed silk, and catalyzed silk with reduction treatment. Transparent thread bundles shown in Figure 1(a) indicate the silk textile; light yellow flakes shown in Figure 1(b) correspond to Pd(acac)2 catalysts, and the small particles with metallic luster shown in Figure 1(c) are the reduced Pd metals. As shown in Figure 1(b), the catalyst is successfully settled on the substrate without damaging the silk structure. After the reduction, small Pd particles finely remained on the substrate shown in Figure 1(c). XRD patterns of the (a) pure silk, (b) catalyzed silk, and (c) catalyzed silk with reduction treatment are shown in Figure 2. The pure silk shows one broad peak at around 2θ = 20° in Figure 2(a). On the other hand, after the catalyzation, all the XRD characteristic peaks can be indexed to the contribution from Pd(acac)2,23,24 shown in Figure 2(b). Figure 2(c) shows the XRD pattern of the reduced catalyst on silk. Four diffraction peaks at 2θ = 40.1°, 46.7°, 68.1°, and 82.1° can be indexed to (111), (200), (220), and (311) planes of the FCC structure of palladium (JCPDS #89-4897). 3.2. Morphology and Structure of the Pt Metallized Silk. The three SEM images shown in Figure 3 demonstrate the trend of coverage of the Pt layer on the silk with various metallization times. Figure 3(a) shows the SEM image of the sample with 80 min of the metallization time, where the bright phase corresponds to nonmetallized silk while the gray phase indicates Pt metal. Since the electron cannot be conveyed on the nonconductive silk, thus the electrons accumulated on the nonmetallized silk would show the charging effect. At 80 min of the metallization time, some parts of silk textile were nonmetallized (pointed by arrows), showing a noncontinuous Pt layer and a smooth surface on the metallized part. Figure 3(b) shows the morphology of the sample with 120 min of the metallization time; a continuous Pt layer was constructed showing gray color thoroughly, and the surface remained smooth. When the metallization time was extended to 180 min (Figure 3(c)), the silk substrate was also completely covered. However, undesired Pt clusters accumulated on the surface, and the metallized surface became rough (pointed by arrows). In the same way, elemental mapping of the Pt metallized silk (Figure 4) shows the critical time demarcation of full coverage, which lies between (a) 80 and (b) 100 min of the metallization time. The least time needed to construct full coverage is defined as the minimum time, which is 100 min. The

Figure 1. OM images of (a) nontreated pure silk, (b) catalyzed silk, and (c) catalyzed silk after reduction.

Figure 2. X-ray diffraction patterns of (a) nontreated pure silk, (b) silk with catalyzation treatment, and (c) catalyzed silk with the reduction treatment.

composition results show a pure Pt layer with merely trace amount of impurity. Figure 5 shows the XRD pattern after the Pt metallization. Five diffraction peaks located at 2θ = 40.2°, 8866

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Figure 4. Elemental mapping of metallized silks at (a) 80 min and (b) 100 min of the metallization time.

Figure 5. X-ray diffraction pattern of Pt metallized silk substrate.

Figure 3. Pt metallization trend demonstrates by SEM images at (a) 80 min, (b) 120 min, and (c) 180 min of the metallization time.

46.8°, 68.4°, 82.4°, and 86.9° can be indexed to the (111), (200), (220), (311), and (222) planes of the FCC structure of platinum (JCPDS #87-0647). The metallization time dependence of the Pt layer thickness is presented in Figure 6. A positive correlation between the metallization time and the thickness is shown. A reliable thickness value could not be measured for the samples with a metallization time less than the minimum time at 100 min due to the poor electrical conductivity. Pt layer thickness at 0.90 μm was obtained with the longest metallization time at 180 min, for which the calculated average growth rate is 1.8 nm/min. 3.3. Immersion Test of the Silk−Pt Material in the SBF. Releasing of the metal ions is an essential factor to provoke the allergic reactions;25 therefore, investigating the released ion concentration in the SBF is a direct way to evaluate the biocompatibility of the silk−Pt composite. After three months of the immersion at human body temperature (37 °C) in the SBF, only 0.011 μg/day and 0.018 μg/day of Pd and Pt were observed, respectively.

Figure 6. Pt layer thickness as a function of the metallization time.

3.4. Polarization Measurements Combined with Adhesion Tests. 3.4.1. Polarization Measurements of Simulated in Vitro (3.5 wt % NaCl) Solution. Figure 7 shows the results of the polarization measurements of the silk− Pt composite materials before and after the adhesion test. The silk substrate with 120 min of the metallization time was chosen for the corrosion resistance evaluation. No significant difference was found among the three polarization curves, indicating excellent corrosion resistance of the Pt layer even after the second adhesion test. The corrosion potential (Ecorr) and 8867

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observed. The Ecorr and Icorr are also summarized in Table 1(b). The morphology after the second adhesion test and polarization measurement is shown in Figure S1(b). Similarly, no obvious corrosion attack was inspected after adhesion test and the corrosion test in the SBF solution. 3.5. Electrical Resistance Measurements Combining with Adhesion Tests. Figure 9 illustrates the electrical

Figure 7. Polarization measurements of the silk−Pt composites before and after the adhesion test in 3.5 wt % NaCl solution.

corrosion current density (Icorr) are summarized in Table 1(a). Moreover, Figure S1(a) shows the morphology after the second Table 1. Corrosion Potentials and Corrosion Currents of the Silk−Pt Composites in (a) 3.5 wt % NaCl and (b) SBF Solutions Ecorr (V) As-deposited Adhesive test Adhesive test As-deposited Adhesive test Adhesive test

(a) Samples (in 3.5 wt % NaCl) −0.090 for once −0.092 for twice −0.093 (b) Samples (in SBF) 0.015 for once 0.018 for twice 0.019

Figure 9. Electrical resistance curve as a function of metallization time of the silk−Pt composites before and after the adhesion test.

Icorr (A/cm2)

resistance as a function of the metallization time before and after the adhesion test. Again, a reliable electrical resistance cannot be obtained owing to the poor conductivity when the metallization time is less than 80 min. The electrical resistance was high in the early stage of the metallization and then decreased with an increasing in the metallization time. After reaching a minimum point at 120 min, the electrical resistance gradually increased and stabilized. The electrical resistances after the adhesion test are shown in Figure 9. The adhesion tests showed serious impact on the samples in the early stage. On the contrary, the electrical resistances persist after the second adhesion test in the last two stages. In addition, there is no distinct difference between the first and the second adhesion test. The stages can be classified into three according to the morphology and the electrical resistance, which will be discussed in the following section.

5.89 × 10−7 6.00 × 10−7 5.93 × 10−7 2.16 × 10−7 2.02 × 10−7 2.02 × 10−7

adhesion test and the polarization measurement. No obvious corrosion attack was found after the adhesion test and the polarization test in 3.5 wt % NaCl solution. For simulating the condition in the human body, similar polarization measurements were carried out in the SBF and the results are shown in the following subsection. 3.4.2. Polarization Measurements of Simulated in Vivo (SBF) Solution. Figure 8 shows the polarization curves of the silk−Pt composite materilas before and after the adhesion test. In the same way, the metallization time at 120 min was chosen for its best suitability to this application. The corrosion resistance also persisted after the second adhesion test, and no significant difference within these three polarization curves was

4. DISCUSSION 4.1. Precursors Embedment and Substrate Activation. The broad peak observed at 2θ = 20° in the XRD pattern indicates the amorphous structure of silk textile as shown in Figure 2(a). With the introduction of sc-CO2, its nonpolar and low viscosity bring the catalyst into the substrate without damaging the substrate structure. The OM image (Figure 1(b)) along with the characteristic peaks in Figure 2(b) reveal that the catalyst was successfully settled on the substrate. Due to the high intensity from the Pd(acac)2 catalysts, the broad peak from silk was suppressed and was not observed in Figure 2(b). According to Vrubel et al.26 and Tanaka et al.,27 the reduction reaction of Pd(acac)2 is believed to be the reactions listed in the following: Pd(C5H 7O2 )2 + 2N2H4 ⇌ Pd2 + + 2C5H8N2 + 2OH− + 2H 2O

(1)

where Pd(C5H7O2)2 = Pd(acac)2 2Pd2 + + N2H4 + 4OH− ⇌ 2Pd + N2 + 4H 2O

(2)

The broad peak at around 2θ = 20° in Figure 2(c) is attributed to the amorphous silk substrate, and the sharp peaks indicate

Figure 8. Polarization measurements of the silk−Pt composites before and after the adhesion test in the SBF. 8868

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4.3. Enhanced Metallization Efficiency and Electrical Resistance. The electroless plating system catalyzed by Pd(acac)2 demonstrates a more efficient process and improved material properties compared to those of the system catalyzed by platinum acteylacetonate (Pt(acac)2).20 In this study, the optimal metallization time needed to reach the lowest electrical resistance is at 120 min, which shows 25% reduction in the time when compared to the study using Pt(acac)2 as the catalyst. The improved efficiency is suggested to be attributed to the high reduction efficiency of Pd(acac)2 to Pd metal. According to the literature,29 Pd(acac)2 owns a lower decomposition temperature than that of Pt(acac)2; it thus can be decomposed or reduced to the metal state more efficiently. After formation of the activation sites and gradual deposition of the Pt layer on the silk surface, the catalysts would be fully covered by the Pt and have no significant influence on the succeeding Pt deposition rate. It is worthy of mention that the silk−Pt composites fabricated in this study showed electrical resistances 36% lower than those of the literature.20 When comparing with the reduction of Pd(acac)2 to Pd metals, reduction of Pt(acac)2 to Pt metals is less efficient and takes a longer time. The nonsynchronized Pt(acac)2 reduction leads to uneven growth of the Pt activation sites and the Pt metallization later on. The irregular sizes of Pt activation sites make the succeeding Pt deposition grow roughly and thus show the high electrical resistance. The relationship between the metallization time and the Pt layer roughness is shown in Figure S3. The rough Pt cross section shown in Figure S3(a) was catalyzed by Pt(acac)2, while the smooth one in Figure S3(b) was catalyzed by Pd(acac)2. By using Pd(acac)2, nucleation and growth of the activation site would be uniform and lead to even growth of the Pt layer. Hence, the electrical resistance could be lowered. 4.4. Metal Ion Releasing Rate and Biocompatibility. In 3.5 wt % NaCl solution, high corrosion resistance of the silk−Pt is shown in the polarization measurements in Figure 7(a), and limited corrosion attacks on the sample surface are revealed in Figure S1(a). On the other hand, the corrosion current densities of the composite materials in the SBF are all lower than those in 3.5 wt % NaCl solution since 3.5 wt % NaCl solution contains more Cl−, and Cl− has significant impact on the corrosion resistance of metals.30 The corrosion potential of silk−Pt composite in the SBF is also more positive than those in 3.5 wt % NaCl solution, indicating that the Pt layer is more inert in the SBF solution. Despite the fact that a thin Pt layer was electroless plated on the nonconductive substrates, the corrosion rates are comparable to those of bulk Pt metal, which indicates the well deposition of the Pt layer on the silk. Overall, the silk−Pt composites show high corrosion resistances in the two solutions. These results suggest that the silk−Pt composite is not merely practical in wearable devices but also in implantable devices. The metal ion concentration in the human body varies with race, environment, and diet. According to several literature reports based on different living conditions, the average Pt input from diet is 1.44 μg/day, and Pt output through the faeces can be up to 1.79 μg/day.31−34 On the other hand, the average Pd intake is 2 μg/day, and the Pd output can be up to 2.5 μg/day.35,36 From the immersion test, only 0.018 μg Pt/day and 0.011 μg Pd/day were observed in the SBF solution. Both values are far less than the intake and have no chance to accumulate in the human body according to the literature and the ICP results. In addition, the Pt ion releasing rate of the

successful reduction of the Pd(acac)2 to form Pd metal. The results of Figure 2(a−c) are in good agreement with Figure 1(a−c). The reduced catalysts can be used as the active sites for the deposition of Pt metals in the metallization step. 4.2. Relationship between Morphology and Electrical Properties. Based on the coverage, surface roughness, and electrical resistance, the Pt metallization of silk substrates is classified into three stages. The partially metallized silk with a discontinuous Pt layer at 80 min of the metallization time is classified to the first stage (Figure 3(a)). The partly metallized surface showed smooth morphology since the metallization is still in the early stage. In this stage, the electrical resistance remained high due to the discontinuous Pt layer. At 120 min of the metallization time (Figure 3(b)), the silk substrate was fully covered with Pt, which is classified into the second stage. Electrical resistance was lowered since a continuous Pt layer was constructed. The metallized surface was smooth, and there were not many redundant clusters on the Pt layer. Hence, the lowest electrical resistance at 55 mΩ was built up in the second stage at the optimal metallization time of 120 min. At 180 min of the metallization time (Figure 3(c)), the silk was still fully covered; however, undesired Pt clusters and the rough metallization resulted in the elevated electrical resistance. The rough Pt deposition on the substrate surface increased unnecessary Joule heating and caused formation of a relative loose structure. SEM images revealing an increase in the roughness as the metallization time increased are shown in Figure S2. A proportional relationship between the electrical resistance and the roughness is indicated in the literature.28 Therefore, coverage of the Pt layer and the surface roughness both have significant influence on the electrical resistance. The influence of morphology on the electrical resistance in different stages is illustrated in Figure 10. In the first stage, the

Figure 10. Relationship between metallization morphology and electrical resistance in three different stages.

silk was not fully metallized, showing a noncontinuous Pt layer. It thus gave the highest electrical resistance. When the metallization time was prolonged to the second stage, full coverage was constructed, showing a continuous Pt layer. Along with the smooth surface, the electrical resistance reached its minimum point at 120 min in the second stage. However, the rough surface emerged as the metallization time extended; the electrical conductivity was thus deteriorated in the third stage. 8869

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silk−Pt composite prepared in this study is lower than those catalyzed by the Pt(acac)2.20 Since the samples catalyzed by Pd(acac)2 show relatively smooth deposition and less defects (Figure S3), they are thus corroded mildly in the immersion tests while compared to those catalyzed by Pt(acac)2. 4.5. Reliability Evaluation by Adhesion Tests. The levels of damage severity from the adhesion test were evaluated by both corrosion and electrical resistance measurements. Generally, defects and cracks would be formed after the adhesion test. Then the corrosion test solution can penetrate into the defects and cracks to give an increase in the current during the polarization measurement. Since the surface area is assumed to be constant, the current density is thus expected to be increased after formation of the defects and cracks. However, no significant difference in the corrosion current density was found after the adhesion test, indicating that the samples stay intact after the adhesion test and hence showing the high adhesive property of the Pt layer on the silk. In the electrical resistance test, the electrical resistance is expected to be elevated if the adhesion test successfully introduces defects into the Pt layer due to an increase in surface roughness of the Pt layer. The results shown in Figure 9 indicate great impacts in the first stage; on the contrary, the electrical resistance persists in the final two stages. In the first stage, the Pt layer is not continuous and the edge of the Pt layer is exposed to the tape during the adhesion tests. The Pt edge would be peeled off more easily than the surface that is fully metallized since it is free from support in the edge. The silk metallized with the optimized time shows perseverance in the second adhesion test, indicating its feasibility in the practice of applications even when under adverse conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01749. OM images after the second adhesion test and the polarization measurement, SEM images showing surface conditions of the silk−Pt composites, and SEM images of cross sections of silk−Pt composites and illustration of the influence of different catalysts on the smoothness of the Pt layer (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-45-924-5631. E-mail: [email protected]. ORCID

Wan-Ting Chiu: 0000-0002-7906-890X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number 26282013) and CREST Project operated by the Japan Science and Technology Agency (JST) (#14531864).

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5. CONCLUSIONS The Pt metallized silk was successfully fabricated from the electroless plating promoted by sc-CO2 in this study. In the catalyzation step, Pd(acac)2 catalyst was successfully settled on the silk substrate without damaging the structure due to the introduction of sc-CO2. The coverage and thickness of the Pt layer increased as a function of the metallization time. Silk textile was fully covered at 100 min of the metallization time, which is the minimum time to achieve full coverage. A maximum thickness of 0.90 μm was obtained at 180 min of the metallization time, and the average growth rate is 1.8 nm/min. The Pt metallization is classified into three stages. In the first stage, the noncontinuous Pt layer on the silk remained highly electrically resistant. On the other hand, the lowest electrical resistance and full coverage were achieved in the second stage. In the end, a rough surface was deposited in the third stage, resulting in deteriorated electrical conductivity. Pt metallized silk showed high corrosion resistance in both 3.5 wt % NaCl and the SBF solutions. The average Pd and Pt ion releasing rates in the SBF were 0.011 μg/day and 0.018 μg/day, respectively, which are negligible when compared with the daily input and output of Pd and Pt of the human body. The lowest electrical resistance at 55 mΩ was reached at the optimal metallization time of 120 min. Strong adhesion of the Pt layer on the silk was revealed by both corrosion test and electrical resistance measurement after the adhesion test. The results indicate the feasibility of this material for medical and wearable devices. 8870

DOI: 10.1021/acs.iecr.7b01749 Ind. Eng. Chem. Res. 2017, 56, 8864−8871

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DOI: 10.1021/acs.iecr.7b01749 Ind. Eng. Chem. Res. 2017, 56, 8864−8871