Metallization on Silk Utilizing Supercritical Carbon Dioxide Assisted Electroless Plating for Wearable Device
Authors: Chiu Wan-Ting , Chen Chun-Yi , Chang Tso-Fu Mark , Hashimoto Tomoko , Kurosu Hiromichi , Sone Masato 
Published: February 2020

According to the International Data Corporation (IDC) report [1], the worldwide wearable devices market will swell to 111.9 million networked devices sold in 2018 projecting the trend and blossom of this potential technology. To fulfill both conductivity and flexibility for wearable device, combining the conductive materials and the flexible fabrics, such as metal and polymer, becomes the greatest challenge. There are many techniques to achieve the combination, such as sputtering and evaporation. Among these, electroless plating is the most potential method owing to its eminent advantages, e.g. low processing temperature, low cost, and simple operation procedures [2]. Electroless plating consists of a pretreatment step to clean and roughen the substrate, a catalyzation step to inlay the catalyst seeds, and a plating step to carry out the metallization on the substrate. A novel catalyzation step with the assist of supercritical carbon dioxide (sc-CO2) was conducted in this study. Distinct from the conventional catalyzation, catalyzing the polymer in sc-CO2 allows exclusion of the pretreatment step and further results a defect-free and smoother surface [3]. With the outstanding self-diffusivity, high compatibility with non-polar materials, and low surface tension characteristics of sc-CO2, organo-metallic catalysts can be embedded into the polymer without damaging the structure. Furthermore, with sc-CO2, the chemical solution used in the catalyzation step can also be withdrawn, which is friendlier to the environment.
In previous studies, we have proposed an electroless plating method involving Ni-P plating step on polyimide followed by catalyzation in pure sc-CO2 containing Pd (II) acetylacetonate (Pd(acac)2) [3,4]. Ni-P solution is a common electrolyte for the plating due to the advantages, such as relatively high deposition rate and high corrosion resistivity. To obtain higher mobility and the application of wearable devices, softer silk is chosen as the substrate in this study. Moreover, silk is a common material in clothes. The combination of Ni-P and silk is thus considered to be a potential candidate for the wearable devices. This work, therefore, executed the electroless plating between Ni-P and silk.
A piece of the silk textile was immersed in the sc-CO2
fluid containing Pd(acac)2 at 80 oC and 15 MPa. Appropriate amount of the Pd(acac)2 catalyst was used to maintain the concentration at the saturation concentration throughout the catalyzation step. No pretreatment was conducted in this study. The plating step was thereupon being carried out on the catalyzed silks at 70 oC and atmospheric pressure in the acidic Ni-P solution. Various lengths of plating times were performed to inspect the relationship between the plating time and the properties. The surface morphology, cross-section and composition were examined by an optical microscope (OM), scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDX). The phase and crystal structure were identified by an X-ray diffractometer (XRD). The electrical resistance was measured by the four-point probe method.
By conducting the catalyzation in sc-CO2, smooth and uniform coverage of Ni-P coatings were successfully plated on the silk textiles under various plating times. The composition, thickness, and coverage trend increase with plating time. The composites turn out to be conductive as the plating time is extended to 2 mins. Lowest electrical resistance at 1.23 ohm can be achieved with plating time at 4 min.

References

[1] R.T. Llamas, Worldwide Wearable Computing Device 2014–2018 Forecast Update: December 2014, U.S.A.: IDC Research, Inc. (2014).
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