The recent escalation in the role of wearable devices for health and disease management has led to their usage for extended periods. Given the extensive usage, recent reports show that these devices can elicit inflammatory reactions and topical irritations1. A commonly reported side effect of continued use of wearable activity trackers is shown to be a skin rash 2.Several research groups around the globe are working on identifying the potential causes for these reactions.Some studies attributed the induced inflammation to nickel leaching. The casings of most of these wearable devices are made of an alloy of stainless-steel containing nickel. The studies show that nickel can cause allergic contact dermatitis 3,4, which can further spread to other parts of the body. Another study reported that the cause of localized contact dermatitis is the leaching of acrylate from the rechargeable battery housing 3. The literature suggests that the management of inflammatory skin conditions triggered by wearable devices can be challenging. The present study thus attempts to understand the source of these inflammatory responses for the future development of biocompatible and sustainable devices.
Conductive inks have been extensively investigated for the development of wearable devices, such as physical or electrochemical sensors 5,6, and flexible electronics 7–9. Screen-printing of these conductive inks has been of particular interest owing to the techniques’ ease of fabrication and low cost. In contrast to other traditional fabrication routes that use conductive inks such as nanoimprint lithography 10,11, and inkjet printing 12–14 screen-printing allows for rapid fabrication and easy design customization, thus attracting a lot of attention 15,16. Typically, these printed sensors interface directly with the user’s skin to acquire the required readings. These conductive inks are generally a heterogeneous mix of several constituent materials, some of which can induce an undesirable biological response if leached into the user’s body. Given that they are placed in direct contact with the skin, the user will be exposed to these materials primarily through the transdermal route. To further add to the problem, depending on the structural properties, certain materials have also been observed to cross the epithelial barrier of the skin and enter the blood circulation 17–19. The translocation of these materials to secondary organs may result in widely distributed toxic effects. We thus hypothesize that material leaching through these conductive inks may be a potential cause for the reported inflammatory responses.
Two primarily used conductive materials for the development of such inks are carbon and silver. Several allotropes of carbon have been used for the synthesis of conductive inks 20,21. Recent studies conducted to understand the potential toxic and immunological effects of carbon-based materials showed that their toxicity is dependent on several factors, such as their size and structure 22. However, given the inherent biocompatible nature of the carbon-based materials used in these inks, the biocompatibility of carbon inks has been of little or no concern. On the other hand, owing to their high electrical and thermal conductivity, silver inks have widespread applications in the fabrication of wearable sensors and conductive traces in flexible electronics 23,24. The biocompatibility of silver, especially in nanoparticle form, has always remained a concern, significantly limiting its practical usage. Silver shows toxicity towards a wide array of biological systems including mammalian cells, bacteria, and fungal cells27. While several studies 25–28 have been conducted to understand its toxicity in various forms, the toxicity of silver leached through printed conductive inks is yet to be assessed. Silver nanoparticles are known to leach out from several products that contain silver when placed in contact with a fluid 29–31. Our study hypothesizes that printed conductive silver inks will leach out silver ions when exposed to a (bio)fluid. Previous studies have shown that physicochemical properties of silver particles, e.g., particle size or surface charge, influence their toxic behavior. Several different studies have unanimously reported that smaller silver particles exhibit higher toxicity when compared to their larger counterparts 25,26. Moreover, a study carried out by Smith et al., showed that silver ions elicit higher toxicity when compared to silver nanoparticles due to their smaller size and surface charge 32. Also, the presence of other ions/metal complexes in the environment has been known to affect the toxicity profile 33,34. One such study showed that the toxic response of silver ions can be mediated by the presence of chloride ions in the medium 35. This study is especially interesting in the context of wearable electrochemical sensors since inks containing silver in combination with silver chloride are commonly used for the fabrication of reference electrodes. In addition to considering the effect of the leached particles’ properties and their environment, the overall packaging of the wearable device should also be accounted for. Seamless interfacing of wearable devices with the complex contours of the tissue requires high flexibility. Polymers are well suited for this purpose and have thus been widely used to encapsulate such wearable devices 36–38. However certain polymers, for instance, isobornyl acrylate, can act as allergens, yet again resulting in an inflammatory skin response 39,40. Assessment of the toxic response of polymer-encapsulated conductive inks is a knowledge gap that is yet to be addressed.
In this work, we explore the biocompatibility of two commercially available conductive inks, namely, carbon, and silver/silver chloride. As a proof of concept, a standard three-electrode electrochemical sensor, screen-printed on a flexible substrate using conductive inks, was selected for this study. The bio-acceptability assessment of the printed sensors was performed through in vitro cell assays. Since keratinocytes primarily constitute the human epidermis, it was used as the surrogate cell line in this study to investigate its response to the printed sensors. The study first aims to understand the effect of encapsulation on material leaching from the two printed inks through a mass spectrometric analysis. Following this, the toxicity profile of the differently encapsulated printed inks was investigated at cellular and intracellular level on an epidermal (HaCaT) cell line to identify the toxicity pathway of the leached particles. The interaction of silver and carbon ink with cells and their concentration-dependent changes in the cellular function, generation of reactive oxygen species, and apoptotic activity of the cells are detailed in this work (Fig. 1). A comprehensive understanding of the toxicity pathway of printed inks developed through this study can be used to inform a sustainable and biocompatible design pathway for future wearable devices.