Human skin provides a wealth of physiological and physical data valuable for health monitoring, prevention, and treatment. Directly interfacing electronic devices with the skin allows for extraction of information like heart rate, muscle condition, skin impedance, and hydration. Wearable bioelectronics have shown promise in epidermal sensing, offering soft, flexible, and stretchable patches for skin contact. However, a significant limitation is susceptibility to motion artifacts due to weak adhesion or imperfect conformability between the electronics and the skin, leading to misinterpretations and misdiagnoses. This study introduces ultra-conformal Drawn-on-Skin (DoS) electronics as a novel bioelectronics platform to address these challenges, offering on-demand multifunctional, motion artifact-free sensing capabilities. Unlike existing wearable or printed bioelectronics requiring specialized equipment, DoS electronics boasts advantages like simple fabrication without specialized equipment, adaptability to dynamic surfaces, construction of active electronics, device/sensor multifunctionality, and motion artifact immunity without additional hardware or computation. Its customizability allows for personalized point-of-care treatments. The DoS electronics are created by drawing liquid functional inks onto stencils applied to human skin, resulting in an ultra-conformal, robust, and stretchable interface immune to motion artifacts. The platform uses Ag flakes/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (Ag-PEDOT:PSS) composite, poly(3-hexylthiophene-2,5-diyl) nanofibrils (P3HT-NF), and ion gel as conductive, semiconducting, and dielectric inks, respectively. The versatility of the platform is showcased through the development of diverse devices: thin-film transistors, strain sensors, temperature sensors, heaters, hydration sensors, and electrophysiological (EP) sensors, all characterized by skin-textured surfaces, curvilinear shapes, and mechanical deformability. A wireless DoS electrocardiogram (ECG) monitoring system is demonstrated, showcasing daily and clinical applicability. Comparative analysis of DoS EP sensors against hospital-grade gel electrodes and ultrathin serpentine mesh electrodes reveals advantages including stable performance in sweat, reliable long-duration EP signal capture, strong skin adherence, and motion artifact immunity.

The introduction extensively reviews existing wearable bioelectronics and their limitations, citing several studies (Liu et al., 2017; Kim et al., 2014; Zhang et al., 2015; Xu et al., 2015) showcasing advances in flexible and stretchable electronics for epidermal sensing. It highlights the prevalent issue of motion artifacts affecting biopotential measurements in wearable devices (Li et al., 2016; Jeong et al., 2014; Nawrocki et al., 2018; Wang et al., 2018; Someya & Amagai, 2019; Bihar et al., 2017) due to inconsistent interfaces between electronics and skin. The review also notes challenges with existing fabrication methods for bioelectronics that often rely on dedicated equipment (Sugiyama et al., 2019; Williams et al., 2019; Zhu et al., 2018; Son et al., 2014; Miyamoto et al., 2017). The authors build upon the existing literature by focusing on the development of a novel approach to address these shortcomings—a simple, robust, and adaptable method.

The study details the preparation of conductive, semiconducting, and dielectric inks. The conductive ink is a mixture of Ag flakes and PEDOT:PSS solution, the semiconducting ink uses P3HT-NF, and ion gel serves as the dielectric. The DoS electronics drawing process involves creating a stencil from Kapton and tape, adhering it to the skin, and drawing the inks into the stencil outlines using a modified ballpoint pen with a 1mm tip diameter. The drawing speed is approximately 10 mm/s, and the stencil is removed after 5 minutes of solvent evaporation. Imperfections can be corrected by redrawing. The morphologies of the resulting films were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). A multifunctional integrated system, including transistors, strain sensors, temperature sensors, heaters, a skin hydration sensor, and electrophysiological (EP) sensors, was drawn to demonstrate the platform's versatility. The mechanical properties, skin compatibility, and electrical performance of the inks were evaluated. The ink line width and resolution were controlled using pens with varying tip diameters and stencils. The robustness of the inks to stretching, twisting, and poking was assessed on polydimethylsiloxane (PDMS) substrates. Skin compatibility was verified through histological staining of mouse skin samples after 48 hours of ink application. The electrical characteristics of the Ag-PEDOT:PSS and P3HT-NF inks, including sheet resistance and its response to strain and temperature, were measured. Transistors, strain sensors, temperature sensors, and heaters were characterized on skin replicas. The transistors' I-V and transfer curves were measured to evaluate their performance under different strain conditions. The strain sensors' resistance change under varying strain levels and cyclic stretching was determined. The temperature sensors' resistance change across a range of temperatures was measured. The heaters' temperature profiles under different applied voltages were evaluated. The Ag-PEDOT:PSS ink was used to fabricate a heater, an RC circuit, a skin hydration sensor, and EP sensors on porcine and human skin. The skin hydration sensor's capacitance was measured and correlated with readings from a commercial hydration meter. Electrophysiological signals (EMG and ECG) were recorded using DoS EP sensors, and the signal-to-noise ratio (SNR) was evaluated. A wireless ECG monitoring system was built using an amplifier, microcontroller, Bluetooth module, and battery. The system's performance was evaluated during a stress test involving standing still and walking. The DoS EP sensors' performance was compared to that of hospital-grade gel electrodes and ultrathin serpentine mesh electrodes in terms of sweat resistance, durability, adhesion, and motion artifact immunity. Motion artifact immunity was assessed during experiments involving skin deformation and vibration-induced motion. The impedance changes at the skin-electrode interface were also evaluated during skin deformation. Finally, accelerated skin wound healing was assessed using a mouse model, with DoS electronics providing electrical stimulation.

The study successfully demonstrated the creation of ultra-conformal, drawn-on-skin electronics using novel electronic inks. The resulting devices exhibit excellent adhesion to the skin and are highly resistant to motion artifacts. Key findings include: 1. **Ink Characterization:** The electronic inks (conductive, semiconducting, and dielectric) demonstrated excellent mechanical flexibility and stability, showing minimal change in performance even under significant strain (up to 30%). Histological analysis showed no adverse skin reactions. 2. **Device Performance:** The various devices fabricated using the DoS method (transistors, strain sensors, temperature sensors, heaters, hydration sensors, and electrophysiological sensors) functioned reliably on the skin, demonstrating the platform's versatility and practicality. The transistors showed typical p-type characteristics, and the strain, temperature, and hydration sensors displayed the expected sensitivity to their respective stimuli. 3. **Electrophysiological Signal Acquisition:** The DoS EP sensors effectively captured high-quality EMG and ECG signals, even under significant stretching and bending of the skin. Importantly, these signals showed remarkable immunity to motion artifacts, as demonstrated by experiments involving both deliberate skin deformation and vibration-induced motion. Comparative studies showed that the DoS EP sensors outperformed both hospital-grade gel electrodes and ultrathin mesh electrodes in terms of stability in the presence of sweat and robustness to motion artifacts. The signal-to-noise ratio remained consistent even when the skin was significantly deformed. 4. **Wireless ECG Monitoring:** The wireless ECG monitoring system successfully captured ECG signals during both resting and walking conditions, enabling the detection of changes in heart rate in response to the activities. This capability demonstrates the feasibility of real-world and clinical application of the technology. 5. **Accelerated Wound Healing:** In a mouse model, electrical stimulation delivered through DoS electronics significantly accelerated skin wound healing, showcasing the platform's potential for point-of-care treatment applications. The treated wounds exhibited significantly reduced scab width compared to control wounds.

The findings of this study address the long-standing challenge of motion artifacts in wearable bioelectronics by introducing a novel platform based on drawing electronic inks directly onto the skin. The ultra-conformality of the DoS electronics ensures a stable and robust interface with the skin, minimizing the effects of motion on signal acquisition. The superior performance of the DoS EP sensors in comparison with conventional electrodes demonstrates the potential of this approach to improve the accuracy and reliability of wearable health monitoring technologies. The ability to create multifunctional devices using a simple and low-cost fabrication method expands the possibilities for creating personalized point-of-care diagnostics and therapeutic applications. The demonstrated efficacy of the DoS electronics in accelerating skin wound healing further underlines the broad scope and impact of this technology. These findings have significant implications for developing next-generation wearable sensors for continuous health monitoring, personalized medicine, and advanced wound care.

This research successfully developed a novel platform of ultra-conformal drawn-on-skin (DoS) electronics, demonstrating its capability for multifunctional, motion artifact-free sensing and point-of-care treatment. The simple and cost-effective fabrication process, combined with the exceptional performance of the devices, showcases the significant potential of DoS electronics for applications in wearable health monitoring, personalized medicine, and advanced wound care. Future research may explore further optimization of ink formulations and device designs, integration with advanced signal processing algorithms, and clinical evaluations for various medical applications.

While the study demonstrates significant advancements, some limitations should be noted. The fabrication process, while simple, is currently manual and may require further automation for high-throughput production. The long-term stability and biocompatibility of the electronic inks require further investigation over more extended periods. The animal study on wound healing used a limited sample size, warranting further investigation with larger sample sizes and different wound models. The wireless ECG system, while functional, could be improved by enhancing the data transmission range and battery life.