A Low‐Cost and Do‐It‐Yourself Pressure Sensor Enable Human Motion Detection and Human–Machine Interface Applications

In this work, cost‐effective and do‐it‐yourself capacitive pressure sensors are fabricated using readily available commercial components. The sensors are created in a single‐step process ‐by simply applying electrically conductive paint onto both sides of a porous melamine sponge. These sensors exhibit a wide‐range pressure sensing capability, spanning from 10 Pa to 100 kPa. The sensors showcase an impressively low limit of detection, detecting pressures as low as 10 Pa, and exhibit a moderate response time of 123 ms. Moreover, the sensors display remarkable repeatability and stability over 10 000 loading and unloading cycles without experiencing fatigue. Notably, these exceptional qualities come at an exceptionally low material cost, with the sensor measuring 20 × 20 × 2 mm. To showcase their potential applications, the fabricated sensors are successfully employed in real‐time human motion detection, proximity detection, and wearable keyboard applications.


Introduction
Over the past decade, wearable pressure sensors have garnered significant attention due to their desirable characteristics including flexibility, high sensitivity, lightweight nature, and conformability. [1,2]These sensors have found application in various fields, such as soft robotics, where they serve as electronic skins, [3] real-time human motion detection, [4] and healthcare monitoring, [5] as well as human-machine interfaces. [6]Previous research studies have presented wearable pressure sensors based on capacitive, piezoresistive, piezoelectric, and triboelectric sensing mechanisms. [7]Among these, capacitive sensors have emerged as a preferred choice due to their straightforward design, enhanced accuracy, lower power consumption, and DOI: 10.1002/adsr.202300162greater independency on external factors, such as temperature and humidity. [8,9]ypically, capacitive flexible sensors consist of two main components: A flexible dielectric layer positioned between two flexible electrodes.When pressure is exerted on the sensor, the thickness of the dielectric layer decreases and the permittivity of the medium increases.This, in turn, leads to an elevation in the capacitance value.Flexible pressure sensors with the capacity to quantify a diverse spectrum of pressures, encompassing subtle pressure (1 Pa-1 kPa), low pressure (1-10 kPa), and medium pressure (10-100 kPa), hold particular significance owing to their pertinence in capturing numerous physiological signals. [10]lexible substrates commonly employed in the fabrication of capacitive sensors include polymer films, such as polydimethylsiloxane (PDMS), [11] Ecoflex, [12] polyester (PET), [13] polyurethane (PU), [14] polyimide (PI), [15] and polyvinyl alcohol (PVA). [16]To create flexible electrodes, solution-processable nanomaterials, such as carbon nanotubes, [12,16,17] graphene, [18,19] Mxenes, [20] and silver nanowires (AgNWs), [11,21] are often coated or printed onto these polymer films.These approaches, however, are characterized by their prolonged manufacturing time requirements, high costs of fabrication for conductive materials, and intricate production procedures.
On the other hand, flexible polymer films exhibiting high dielectric constants, such as PDMS, [22] Ecoflex, [23] polyvinylidene fluoride, [24] polyimide, [25] and polymethyl methacrylate, [21] have been widely employed as dielectric materials in flexible pressure sensor applications.However, to enhance the sensing performance, it is sometimes necessary to modify the structure of the polymer dielectric materials.Introducing porosity into the dielectric layer has emerged as a prominent research avenue for improving sensitivity.The incorporation of porosity results in easier deformation of the dielectric layer due to a decreased Young's modulus, which in turn enhances the effective dielectric constant and benefits the sensitivity. [26]However, introducing porosity to polymer films poses challenges in terms of multi-step process and cost.Additionally, the process of laminating or encapsulating the electrodes and dielectric layer presents inherent challenges and complexities.
In a recent study by Cicek et al., [27] a seamless monolithic design was introduced, eliminating the need for lamination or encapsulation and offering a simplified approach to sensor fabrication.The outer layers of a melamine sponge were coated with Ag-NWs to serve as electrodes, while the body of the sponge was utilized as the dielectric layer, resulting in a highly flexible and sensitive sensor configuration.While this approach brought various benefits including breathability and washability often neglected in applications intended to be wearable, the usage of facile and low-cost processing techniques and materials is still required.Briefly, a protective paraffin mask was applied to the interior regions of the foam, followed by multiple cycles of dip-and-dry processes involving AgNWs coating on both sides of the melamine sponge.This procedure created the upper and lower electrode layers of the sensor.Subsequent elimination of the residual paraffin mask using n-hexane yielded a seamless monolithic structure.Nevertheless, this procedure entails complexity, involves the usage of relatively dangerous solvents (such as n-hexane) thus rendering it not recommendable for a do-it-yourself approach, and consumes time.Besides, AgNWs exhibit limitations in applications requiring contact with human skin, as they can translocate through epithelial barriers and have demonstrated cytotoxicity. [28]dditionally, the complexities in the manufacturing processes impose constraints on the scalability of the process.
Therefore, the objective of this work was to develop lowcost, scalable, environmentally friendly, and do-it-yourself capacitive pressure sensors by employing easily accessible commercial components.From the fabrication safety viewpoint, ethanol is the only solvent needed and it is only used for cleaning the melamine sponge.Following the thorough characterization of the sensor's properties, its applicability was demonstrated in various domains, such as human motion detection, proximity sensing, and human-machine interface applications.

Characterization of the Electrode
The application of electrically conductive paint onto a glass slide with an approximate thickness of 50 ± 4 μm was accomplished using a brush painting technique.This coating exhibited an electrical resistivity of 1.05 ± 0.1 Ω⋅cm, which is sufficiently low to power the light-emitting diode (LED), as depicted in Figure S2, Supporting Information, and the resistivity value of the paint is comparable to that of carbon paste utilizing activated carbon as the conducting material. [29]Both sides of the 2 mm thick melamine sponge were brushed with the paint to fabricate the sensor, as illustrated in Figure 1A.Consequently, the resulting sensor displayed exceptional compressibility, conformity, and flexibility (Figure 1B).The density of the melamine sponge and the electrical paint is 0.0096 and 1.16 gr cm −3 , respectively, which resulted in a lightweight sensor as shown in Figure 1C.
The melamine sponge exhibited a highly porous microstructure characterized by pore sizes ranging from ≈30 to 120 μm, with wall thicknesses of ≈5 to 10 μm, as shown in Figure 1D.Due to its high viscosity, the electrical paint coating on the melamine sponge resulted in the formation of a thin film with a rough surface structure, as illustrated in Figure 1E.Cross-sectional SEM analysis further revealed the penetration of the paint into the top surface of the sponge, forming a thin film layer with a sheet resistance of 50 ± 2 Ω sq −1 and a thickness of ≈200 μm (Figure 1F).

Characterization of the Sensors
The economic evaluation of material costs for the sensor is presented in Table S1, Supporting Information, which unveils that the cost associated with a sensor measuring 20 × 20 × 2 mm amounts to 7.3 Euro cents, demonstrating its notable potential as a cost-effective solution in various applications.In addition, it is worth noting that the sensor fabrication process is free of any harmful procedures or chemicals, thereby proving the environmentally benign nature of the manufacturing process.
The sensor was subjected to comprehensive pressure testing, covering a wide range from 0.1 to 100 kPa, which closely corresponds to the pressure levels experienced by human skin during typical daily activities. [30]A texture analyzer equipped with a 12.8 mm diameter cylindrical probe was utilized to incrementally apply pressure to the sensor.The mechanical properties of the melamine sponge, including its compressibility and elasticity, are of substantial significance in understanding its suitability for sensor applications.With a modulus (1% tangent) of 40.9 kPa, the melamine sponge exhibits notable compressibility, enabling deformation under external forces and subsequent recovery to its original form (as shown in Figure S3, Supporting Information), thus ensuring its potential for precise and reliable pressure measurements.The application of a compressive load on the sensor facilitates a reduction in the distance between the two parallel electrodes, leading to an increase in capacitance.Furthermore, when subjected to compressive loads, the air present within the pore structure of the system is displaced by the surrounding melamine skeleton.As a result, in addition to the decreased distance between the parallel electrodes of the capacitive sensor, the low relative permittivity air is replaced by the higher relative permittivity of the melamine material. [27]nitially, the response of the sensor to various pressure levels and its repeatability were assessed, as depicted in Figure 2A.The capacitance value exhibited a positive correlation with the applied pressure, and consistent response signals were obtained throughout five consecutive loading/unloading cycles.These findings demonstrate the stability and repeatability of the sensors, with a notable response performance.Figure 2B illustrates the relative change in capacitance of the sensor as a function of pressure in the range of 0.1 to 100 kPa.The sensor displayed a linear increase in capacitance attributed to the reduction in thickness resulting from increasing pressure, along with two linear segments characterized by a decreasing slope, denoted as the pressure sensing sensitivity.The sensor exhibited sensitivities of 0.279, 0.066, and 0.0045 kPa −1 in the subtle (10 Pa-1 kPa), low (1-10 kPa), and moderate pressure regimes (10-100 kPa), respectively.The detailed graphs for sensitivity calculations in different pressure regimes are demonstrated in Figure S4, Supporting Information.
The robustness of the sensor was evaluated by subjecting it to 10 000 loading/unloading cycles with a pressure of 5 kPa, revealing that the sensor maintained its functionality without significant signs of fatigue (Figure 2C).Response and recovery times were assessed by applying and maintaining a pressure of 100 Pa for 5 s, as depicted in Figure 2D.The sensor demonstrated a rapid increase in capacitance with a fast response time of 123 ms, slightly higher than that of human skin (20-40 ms). [31]Upon pressure release, the sensor promptly exhibited a decrease in capacitance with a short recovery component of ≈148 ms, gradually attenuating its initial value.Our sensor demonstrated a limit of detection (LOD) value of 10 Pa, as shown in Figure S5, Supporting Information.However, achieving reproducible and consistent results below 10 Pa proved to be challenging.
The provided table compares the results obtained from the work conducted in this study with the results reported in the literature for various porous dielectric materials used in capacitive sensors (Table 1).The dielectric material's elastic modulus and the pore size distribution are two important parameters influencing the sensitivity of the sensors.In terms of maximum sensi-tivity, the melamine sponge-based sensor in this study achieved a sensitivity of 0.279 kPa −1 in the subtle pressure range of 0-1 kPa.Comparatively, the literature cites maximum sensitivities ranging from 0.01 1.285 kPa −1 , depending on the specific material and pressure range considered.Response times reported in the literature range from 15 to 155 ms, while the response time achieved in this study is 123 ms.The results obtained in this study with the melamine sponge-based sensor demonstrate a relatively moderate sensitivity and response time in comparison to the literature-reported sensors.It is worth noting that it would be difficult to implement electronic skin for health monitoring applications with the sensor response time in this work.The achieved sensor response time in this work could pose challenges for the implementation of the sensors as electronic skin in health monitoring applications.Similarly to our work, Cicek et al. employed a melamine sponge as a dielectric layer, however, they coated AgNWs on the external layers of both surfaces. [27]As outlined in Table 1, they determined the maximum sensitivity to be 1.285 kPa −1 (0-100 Pa) with a response time of 18 ms.Nonetheless, upon examining the sensitivity within the range of 0-1 kPa, it was ascertained to be ≈0.3 kPa −1 , which is comparable to our work.The relatively swifter response time in their study might be attributed to variances in stiffness between the AgNWs and the electric paint utilized in our investigation.We believe that the electric paint led to a rigid continuous film covering the whole outer layer of the melamine sponge in our work, while in the research by Cicek et al., AgNWs were solely deposited along the melamine sponge's wall structures. [27]otably, our approach not only offers a facile, scalable, and cost-effective solution but also aligns with environmental considerations.

Human Motion Detection
The sensor demonstrated several promising properties, such as the capacity to respond to multiple pressure ranges, high sensitivity, lightweight design, and flexibility, making it a suitable candidate for human motion detection.To showcase its potential applications, the sensor was affixed to various locations on the human body, including the bicep, wrist, finger joint, face, knee, and under the foot.Real-time monitoring of capacitance changes was performed during specific movements.
During the contraction and relaxation of the bicep muscle, the sensor exhibited an increase in capacitance, indicating the  applied pressure resulting from muscle inflation.Figure 3A illustrates the observed capacitance changes in response to bicep movement.Consistent responses were obtained upon repeating the same activity.Furthermore, the sensor was positioned on the wrist to monitor its movement (Figure 3B).Bending the wrist by 90°induced an increase in capacitance value, which returned to its original value upon returning to the initial position after each bending motion.Similarly, when placed on the joint of the index finger, the sensor detected bending of the finger joint, resulting in an increase in capacitance (Figure 3C).Reliable and consistent results were obtained during repeated bending and unbending of the finger joint.Moreover, by attaching the sensor to the facial muscle responsible for smiling, the immediate response was observed upon smiling, with the capacitance returning to its original value when the muscle relaxed back to its initial position (Figure 3D).Furthermore, when placed on the knee joint to detect leg flexion, the sensor exhibited recovery of capacitance after each flexion cycle.This indicates the highly reproducible signals and exceptional stability of the fabricated sensors during continuous leg flexion 90°and relaxation (Figure 3E).
Ultimately, the sensor was positioned beneath the shoe to monitor pressure variations during walking, as illustrated in Figure 3F.The sensor exhibited consistent and reproducible signals throughout the walking activity, showcasing the exceptional stability of the sensors under applied pressure.

Proximity Detection
Capacitive sensors possess the capability to detect the presence and position of objects or humans without the need for physi-cal contact.This is achieved by monitoring the electrical charge carried by the objects or individuals in question.The underlying principle relies on the phenomenon of capacitive coupling, which occurs between the sensors, objects, and people involved. [38]hen an object approaches the capacitive sensors, it perturbs the distribution of the electric field between the electrodes.As the object draws nearer, it acts as a grounded conductor, causing the electric field lines to extend and pass through the object.As a result, the fringing electric field leads to a reduction in the measured capacitance of the capacitive sensor compared to its base capacitance, which corresponds to the absence of any proximity. [38]n the present study, the developed sensors were tested by vertically approaching the human hand (inset, Figure 4A).The experimental results revealed that the sensors could detect proximity within a range of up to 20 cm, as illustrated in Figure 4A.The maximum sensitivity achieved was measured as 0.049 cm −1 for distances below 6 cm, which is comparable to the findings reported in previous research. [37]Furthermore, a cyclic test was conducted by progressively bringing the hand closer to the sensor surface, ranging from 25 cm down to 0.5 cm (Figure 4B).The measurements consistently yielded reproducible signals, indicating the remarkable stability of the sensor in response to hand proximity.

Human-Machine Interface Application
A 3 × 3 sensor grid was successfully fabricated on a cotton fabric for application in a wearable keyboard, as shown in Figure 4C.The Arduino platform was selected for electronic prototyping due to its open-source nature, aligning with the do-it-yourself  concept of this research.The display of the keyboard featured the abbreviation "MMD", representing our research group.The flexibility and conformability of the keyboard were demonstrated excellently, as shown in Figure 4D, and it retained its functionality when placed on a human arm, allowing for freedom of movement for end users.Videos showcasing real-time writing the name of the co-authors ("Jaana", "Mari", and "Kim") with the keyboard are available in the supporting information.To address capacitance fluctuations caused by the proximity of human fingers to the sensors, an additional script was incorporated into the Arduino

Conclusion
In conclusion, this work successfully developed cost-effective, environmentally friendly, and do-it-yourself capacitive pressure sensors using readily available commercial components.The fabricated sensors exhibited a wide pressure range (10 Pa-100 kPa), moderate sensitivity (0.279 kPa −1 ), low limit of detection (10 Pa), and fast response time (123 ms).They demonstrated remarkable stability and repeatability over 10 000 loading and unloading cycles, highlighting their robust nature.The sensors were successfully employed in various applications, including realtime human motion detection, proximity sensing, and wearable keyboard functionality.These findings contribute to the advancement of flexible and wearable sensor technology, showcasing the potential for their integration in human-machine interfaces, healthcare monitoring, and other related fields.Further research and optimization efforts can enhance the performance and expand the applications of these capacitive pressure sensors.

Experimental Section
Materials: The electric paint utilized in this study was procured from Bare Conductive.The highly porous melamine sponge, Shieldex Nanking RS, Kapton tape, Cu tape, paintbrush, medical-grade bandage, and Arduino UNO R3 microcontroller board were sourced from online stores.Denatured ethanol, a necessary component, was obtained from a local market.
Fabrication of Pressure Sensors: After cutting the melamine sponge with desired size (2 cm × 2 cm × 2 mm) with a razor blade, it was washed with ethanol several times and allowed to dry at RT overnight.Then, briefly, ≈0.1 gr of electrical paint was placed on the surface of the melamine sponge with a spatula and painted with a brush (size no: 6) several times until a uniform coating on the surface was achieved, followed by drying at RT for at least 30 min.The same procedure was repeated for the opposite surface.Subsequently, Cu tapes were connected at both ends as electrical contacts for further measurement and the sensors were finally sealed with a Kapton tape.It was important to emphasize that all experimental procedures were carried out in a standard laboratory environment, and no specialized conditions, such as a cleanroom, were required.
Human Motion Detection Measurements: The sensors were strategically positioned on specific locations of the human body, such as the bicep, wrist, index finger joint, face, and under the shoe.To secure the sensors in place, a porous and elastic bandage was utilized.This bandage facilitated the fixation of the sensors onto the designated points.The monitoring of capacitance changes was conducted concurrently with the movement of the corresponding joints or muscles, allowing for real-time assessment of the sensor response to these movements.Written consent from all participants was obtained prior to the research (Aalto University Research Ethics Committee with D/1302/03.04/2022WEARSENSNANO decision number) Proximity Sensing: To evaluate the proximity sensing capabilities of the fabricated capacitive pressure sensors, experiments were conducted by vertically approaching the human hand towards the sensor surface.A setup was prepared where the sensor was positioned in close proximity to the approaching hand, and the distance between the hand and the sensor was carefully controlled.The sensor's base capacitance, Adv.Sensor Res.2024, 2300162

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© 2024 The Authors.Advanced Sensor Research published by Wiley-VCH GmbH corresponding to the absence of any proximity, was measured as a reference.The capacitance variations were recorded as the hand was progressively brought closer to the sensor surface in incremental steps.
Fabrication of Wearable Keyboard: flexible 3 × 3 sensor grid was fabricated on a cotton fabric, with the layout depicted in Figure S6, Supporting Information.To outline the fabrication process briefly, three strips of Shieldex Nanking RS, each measuring 7 mm in width and possessing electrical conductivity, were horizontally positioned the cotton fabric.Subsequently, sensors encapsulated with Kapton tape were centrally placed on the conductive strips.Following this, another set of three strips of Shieldex Nanking RS, also measuring 7 mm in width and possessing electrical conductivity, were vertically positioned on top of the sensors.The final structure was then laminated using a hot press at a temperature of 120 °C for a duration of 10 min.Ultimately, the sensor grid was coupled with the Arduino system for further integration and functionality.The Arduino code developed during this research study has been made available at the end of Supporting Information.
Characterization: The morphological characterization of both the melamine sponge and the fabricated sensors was conducted using a scanning electron microscope (SEM, Tescan Mira3).The capacitance values of the sensors were measured using an LCR meter (E4980AL, KEYSIGHT) with an applied AC voltage of 1 V and a frequency of 10 kHz.External pressure was applied to the sensors using a mechanical analyzer equipped with a computer-controlled stage (TA.XTplus, Stable Micro Systems).The resistance values of the electrodes were measured using a digital multimeter.The mechanical properties of the melamine sponge were measured using a universal mechanical testing machine (Instron 4204).Evaluation of the response and recovery times was performed using OriginLab Pro software, specifically utilizing the Rise Time Gadget.To ensure accuracy in determining the response and recovery times, and to mitigate any irregularities associated with signal transition corners, these times were defined as the duration between 10% and 90% of the amplitude.
Statistical Analysis: Resistivity, sheet resistance, and thickness of electrical paint were measured over three identical samples for each datapoint, and data was presented as average and SD.Sensor performance tests (Figure 2) were characterized with one sample.Human motion detection (Figure 3) and proximity sensing (Figure 4A,B) characterizations were performed over one sample.All the data analyses were performed using Matlab and Excel.

Figure 1 .
Figure 1.A) Schematic illustration of sensor preparation, digital photos of B) the sensor under mechanical deformation and C) sensor placed on a flower, SEM images of D) surface morphology of bare and E) coated melamine sponge, and F) cross-sectional view of the coated melamine sponge.
Authors.Advanced Sensor Research published by Wiley-VCH GmbH

Figure 2 .
Figure 2. Properties of the pressure sensor.A) Repeated real-time responses of the sensor to various pressures, B) sensitivity of the sensor in different pressure regimes, C) stability of the sensor tested for 10 000 cycles under an applied pressure of 5 kPa, and D) response and recovery time of sensor.

Figure 3 .
Figure 3. Real-time monitoring of human motion A) relaxed and fully inflated bicep, B) wrist bending, C) finger joint bending, D) smiling, E) knee bending, and F) walking.
Authors.Advanced Sensor Research published by Wiley-VCH GmbH

Figure 4 .
Figure 4. A) Flexible 3 × 3 sensor grid coupled with the Arduino platform for wearable keyboard application and B) the keyboard mounted on the human arm.

Table 1 .
Performance comparison of the capacitive pressure sensors using porous dielectric layers.