Highly Sensitive and Linear Vibration‐Based Flexible Modulus Sensing System for Human Modulus Monitoring and Disease Prevention

It is important to directly characterize the modulus of objects through wearable systems, which can further develop electronic skin and human‐computer interaction. In this work, a modulus sensing system based on a pressure sensor and vibration module is developed. This resistive pressure sensor exhibits good linearity and sensitivity. Combined with the vibration module to adjust the applied stress and strain, the modulus value of the objects can be directly calculated. The modulus sensing system demonstrates excellent modulus monitoring capabilities in the range of 60‒3000 kPa modulus and high linearity. In addition, it is used to detect modulus values for different parts of the human body and integrate a concept eye mask to prevent and monitor migraine. The system shows the potential to be further applied to wearable devices for human health monitoring and disease prevention.


Introduction
Softness perception is an intrinsic function of human skin.[17] The measured parameters of human DOI: 10.1002/adsr.202300148soft tissue are also modulus values, which are used to determine the extent of certain diseases.20][21][22][23] Based on the complex conversion mechanism of modulus, how to directly characterize the modulus of objects based on flexible wearable sensors is a major difficulty.[26][27][28][29] One solution is to build a modulus sensing system by multiple sensor integrations. [24]The stress and strain are tested respectively by the pressure sensor and the displacement sensor, and the modulus value is calculated.For example, a thin-layer modulus sensor based on a strain sensor and a pressure sensor is developed. [25]The sensor is able to distinguish touch materials with different moduli.It also can be mounted on human skin to compare and evaluate the skin modulus of healthy and dermatological patients.This solution can effectively characterize the modulus value of the object, but there are still problems such as multi-signal coupling of sensors, excessive volume of precision displacement sensors, high design cost, and inconvenient integration.It is suitable for industrial testing environments.
Another solution is to design a specific physical structure of the system to immobilize the strain during the test. [26]The modulus value can be calculated from the stress measured by a single pressure sensor.A haptically self-locking measurement-based modulus sensing system is designed. [27]The system is based on the self-locking effect of the Hertz model, enabling a single stretchable strain sensor to test modulus with a single touch.Integrating the system on a prosthetic limb, the modulus of swollen tissue in patients with fractures can be detected, showing objective and reliable palpation.This method exhibits high integration, but it needs to apply a stable external force source, which is difficult to measure stably for a long time.
A novel solution is a modulus sensing system based on Micro-Electro-Mechanical System (MEMS) design. [28]A simple, miniature electromechanical system that can interface with biological tissues for precise elastic modulus is proposed.The system detects modulus in different parts of the human body, as well as biomaterials with a modulus comparable to that of human soft tissue.It can be used in applications related to the clinical evaluation of patients with skin diseases.This design scheme is more advantageous in the application of human body modulus monitoring.
Herein, a high sensitivity and linearity modulus sensing system based on a pressure sensor and vibration module is proposed.The system can accurately and quickly evaluate the elastic modulus of the objects within a certain frequency and modulus range (a single measurement can be up to 1 s).The vibration module provides adjustable fixed strain at a constant frequency.The pressure sensor is responsible for reading the stress change.Through the designed readout circuit system, the modulus of the objects under test can be directly calculated.In addition, the human body modulus is also evaluated based on the system, including changes in different parts of the human body and movement.A smart eye mask based on the modulus sensing system is proposed, which can be used for auxiliary treatment and prevention of migraine.

Fabrication of the Modulus Sensing System
Figure 1a is the schematic illustration of the modulus sensing system.The system mainly includes three modules: 1) Vibration module for automatic and periodic stressing of an underlying pressure sensor; 2) Resistive pressure sensor for sensing pressure, mainly including graphene/ polydimethylsiloxane (PDMS) sensing layer, silver electrode, and Polyimide (PI) for packaging (Figure 1b); 3) A elastomer with a certain elasticity modulus for modulus detection.Figure 1c demonstrates a schematic diagram of the modulus sensing system overview.The detailed preparation procedures are discussed in the experimental section and Figure S1 (Supporting Information).A top-view optical picture of the modulus sensing system is shown in Figure S2 (Supporting Information).
Figure 1d explains how to build a modulus sensing system with a vibration module and a pressure sensor.Apply a stable and adjustable voltage (2.5 V in this case) to the vibration module through a voltage source to control the vibration module, which applies pressure to the pressure sensor at a settled frequency.
When the modulus of the elastic modulus below changes, the resistance change range of the pressure sensor changes accord-ingly, and the modulus can be measured by the resistance change range.The resistance value of the pressure sensor changes continuously when an external force is applied.Through the resistance change of the pressure sensor, the relationship between force and modulus can be further analyzed.The modulus value of the objects to be measured is obtained by reading the resistance signal of the pressure sensor through the designed circuit and calculating it.The detailed readout and calculation processing circuit design is demonstrated in the experimental section.

Performance of the Modulus Sensing System
Figure 2 shows the mechanical-electrical properties of the pressure sensor in sensing layer of the modulus sensing system.This pressure sensor is based on a resistive sensing mechanism.The sensitivity of a resistance-type pressure sensor is defined as Formula 1. R 0 and ΔR are the initial and relative resistance change values corresponding to before and after pressure (P).A resistive pressure sensor with high sensitivity is susceptive to pressure changes (ΔP), leading to a significant variation of the resistance (ΔR) concurrently.
As shown in Figure 2a, the pressure sensor exhibits high sensitivity and linearity over a specific pressure range.The averaged sensitivity is 2.61, 0.15, and 0.02 kPa −1 within the pressure range of 0-50, 50-100, and 100-300 kPa, respectively.The corresponding linearities are 0.99, 0.92, and 0.99, respectively.Considering the high sensitivity and linearity of the range of 0-50 kPa, this range is used in the subsequent modulus sensing system design.Figure S3 (Supporting Information) also shows the Gauge factor of the pressure sensor.Benefiting from the excellent performance in this range, the pressure sensor exhibits a low detection (LOD) limit of ≈1 Pa, as evidenced in Figure 2b.
The dynamic response speed is a crucial parameter of the pressure sensor.Figure 2c shows the typical relative resistancepressure curves under 10 kPa pressure of different frequencies (0.25, 0.5, and 1 Hz, respectively), indicating the rapid response capability.As shown in Figure 2d, a 100 kPa pressure is suddenly squeezed on the pressure sensor and quick release, revealing a 10 ms rise time and a 12 ms fall time, demonstrating a fast response ability.The zoom-in view of the rise and fall time for the pressure sensor is shown in Figure S4 (Supporting Information).
Repeatability and stability are crucial properties of the pressure sensor for further system use and design.Figure 2e demonstrates negligible signal drift or fluctuation after over 3000 cycles with 0-10 kPa pressure cycles, illustrating high repeatability.Figure S5 (Supporting Information) exhibits the enlarged view of resistance change at 101-110 cycles and 1901-1910 cycles.
Figure 3 shows the modulus test performance of the modulus sensing system.The modulus is the ability of an object to resist elastic deformation, which is defined as the ratio of stress to strain under stress, as shown in Formula 2.
In this work, the modulus sensing system is based on the resistive pressure sensor and vibration module.Stress can be expressed by the ratio of the relative resistance change rate of the sensor to the sensitivity.Strain is the deformation quantity to the thickness of the object.The thickness of the object can be directly measured, and the deformation quantity can be obtained by the amplitude of the vibration module.Therefore, the expression for the modulus can be converted to the form as shown in Formula 3.
where the S is the sensitivity in the linear region of the sensor, ΔR/R is the relative resistance change rate of the sensor stressed, and ΔL/L is the strain of the object.
Figure 3a demonstrates the relationship between the relative resistance change and modulus with the same amplitude for the vibration module and different thicknesses for the objects.It shows that when the amplitude of the vibration module remains constant (the deformation quantity of the object remains unchanged), the modulus of the object with the same thickness is proportional to the relative resistance change of the sensor.In addition, for the same modulus and different thicknesses of the objects, the relative resistance change decreases with the increase in thickness.Considering that the range of human body modulus is ≈0-500 kPa, a further test is measured in this range, and the results are shown in Figure 3b, which is consistent with the results in Figure 3a.The relationship between the relative resistance change and modulus with different amplitude for the vibration module, the same thickness for the objects is also shown in Figure 3c.It further proves the proportional relationship between the relative resistance change of the sensor and the modulus of the objects under the same strain.
Figure 3d exhibits the relationship between the relative resistance change and amplitude for the vibration module with the same thickness for the objects.The test is carried out on the same object, and the amplitude of the vibration module is constantly adjusted.The relative resistance change increases with the increase of the amplitude, which explains that the amplitude of the vibration module is related to the deformation quantity of the objects.The relationship between the relative resistance change and thickness for the objects with the same amplitude for the vibration module is also shown in Figure 3e, indicating the inverse relationship between the thickness and relative resistance change.
Figure 3f shows the detection ranges and limits of the modulus sensor system.The results show that the modulus sensor system exhibits a good and linear modulus test capability in the modulus range of 60-3000 kPa. Figure S6 (Supporting Information) shows the ranges of failure in the modulus sensing systems.
[32][33] The 3D simulation model of the modulus sensing system is shown in Figure S7a (Supporting Information).The model is mainly divided into three parts, including the vibration module, pressure sensor, and objects.The corresponding dimensions of each part and the overall side view are shown in Figure S7b (Supporting Information).The vibration module is set as a rigid cylinder with a circle diameter of 8 mm and a height of 2.7 mm.The parameters of the pressure sensor are a rectangular parallelepiped elastic film with a length and width of 5 mm, a thickness of 100 m, and a modulus of 200 kPa.The parameters of the objects are rectangular parallelepiped.The length, width, and height of elastomer samples are both 8 mm.The modulus parameters of the elastomer samples are 100, 500, and 1000 kPa, respectively.
Figure 4a-c demonstrates the results of finite element simulation of different modulus objects.When the same pressure is applied to the objects through the vibration module, the crosssectional stress distribution cloud images of the test object with different moduli are also different.The stress distribution of high-modulus objects is larger and the distribution depth is shal-  lower than that of low-modulus objects.This exhibits that by adjusting the stress, the test depth of different modulus objects can be further adjusted.

Applications of the Modulus Sensing System
[36][37] Different parts of the human body have different tissue structures, exhibiting significant differences in modulus values.It is feasible to judge the human body part or motion state through the modulus value.In order to verify that the pressure sensor is not affected by bending or stretching when attached to different parts of the human body, the initial resistance change of the pressure sensor is tested.The results are shown in Figure S8.The initial resistance change of the pressure sensor is negligible, indicating that stretching and bending caused by human body wearability have no effect on the pressure sensor.The modulus sensor system is able to directly detect the modulus of humans on the skin, which is shown in Figure 5.It should be noted that the tissue thickness is uniformly defined as 2 mm in this experiment, which is the approximate thickness of the human skin layer.
As shown in Figure 5, five different locations on the human body (abdomen, forearm, biceps, calf, and focile) are selected for modulus detection.These areas contain the main movements and functions of the human body.The results exhibit that the resistance value change of the modulus sensing system reproduced significantly differently in different human body regions (Figure 5a).In the same position, the stable resistance change value indicates the repeatability and stability of the measurement.Figure 5b shows the modulus value after calculation, all modulus values are the average of the results of more than ten times to ensure accuracy.It is obvious that the modulus values of these different regions of the human body are significantly different (abdomen≈80 kPa, calf≈162 kPa, forearm≈200 kPa, biceps≈325 kPa, focile≈513 kPa).In addition, the above five different regions of the human body are respectively tested on different subjects with different Body Mass Index (BMI) and Body Fat Percentages (BFR), and the results are shown in Figures S9-S11 (Supporting Information).
Further, the movement state of a certain part of the human body can be estimated by the modulus sensing system.As shown in Figure 5c, the modulus value is measured at the same location on the forearm when the human hand is repeatedly relaxed and tightened.The modulus values of the forearm change significantly, which is caused by the tense and relaxed states of the muscle.When the hand is relaxed, the forearm muscles are in a relaxed state, and the modulus value is low.When the hand is held tightly, the forearm muscles are in a tense state, and the modulus value increases significantly.Figure 5d is an enlarged data graph for a single-hand relaxation and clenching process, further proving the stability of the system.More details can be found in Movie S1 (Supporting Information).
With age, human skin constantly loses water, which reduces skin tension and increases modulus values.Determining the moisture content in human skin from the modulus value is an interesting application.As shown in Figure 5e, hydrogels with different hydration levels are prepared to simulate human skin with different water contents for modulus testing.The results demonstrate that the modulus values gradually decrease with the increase of the hydrogel hydration level, which is consistent with the expected results.Figure S12 (Supporting Information)shows the different modulus values of pork with different water content, which is able to detect the water-injected pork.The modulus sensor system also can distinguish the modulus value of pork and pig viscera (Figure S13, Supporting Information).Figure 5f exhibits the comparison of measured and calculated modulus values under different hydrogel hydration levels.It further demonstrates the accuracy of the modulus sensing system.
[40] By monitoring the modulus changes of human temples, the disease of migraine can be analyzed and prevented.The designed smart eye mask is based on the modulus sensing system, which is used to monitor the modulus changes in human temples.Through circuit design, the sensor signal is able to be displayed in real-time on a small display screen, and the corresponding modulus value is also calculated and demonstrated.Figure S16 (Supporting Information) shows the modulus changes of the temples measured by the same subject at different periods in one day and Figure S16b (Supporting Information) is the enlarged picture.The results demonstrate the modulus of the human temple changes with the degree of fatigue, showing the potential of the modulus sensing system for migraine detection and prevention.
[43][44] Modulus classification and identification of human body parts using a Transformer Network based on the modulus sensing system.The network architecture is shown in Figure 6b, mainly using the multi-head attention algorithm.By training the network model on the collected samples (32 subjects, 4000 samples), the confusion matrix results are shown in Figure 6c.The labels (I-V) correspond to abdomen, calf, forearm, biceps, and focile, respectively.In the cross-validation of these five different human body parts, a classification and recognition high accuracy of 94.3% is achieved.
Figure S17 (Supporting Information) also exhibits the collected statistics on the modulus value of the five human body parts and the classification accuracy for each part.The above results demonstrate the great potential of the modulus sensing system in intelligent applications.

Conclusion
In conclusion, a modulus sensing system based on a pressure sensor and vibration module is developed.This resistive pressure sensor exhibits good linearity and sensitivity.Combined with the vibration module to adjust the applied stress and strain, the modulus value of the objects is able to be directly calculated.The modulus sensing system demonstrates excellent modulus monitoring capabilities in the range of 60-3000 kPa modulus and high linearity.In addition, it is used to detect human body modulus and integrate a concept eye mask to prevent and monitor migraine.The system shows the potential to be further applied to wearable devices for smart medical care and disease prevention.

Experimental Section
Materials and Chemicals: The hydro graphene was purchased from Carbon Valley Technology Co., Ltd.(Jiangsu, China).SYLGARD 184 Silicone Elastomers were obtained from Sigma-Aldrich Trading Co., Ltd.(Shanghai, China).The vibration modules were purchased from Ruishengwei Technology Co., Ltd.(Shenzhen, China).All the materials and chemicals were used as received.
Fabrication of the Pressure Sensor: The fabrication process of the pressure sensor is shown in Figure S1 (Supporting Information).Apply the prepared PDMS solution (agent A to B ratio of 10 to 1) evenly on the sandpaper template.The template was placed in an oven at 85 °C for 1 h.After curing, the PDMS was peeled off from the template to obtain a PDMS film substrate.Afterwards, the hydrographene film was transferred to the PDMS film substrate by the Langmuir-Blodgett film method.For the detailed method of the LB method hydrographene film transfer, please refer to the previous work: "Synthesis of Sensitive Graphene Film on Substrate" section.

Preparation of Objects and Standard Modulus Test:
The objects with different modulus were adjusted by the ratio of SYLGARD 184 agents A and B. The ratio of reagent A to B is able to be adjusted from 3:1 to 15:1, and the modulus range is 10 kPa-3500 kPa.The modulus value of the objects was directly calculated by the pressure and displacement.The standard pressure sensor was purchased from LEGACT Technology Co., Ltd.(Shenzhen, China).The displacement was tested by a high-precision electronic testing machine (UH6502, Youhong).Different thicknesses and sizes of the objects could be regulated by grinding tools.
Preparation of Hydrogels: Weigh 10 g of corn starch and add it to 30 ml of deionized water.Stir the solution with a stirring speed of 200 rpm min −1 at 80 °C for 10 min to form a viscous gel.Transfer the viscous substance to a petri dish, and place it in an environment of -20 °C for two hours.Then take it out and place in a room temperature, and the hydrogel preparation was completed.Different hydration levels were achieved by regulating the content of deionized water.The weight of corn starch was fixed at 10 g, and the amount of deionized water was adjusted to 30 -80 ml, corresponding to a change in hydration levels of 30%−80%.
Assembly of the Modulus Sensor System: Silver electrodes were added to the prepared pressure sensor and packaged with Polyimide (PI).Glue the vibration module directly on top of the pressure sensor.Put the vibration module and pressure sensor directly on different objects, and the modulus measurement was ready.
Design of the Modulus Application System: The block diagram of the modulus application system was demonstrated in Figure S14 (Supporting Information).There were three steps to calculate the modulus.In the first step, A vibrator motor combined with an adjustable resistor produces stress, which had constant amplitude acting on the sensor, and the adjustable resistor was used to control the magnitude of stress.Then, the sensor responds to the stress by acting as a variable resistor, and the change of resistor was converted to voltage using a voltage division circuit.In the end, the voltage was sampled by an on-chip ADC, and the microprocessor employs an envelope detection algorithm to calculate the modulus.The calculated data and raw data were finally wirelessly transmitted and displayed on a PC.
Figure S15 (Supporting Information) shows the detail of the entire circuit.The adjustable resistor connected to the vibrator motor limits the start-up current of the motor and thus controls the magnitude of the stress generated by the motor and the model of the vibrator motor was an 0827-coin type vibration motor.The voltage division circuit consisting of a Wheatstone bridge and an instrument amplifier converted the variable resistor to the voltage signal.The type of operational amplification used in the circuit was AD8541.The converted voltage was finally sampled by a 12-bit on-chip SAR ADC which was inside the selected microprocessor nRF52840.
Characterizations and Measurement: The scanning electron microscopy (SEM) model was Zeiss GeminiSEM450 and a 3 kV accelerating voltage was used to acquire the SEM images.A high-precision electronic testing machine (UH6502, Youhong) controlled by a computer recorded mechanical properties while the real-time electrical data was measured by an LCR measurement instrument (TH2810d, Tonghui).

Figure 1 .
Figure 1.a) Exploded-view schematic illustration of the modulus sensing system.b) The schematic diagram of the pressure sensor in the sensing layer.c) Overview schematic illustration of the modulus sensing system.d) The schematic diagram of the measurement mechanism for the modulus sensing system.

Figure 2 .
Figure 2. Pressure sensor properties and characteristics of the modulus sensor system.a) A curve of relative resistance changes under increasing pressure and the corresponding sensitivities, showing three linear working regions.b) Resistance change versus pressure curve under a low limit of detection (LOD) of 1 Pa.c) Typical relative resistance change-strain curves under 10 kPa of 0.25, 0.5, and 1 Hz.d) The rise and fall time of the pressure sensor under loading and unloading 100 kPa pressure.e) Relative resistance changes under repeated compressing and releasing for over 3000 cycles under 0 -10 kPa.

Figure 3 .
Figure 3.The modulus test performance of the modulus sensing system.a-c) The relationship curve between the relative resistance change and modulus with a) same amplitude for vibration module, different thickness for the objects; b) enlargement of Figure 3a in human body modulus (0-500 kPa); c) different amplitude for vibration module, same thickness for the objects.d) The relationship curve between the relative resistance change and amplitude for the vibration module with the same thickness for the objects.e) The relationship curve between the relative resistance change and thickness for the objects with the same amplitude for the vibration module.f) Detection ranges and limits of the modulus sensor system.

Figure 4 .
Figure 4. a-c) Normalized FEA results for distributions of strain throughout the structure with different moduli of a) 100 kPa modulus; b) 500 kPa modulus; c) 1000 kPa modulus.

Figure 5 .
Figure 5. Modulus detection of the human body skin.a) Sensor resistance change results in five different body parts.b) Modulus detection results on five different body parts.c) Changes in sensor resistance when the muscles in the human arm change and d) is an enlarged image.e) Modulus changes induced by changes in the hydration level of human skin simulated by hydrogels.f) The relationship between the modulus value and the hydration level, compared with the experimental value, and the calculated value.

Figure 6 .
Figure 6.a) Schematic diagram of the relationship between migraine and temple modulus changes and design of the modulus application system.b) Human body part recognition network architecture based on modulus sensing system, using multi-head attention algorithm for classification.c) The confusion matrix for the human body part classification and identification.