The team of Professor Pan Tingrui of USTC developed a wearable adaptive penile hardness monitoring system (WARM). The system uses the principle of active load measurement, and the two-ring sensor consists of strain sensing rings with different elastic moduli, providing high resolution, stability, and immunity to interference. Erectile dysfunction is assessed by transmitting a signal via Bluetooth to the host computer. Compared to the conventional RigiScan, the WARM system is smaller and simpler. Tested on penis models and volunteers, the WARM system is as accurate as standard clinical equipment with minimal interference with erections, providing a reliable and convenient protocol for clinical diagnosis of ED.
Erectile dysfunction (ED) is a common sexual dysfunction, and continuous monitoring of penile erection and stiffness with spontaneous nocturnal erections is essential for its diagnosis and classification. However, current clinical standard devices are bulky and non-wearable due to the limitations of their active mechanical loads, strongly interfering with erections, which both affects the reliability of monitoring and reduces patient compliance.
Recently, the team of Professor Pan Tingrui of the University of Science and Technology of China has developed a wearable adaptive rigidity monitoring (WARM) system, which uses the measurement principle without active load to evaluate penile erection and hardness through a specially designed elastic double-ring sensor. This double-ring transducer consists of two strain sensing rings with different elastic modulies, providing high resolution (0.1%), robust mechanical and electrical stability (lasting more than 1000 cycles), and robust immunity to interference. An integrated flexible printed circuit (FPC) collects and processes the sensed signal, which is then transmitted via Bluetooth to a host computer for ED evaluation. In addition, the system facilitates simultaneous sleep monitoring during the nocturnal erection test, continuously assessing penile erection while minimizing the ability to interfere with nocturnal erections. The system provides a fully integrated, wearable solution for continuous, precise, and patient-friendly measurement of penile erection and hardness. The work was published in the latest issue of the Nature journal Microsystems & Nanoengineering in a paper titled "A wearable adaptive penile rigidity monitoring system for assessment of erectile dysfunction".
Penile erection hardness measurement principle and WARM system
The operating principle of the WARM system is first compared with the widely recognized clinical device RigiScan. The principle used by RigiScan to assess penile erection and hardness is based on dynamic mechanical measurements. Specifically, the evaluated erection reflects the change in penile circumference during erection, and the evaluated stiffness reflects the ability to resist radial deformation under known compressive forces, as shown in Figure 1a. It can be observed that the RigiScan exerts a tensile force in the tangential direction. Such a tangential stretch can then be converted to radial compression, which changes the radial shape of the penis and measures the corresponding circumference. To implement such measurements, RigiScan employs a two-step process. Initially, a gentle reference load (Tr) is applied at regular intervals to measure the circumference (Dr) of the penis without causing noticeable deformation. When a significant change in penile circumference is detected, the RigiScan device can then apply a more robust compressive load (Tm) while continuously assessing the corresponding circumferential displacement (Dr-Dm), which makes it possible to determine the radial hardness (S) of S = (Tm-Tr)/(Dr-Dm). Therefore, it is essential to include a motorized loading component with a high-precision length measurement unit in the system design, which increases the complexity of the system and limits its use in clinical settings.
Figure 1b shows a schematic diagram of the WARM system, which is designed to monitor penile erection. The wearable sensing device provides real-time information on penile hardness and erection, which is instantly transmitted to the mobile device via the Bluetooth module, allowing doctors to conduct more accurate analysis and diagnosis. As shown in Figure 1c, the system consists of three main components: an accurate and stable double-ring sensor consisting of two sensing units with different moduli of elasticity, the reference ring and the measuring ring. Specifically, flexible double-ring sensors with different moduli of elasticity can adaptively apply different levels of load to the penis and convert circumferential data into capacitive output. The sensor is reversibly connected to the FPC board via a standard connector, which facilitates signal acquisition by the capacitance detection module. Subsequently, a microcontroller unit (MCU) performs a real-time assessment of penile stiffness and erection. Figure 1d shows a prototype of the WARM system, which has a more compact size, reduced weight, and simplified design than the RigiScan. The double-loop method allows the elastomeric sensor to simultaneously apply radial compression and provide continuous circumferential measurements.
Figure 1. Wearable Adaptive Stiffness Monitoring (WARM) system.
Design, optimization, and characterization of elastic double-loop sensors
Figure 2a illustrates the planar layout of an elastic double-ring sensor, which consists of a reference ring and a measuring ring, both of which are based on the parallel plate capacitance principle for strain measurements. The parallel-plate sensing architecture has been shown to be highly mechanically and electrically stable in continuous measurements. It is designed as a 7-layer structural assembly, in which all layers are made of highly elastic polymer material, and the conductive layers are always separated by adjacent insulating layers. The capacitance between the middle conductive layer and the two outer layers is measured to assess penile erection and hardness. This structural design also increases the initial capacitance and device sensitivity by doubling the capacitance overlap area. The conductive layer can be made using a highly conductive and elastic ionic gel or a commercially available stretchable conductive silver paste, while the protective and dielectric layers can be made from off-the-shelf silicone rubber. Thanks to the fully flexible design, the overall structure of the sensor can withstand bending and stretching without mechanical fracture (Figure 2B).
Determining the optimal geometry of a double-ring sensor, including dielectric layer thickness and electrode layer width, requires specific mechanical and electrical requirements to meet the design principle. In addition, it is crucial to choose the right material for each layer. After evaluation, the authors chose a dielectric layer thickness of 200 microns. This thickness provides the best balance between sensitivity and reliability, as shown in Figure 2c, with sensitivity and initial capacitance of 95.86 pF and 1.26 pF/mm, respectively. In addition, in order to achieve the required accuracy in penile circumference measurements, the sensor must exhibit a significant and stable capacitance change at a deformation of 0.1 mm, which corresponds to approximately 0.1% of the sensor strain. As shown in Figure 2d, samples with electrode widths of 5 mm and 7 mm showed a steady increase in capacitance at 0.1 mm deformation of 0.172 pF and 0.258 pF, respectively.
Once the geometry of the sensor has been determined, it is critical to select the right material to ensure that the mechanical properties meet the design requirements. The authors prepared samples of silicone rubber with different elastic modulus and tested them. The test results shown in Figure 2e show that the maximum tensile load varies greatly from 0.248 N to 2.590 N for different silicone rubber samples. Based on the design principle of the double-ring sensor, Ecoflex 0020 and Dragon Skin 30 were chosen as the insulating materials for the reference and measuring rings, respectively. Specifically, Ecoflex 0020 and Dragon Skin 30 exhibit a minimum and maximum elastic modulus, with maximum tensile loads of 0.248 N and 2.590 N, respectively, both below the upper limit of the design. In addition, both ionic gels and conductive silver pastes were tested as electrode layer materials and maintained good conductivity after 1000 tensile cycles; Therefore, both materials are suitable as electrode layers. Ionic gels are easy to prepare, cost-effective, and transparent, while conductive silver pastes have lower electrical resistance. With these material choices for insulation and electrode layers, the dual-ring sensor meets the design requirements in terms of mechanical properties, wearing comfort, and conductivity.
After optimization, the double-ring sensor is fabricated and characterized to evaluate its mechanical and electrical properties. Figure 2b shows an optical image of a sensor fabricated using ionic gels and conductive silver paste. Figure 2f shows the capacitance-circumferential displacement and tension-circumferential displacement responses of the reference and measuring rings. Even when the maximum circumferential displacement is 37 mm stretched, the maximum tensile forces observed for the reference and measuring rings are 0.365 N and 2.528 N, respectively, which is lower than the RigiScan's 1.11 N and 2.78 N. This reduces the measurement load to reduce interference with the erection. In addition, the adaptive load of the dual-ring sensor is more conducive to continuous and imperceptible monitoring of penile hardness and erection.
Figure 2g illustrates the capacitance and tension records of a sensor using a measuring ring under a step change of 1 mm of deformation. The results show that the capacitance and tensile load of the sensor change significantly with each small deformation and remain stable for the specified duration. In Figure 2h, the initial capacitance of the measuring rings of five volunteers in the non-contact and contact states is compared. The initial capacitance values showed a small difference (less than 0.33 pF), suggesting that the parasitic capacitance changes caused by different patients would not significantly affect the actual measurements of the sensor. The repeatability of the elastic double-ring sensor was evaluated by performing a tensile cycle test on two rings. Figure 2i illustrates the consistent mechanical and electrical properties of the measurement and reference loops with no significant baseline drift after 1000 cycles.
Figure 2. Design and characterization of dual-ring sensors.
Feasibility test of the WARM system as well as penile erection assessment
The authors developed a penile erection simulator to effectively characterize the WARM system. It allows for stable and programmable adjustments to the firmness and erection of expandable penis models, providing a standardized test platform to evaluate the WARM system. Figure 3a illustrates the simulator, which includes an expandable penis model, a flow controller for indirectly adjusting the stiffness and circumference of the model, and a computer equipped with the corresponding control software. The penis model is a commercial hollow round cylinder made of elastic silicone that reflects the physiological characteristics of penile hardness and erection that change with pressure within the cavernosa body. By adjusting its internal pressure, the hardness and erection of the model can be flexibly controlled. When the pressure control unit (i.e. flow controller) is connected, the stiffness and erection of the model can be precisely adjusted by computer programming.
Figure 3c shows a comparison of hardness measurements obtained from the WARM system and RigiScan on a penis model under varying internal pressures. The RigiScan has a poor ability to continuously monitor erections, while in contrast, the WARM system maintains a tensile load of less than 1 Newton for hardness measurement when the internal pressure of the model is below 20 mmHg due to the adaptive tensile load applied by the elastic double-loop sensor. This adaptability allows the WARM system to continuously monitor penile stiffness in a patient-friendly manner without the need for a full erection.
The linear regression analysis of the hardness measurements of the two systems in Figure 3d showed a high degree of agreement with a correlation coefficient of 0.98. This is because both the WARM system and the RigiScan use the same radial hardness definition and calculation formula. However, their measuring principles differ: the WARM system uses elastic double-loop measurements, while the RigiScan uses a traditional one-step force-displacement process. To simulate the penile erection-regression process during actual erection monitoring, the penile model underwent multiple cycles of inflation and deflation of varying durations. The RigiScan and WARM systems monitor this process at the same time, and the results are shown in Figure 3e.
Figure 3. Feasibility testing of the WARM system.
Clinical testing in volunteers
To further evaluate the clinical utility of the WARM system in monitoring erections and diagnosing ED, the authors recruited two volunteers for AVSS and NPTR tests. Currently, AVSS and NPTR tests serve as the two main diagnostic methods to distinguish between psychological and organic ED, as they are non-invasive and objective. The results of the AVSS test are shown in Figure 4a. It can be observed that after 2 minutes of audiovisual stimulation, there is a significant increase in the erection and hardness of the penis, indicating that an erection has occurred. This erection lasts until the 22.5th minute, followed by a gradual decrease in erection and hardness, indicating the end of the erection. Comparative analysis showed that the WARM system provided superior real-time tracking of penile erection status, providing more accurate circumferential assessment and high-precision hardness measurement than the RigiScan system.
Nocturnal penile erection is a normal physiological phenomenon that usually occurs during rapid eye movement (REM) sleep. To confirm the ability of the WARM system to minimize disturbances to sleep and nocturnal erections, a second volunteer initially wore a Fitbit continuously for monitoring during three nights of sleep. On the fourth night, he was told to wear both the Fitbit and WARM systems for a combined NPTR-sleep assessment (Figure 4c). Fitbit is a wearable sleep monitor commonly used in medical research that provides detection of sleep stages. As shown in Figure 4d, the data showed that the use of the WARM system did not affect sleep quality, as no reduction in time was observed in the deep sleep or REM phases, and there was no increase in the time spent in the waking phase. Figure 4e provides a detailed analysis of sleep data and NPTR results, highlighting areas where penile circumference has increased by more than 6 mm or hardness has exceeded 60%. The comparison results verify the reliability of the WARM system in monitoring NPTR. During the seven-hour NPTR monitoring period, the WARM system demonstrated high fidelity and accuracy with minimal interference with erections, thereby enhancing its clinical value in the diagnosis of ED.
Figure 4. Volunteer test for continuous penile erection assessment using the WARM system.
brief summary
The study developed a wearable adaptive hardness monitoring (WARM) system capable of continuously monitoring penile stiffness and erection for clinical ED diagnosis. Specifically, the authors propose a differential elastic measurement principle that replaces the traditional two-step force-displacement process by using the double-ring method. This approach allows for the assessment of penile erection by a specially designed elastic double-ring sensor, addressing the challenges of measurement reliability and patient compliance associated with mechanical moving parts. The two-ring sensor consists of two strain sensing rings with different elastic moduli, providing high resolution (0.1%), robust mechanical and electrical stability (lasting more than 1000 cycles) and strong immunity to interference. An integrated flexible printed circuit (FPC) collects and processes sensor signals in real time and transmits them via Bluetooth to the terminal for ED evaluation. To verify the reliability of the WARM system, the authors conducted an exhaustive comparison with RigiScan through experiments with penile models and human subjects. The study found that the WARM system can skillfully monitor penile stiffness and erection, providing accuracy comparable to that of standard clinical equipment (R2 > 0.98) while ensuring minimal interference with erections. The system provides a fully wearable, integrated solution for measuring penile stiffness and erection in a continuous, accurate, and patient-friendly manner, potentially leading to more reliable and convenient results in the clinical diagnosis of ED.
It is recommended to transfer it to the brother who needs it, so as to achieve early detection and early treatment
Article Links:
https://www.nature.com/articles/s41378-024-00721-5