{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,17]],"date-time":"2026-10-17T15:10:29Z","timestamp":1792249829838,"version":"4.4.0"},"reference-count":38,"publisher":"MDPI AG","issue":"20","license":[{"start":{"date-parts":[[2025,10,13]],"date-time":"2025-10-13T00:00:00Z","timestamp":1760313600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"crossref","award":["62404134"],"award-info":[{"award-number":["62404134"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"crossref"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Cardiovascular disease (CVD) remains the leading cause of global mortality, highlighting the need for continuous vital sign monitoring. Photoplethysmography (PPG) is well suited for wearable devices. Smart rings, benefiting from dense capillary distribution and minimal tissue interference, can capture high-quality PPG signals with comfort, making them a promising next-generation wearable. However, ring rotation relative to the finger alters the optical path, especially for multi-wavelength light, thus reducing accuracy. This paper proposes a rotation-robust PPG sensor for smart rings. Monte Carlo simulations analyze photon transmission under different LED\u2013photodiode (PD) angles, showing that at \u00b160\u00b0, green, red, and infrared light achieve optimal penetration into the microcirculation layer. Considering non-ideal conditions, the green-light angle is adjusted to \u00b130\u00b0, and a symmetrical sensor design is adopted. A prototype smart ring is developed, capable of recording 4-channel PPG, 3-axis acceleration, and 4-channel temperature signals at 100, 25, and 0.2 Hz, respectively. The system achieves reliable PPG acquisition with only 0.59 mA average current consumption. In continuous testing, heart rate estimation reached mean absolute errors of 0.82, 0.79, and 0.78 bpm for green, red, and IR light. The results provide a reference for future smart ring development.<\/jats:p>","DOI":"10.3390\/s25206326","type":"journal-article","created":{"date-parts":[[2025,10,15]],"date-time":"2025-10-15T08:34:26Z","timestamp":1760517266000},"page":"6326","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":7,"title":["Development of a Rotation-Robust PPG Sensor for a Smart Ring"],"prefix":"10.3390","volume":"25","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-0972-8810","authenticated-orcid":false,"given":"Min","family":"Wang","sequence":"first","affiliation":[{"name":"School of Integrated Circuits (School of Information Science and Electronic Engineering), Shanghai Jiao Tong University, Shanghai 200240, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Wenqi","family":"Shi","sequence":"additional","affiliation":[{"name":"School of Integrated Circuits (School of Information Science and Electronic Engineering), Shanghai Jiao Tong University, Shanghai 200240, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jianyu","family":"Zhang","sequence":"additional","affiliation":[{"name":"School of Integrated Circuits (School of Information Science and Electronic Engineering), Shanghai Jiao Tong University, Shanghai 200240, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7150-5239","authenticated-orcid":false,"given":"Jiarong","family":"Chen","sequence":"additional","affiliation":[{"name":"School of Integrated Circuits (School of Information Science and Electronic Engineering), Shanghai Jiao Tong University, Shanghai 200240, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Qingliang","family":"Lin","sequence":"additional","affiliation":[{"name":"Shenzhen Ninenovo Technology Limited, Shenzhen 518000, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Cheng","family":"Chen","sequence":"additional","affiliation":[{"name":"School of Integrated Circuits (School of Information Science and Electronic Engineering), Shanghai Jiao Tong University, Shanghai 200240, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Guoxing","family":"Wang","sequence":"additional","affiliation":[{"name":"School of Integrated Circuits (School of Information Science and Electronic Engineering), Shanghai Jiao Tong University, Shanghai 200240, China"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2025,10,13]]},"reference":[{"key":"ref_1","first-page":"1151","article-title":"A 3-Lead ECG-on-Chip with QRS Detection and Lossless Compression for Wireless Sensors","volume":"63","author":"Deepu","year":"2016","journal-title":"IEEE Trans. Circuits Syst. II Express Briefs"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1109\/TIM.2023.3267378","article-title":"Continuous Blood Pressure Monitoring Using Nonpulsatile Photoplethysmographic Components for Low-Frequency Vascular Unloading","volume":"72","author":"Panula","year":"2023","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"608","DOI":"10.1016\/j.jacc.2021.05.048","article-title":"Sleep Disordered Breathing and Cardiovascular Disease: JACC state-of-the-art review","volume":"78","author":"Cowie","year":"2021","journal-title":"J. Am. Coll. Cardiol."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"5772","DOI":"10.1109\/JBHI.2022.3171918","article-title":"Edge2Analysis: A novel AIoT platform for atrial fibrillation recognition and detection","volume":"26","author":"Chen","year":"2022","journal-title":"IEEE J. Biomed. Health Inform."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"0176","DOI":"10.34133\/hds.0176","article-title":"Moving beyond medical statistics: A systematic review on missing data handling in electronic health records","volume":"4","author":"Ren","year":"2024","journal-title":"Health Data Sci."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"R1","DOI":"10.1088\/0967-3334\/28\/3\/R01","article-title":"Photoplethysmography and its application in clinical physiological measurement","volume":"28","author":"Allen","year":"2007","journal-title":"Physiol. Meas."},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Hartmann, V., Liu, H., Chen, F., Qiu, Q., Hughes, S., and Zheng, D. (2019). Quantitative Comparison of Photoplethysmographic Waveform Characteristics: Effect of Measurement Site. Front. Physiol., 10.","DOI":"10.3389\/fphys.2019.00198"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"115400","DOI":"10.1109\/ACCESS.2019.2928055","article-title":"Self-Sustainable Smart Ring for Long-Term Monitoring of Blood Oxygenation","volume":"7","author":"Magno","year":"2019","journal-title":"IEEE Access"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"14360","DOI":"10.1109\/JSEN.2020.3041072","article-title":"Study of a Ring-Type Surgical Pleth Index Monitoring System Based on Flexible PPG Sensor","volume":"21","author":"Zhou","year":"2021","journal-title":"IEEE Sens. J."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"127217","DOI":"10.1109\/ACCESS.2021.3111956","article-title":"Ring-Embedded Micro-Power mm-Sized Optical Sensor for Accurate Heart Beat Monitoring","volume":"9","author":"Boukhayma","year":"2021","journal-title":"IEEE Access"},{"key":"ref_11","unstructured":"Smart Ring News (2025, January 20). Latest Updates on Smart Ring Technology. Available online: https:\/\/smartringnews.com\/."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"0096","DOI":"10.34133\/hds.0096","article-title":"Recent progress in wearable brain\u2013computer interface (BCI) devices based on electroencephalogram (EEG) for medical applications: A review","volume":"3","author":"Zhang","year":"2023","journal-title":"Health Data Sci."},{"key":"ref_13","doi-asserted-by":"crossref","unstructured":"Wang, M., Chen, C., Wu, H., Zhang, J., Zhou, S., and Wang, G. (2024, January 24\u201326). Will Smart Ring be Next Wave of Wearables?. Proceedings of the 2024 IEEE Biomedical Circuits and Systems Conference (BioCAS), Xi\u2019an, China.","DOI":"10.1109\/BioCAS61083.2024.10798374"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"27106","DOI":"10.1109\/JSEN.2021.3123243","article-title":"Deep Learning Fused Wearable Pressure and PPG Data for Accurate Heart Rate Monitoring","volume":"21","author":"Mehrgardt","year":"2021","journal-title":"IEEE Sens. J."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"11653","DOI":"10.1109\/JSEN.2022.3170069","article-title":"SpO2 Measurement: Non-Idealities and Ways to Improve Estimation Accuracy in Wearable Pulse Oximeters","volume":"22","author":"Berwal","year":"2022","journal-title":"IEEE Sens. J."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"1257","DOI":"10.1109\/TBCAS.2023.3334960","article-title":"Meta-Analysis of Pulse Transition Features in Non-Invasive Blood Pressure Estimation Systems: Bridging Physiology and Engineering Perspectives","volume":"17","author":"Mohammed","year":"2023","journal-title":"IEEE Trans. Biomed. Circuits Syst."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Guo, H., Wu, H., Xia, J., Cheng, Y., Guo, Q., Chen, Y., Xu, T., Wang, J., and Wang, G. (2024, January 22\u201325). OSAHS Detection Capabilities of RingConn Smart Ring: A Feasibility Study. Proceedings of the 2024 IEEE 6th International Conference on AI Circuits and Systems (AICAS), Abu Dhabi, United Arab Emirates.","DOI":"10.1109\/AICAS59952.2024.10595903"},{"key":"ref_18","first-page":"69000210","article-title":"Design and Validation of a New PPG Module to Acquire High-Quality Physiological Signals for High-Accuracy Biomedical Sensing","volume":"25","author":"Kao","year":"2018","journal-title":"IEEE J. Sel. Top. Quantum Electron."},{"key":"ref_19","unstructured":"Kao, Y.H., Chao, P.C.P., Hung, Y., and Wey, C.L. (November, January 29). A New Reflective PPG LED-PD Sensor Module for Cuffless Blood Pressure Measurement at Wrist Artery. Proceedings of the IEEE SENSORS, Glasgow, UK."},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Lin, Y., Chen, C., Ma, Z., Sabor, N., Wei, Y., Zhang, T., Sawan, M., Wang, G., and Zhao, J. (2023). Emulation of brain metabolic activities based on a dynamically controllable optical phantom. Cyborg Bionic Syst., 4.","DOI":"10.34133\/cbsystems.0047"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"122","DOI":"10.1109\/RBME.2017.2739801","article-title":"Advances in Monte Carlo simulation for light propagation in tissue","volume":"10","author":"Periyasamy","year":"2017","journal-title":"IEEE Rev. Biomed. Eng."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"869","DOI":"10.1088\/0967-3334\/30\/9\/001","article-title":"Vessel calibre and haemoglobin effects on pulse oximetry","volume":"30","author":"McEwen","year":"2009","journal-title":"Physiol. Meas."},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Elgendi, M. (2016). Optimal Signal Quality Index for Photoplethysmogram Signals. Bioengineering, 3.","DOI":"10.3390\/bioengineering3040021"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"6351","DOI":"10.1109\/TIM.2020.2971132","article-title":"On-Device Integrated PPG Quality Assessment and Sensor Disconnection\/Saturation Detection System for IoT Health Monitoring","volume":"69","author":"Reddy","year":"2020","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"1867","DOI":"10.1109\/TBME.2009.2039568","article-title":"Two-stage approach for detection and reduction of motion artifacts in photoplethysmographic data","volume":"57","author":"Krishnan","year":"2010","journal-title":"IEEE Trans. Biomed. Eng."},{"key":"ref_26","doi-asserted-by":"crossref","unstructured":"Orphanidou, C. (2018). Signal Quality Assessment in Physiological Monitoring, Springer International Publishing. [1st ed.].","DOI":"10.1007\/978-3-319-68415-4"},{"key":"ref_27","first-page":"1910","article-title":"Real-Time PPG Signal Quality Assessment System for Improving Battery Life and False Alarms","volume":"66","author":"Vadrevu","year":"2019","journal-title":"IEEE Trans. Circuits Syst. II Express Briefs"},{"key":"ref_28","doi-asserted-by":"crossref","unstructured":"Kamshilin, A.A., Nippolainen, E., Sidorov, I.S., Vasilev, P.V., Erofeev, N.P., Podolian, N.P., and Romashko, R.V. (2015). A new look at the essence of the imaging photoplethysmography. Sci. Rep., 5.","DOI":"10.1038\/srep10494"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"501","DOI":"10.1007\/s11517-006-0070-0","article-title":"Non-invasive continuous estimation of blood flow changes in human patellar bone","volume":"44","author":"Pettersson","year":"2006","journal-title":"Med. Biol. Eng. Comput."},{"key":"ref_30","unstructured":"Kyriacou, P.A., and Allen, J. (2021). Photoplethysmography: Technology, Signal Analysis and Applications, Academic Press."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Volkov, M.V., Margaryants, N.B., Potemkin, A.V., Volynsky, M.A., Gurov, I.P., Mamontov, O.V., and Kamshilin, A.A. (2017). Video capillaroscopy clarifies mechanism of the photoplethysmographic waveform appearance. Sci. Rep., 7.","DOI":"10.1038\/s41598-017-13552-4"},{"key":"ref_32","doi-asserted-by":"crossref","unstructured":"Wang, M., Li, Z., Zhang, Q., and Wang, G. (2019). Removal of Motion Artifacts in Photoplethysmograph Sensors During Intensive Exercise for Accurate Heart Rate Calculation Based on Frequency Estimation and Notch Filtering. Sensors, 19.","DOI":"10.3390\/s19153312"},{"key":"ref_33","doi-asserted-by":"crossref","unstructured":"Zhou, H., Lu, T., Liu, Y., Zhang, S., Liu, R., and Gowda, M. (2023, January 9\u201312). One Ring to Rule Them All: An Open Source Smartring Platform for Finger Motion Analytics and Healthcare Applications. Proceedings of the 8th ACM\/IEEE Conference on Internet of Things Design and Implementation, New York, NY, USA. IoTDI \u201923.","DOI":"10.1145\/3576842.3582382"},{"key":"ref_34","first-page":"1","article-title":"Ring-a-Pose: A Ring for Continuous Hand Pose Tracking","volume":"8","author":"Yu","year":"2024","journal-title":"Proc. ACM Interact. Mob. Wearable Ubiquitous Technol."},{"key":"ref_35","doi-asserted-by":"crossref","unstructured":"Bi, R., Du, Y., Singh, G., Ho, C.J.H., Zhang, S., Attia, A.B.E., Li, X., and Olivo, M. (2020). Fast pulsatile blood flow measurement in deep tissue through a multimode detection fiber. J. Biomed. Opt., 25.","DOI":"10.1117\/1.JBO.25.5.055003"},{"key":"ref_36","doi-asserted-by":"crossref","unstructured":"Bi, R., Zhang, R., Meng, L., Du, Y., Low, J., Qi, Y., Rajarahm, P., Lai, A.Y.F., Tan, V.S.Y., and Ho, P. (2024). A portable optical pulsatile flowmetry demonstrates strong clinical relevance for diabetic foot perfusion assessment. APL Bioeng., 8.","DOI":"10.1063\/5.0182670"},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"0182","DOI":"10.34133\/hds.0182","article-title":"Deep learning in heart sound analysis: From techniques to clinical applications","volume":"4","author":"Zhao","year":"2024","journal-title":"Health Data Sci."},{"key":"ref_38","doi-asserted-by":"crossref","unstructured":"Ma, Y., Xu, X., Yu, Q., Zhang, Y., Li, Y., Zhao, J., and Wang, G. (2019, January 17\u201319). Lungbrn: A smart digital stethoscope for detecting respiratory disease using bi-resnet deep learning algorithm. Proceedings of the 2019 IEEE Biomedical Circuits and Systems Conference (BioCAS), Nara, Japan.","DOI":"10.1109\/BIOCAS.2019.8919021"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/25\/20\/6326\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,16]],"date-time":"2025-10-16T04:47:28Z","timestamp":1760590048000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/25\/20\/6326"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2025,10,13]]},"references-count":38,"journal-issue":{"issue":"20","published-online":{"date-parts":[[2025,10]]}},"alternative-id":["s25206326"],"URL":"https:\/\/doi.org\/10.3390\/s25206326","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2025,10,13]]}}}