{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,15]],"date-time":"2026-10-15T13:02:32Z","timestamp":1792069352966,"version":"4.4.0"},"reference-count":23,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2026,2,15]],"date-time":"2026-02-15T00:00:00Z","timestamp":1771113600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Continuous blood pressure (BP) monitoring remains a major challenge in wearable healthcare systems, as conventional cuff-based sphygmomanometers are intermittent and unsuitable for long-term use. This study presents a Smart Sock platform for cuffless BP estimation using single-site photoplethysmography (PPG). Unlike approaches based on pulse transit time or fiducial point detection, the proposed framework relies on peak-independent features extracted from PPG and its first and second derivatives, capturing blood volume and hemodynamic dynamics in the lower limb. PPG signals from 60 participants were segmented into overlapping 30 s windows and processed through a unified preprocessing pipeline. A compact set of physiologically meaningful statistical and information-theoretic features was extracted from each window, and temporal lag modelling (5\u201315 s) was employed to encode short-term hemodynamic memory without explicit peak detection. Multiple regression models were assessed using leakage-safe cross-validation strategies. In a subject-independent diagnosis scenario, the system achieved errors of 8.60 mmHg for systolic BP and 6.42 mmHg for diastolic BP. In a monitoring scenario with single-point calibration, performance substantially improved, yielding mean absolute errors of 1.3\u20131.7 mmHg and R2 &gt; 0.90. These results demonstrate that foot-based PPG, combined with peak-independent feature engineering and temporal context modeling, enables accurate and comfortable continuous personalized blood pressure monitoring after calibration, while subject-independent estimation remains more challenging.<\/jats:p>","DOI":"10.3390\/s26041269","type":"journal-article","created":{"date-parts":[[2026,2,16]],"date-time":"2026-02-16T08:38:39Z","timestamp":1771231119000},"page":"1269","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Peak-Independent Cuffless Blood Pressure Monitoring Using a Smart Sock: The Role of Temporal Lag Modeling in Foot-Based PPG"],"prefix":"10.3390","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0009-0004-8130-9639","authenticated-orcid":false,"given":"Hamed","family":"Abdollahzadeh","sequence":"first","affiliation":[{"name":"Department of Industrial and Information Engineering and Economics, University of L\u2019Aquila, 67100 L\u2019Aquila, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0000-9927-7816","authenticated-orcid":false,"given":"Elisa","family":"Montaldi","sequence":"additional","affiliation":[{"name":"Department of Industrial and Information Engineering and Economics, University of L\u2019Aquila, 67100 L\u2019Aquila, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0004-5494-9374","authenticated-orcid":false,"given":"Riccardo","family":"Olivieri","sequence":"additional","affiliation":[{"name":"Department of Industrial and Information Engineering and Economics, University of L\u2019Aquila, 67100 L\u2019Aquila, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0004-5574-1014","authenticated-orcid":false,"given":"Paolo","family":"Esposito","sequence":"additional","affiliation":[{"name":"Department of Industrial and Information Engineering and Economics, University of L\u2019Aquila, 67100 L\u2019Aquila, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4937-0398","authenticated-orcid":false,"given":"Gianluca","family":"Barile","sequence":"additional","affiliation":[{"name":"Department of Industrial and Information Engineering and Economics, University of L\u2019Aquila, 67100 L\u2019Aquila, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8060-9558","authenticated-orcid":false,"given":"Giuseppe","family":"Ferri","sequence":"additional","affiliation":[{"name":"Department of Industrial and Information Engineering and Economics, University of L\u2019Aquila, 67100 L\u2019Aquila, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7082-9429","authenticated-orcid":false,"given":"Vincenzo","family":"Stornelli","sequence":"additional","affiliation":[{"name":"Department of Industrial and Information Engineering and Economics, University of L\u2019Aquila, 67100 L\u2019Aquila, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,2,15]]},"reference":[{"key":"ref_1","unstructured":"World Health Organization (2026, January 07). Hypertension. Available online: https:\/\/www.who.int\/news-room\/fact-sheets\/detail\/hypertension."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"e127","DOI":"10.1016\/j.jacc.2017.11.006","article-title":"2017 ACC\/AHA\/AAPA\/ABC\/ACPM\/AGS\/APhA\/ASH\/ASPC\/NMA\/PCNA guideline for the prevention, detection, evaluation, and management of high blood pressure in adults: A report of the American College of Cardiology\/American Heart Association Task Force on Clinical Practice Guidelines","volume":"71","author":"Whelton","year":"2018","journal-title":"J. Am. Coll. Cardiol."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"1879","DOI":"10.1109\/TBME.2015.2441951","article-title":"Toward Ubiquitous Blood Pressure Monitoring via Pulse Transit Time: Theory and Practice","volume":"62","author":"Mukkamala","year":"2015","journal-title":"IEEE Trans. Biomed. Eng."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"101870","DOI":"10.1016\/j.bspc.2020.101870","article-title":"A Review of Machine Learning Techniques in Photoplethysmography for the Non-Invasive Cuff-Less Measurement of Blood Pressure","volume":"58","author":"Kyriacou","year":"2020","journal-title":"Biomed. Signal Process. Control"},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"Hosanee, M., Chan, G., Welykholowa, K., Cooper, R., Kyriacou, P.A., Zheng, D., Allen, J., Abbott, D., Menon, C., and Lovell, N.H. (2020). Cuffless Single-Site Photoplethysmography for Blood Pressure Monitoring. J. Clin. Med., 9.","DOI":"10.3390\/jcm9030723"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"685","DOI":"10.1088\/0967-3334\/27\/8\/003","article-title":"Evaluation of Blood Pressure Changes Using Vascular Transit Time","volume":"27","author":"Foo","year":"2006","journal-title":"Physiol. Meas."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"136","DOI":"10.1152\/japplphysiol.00657.2005","article-title":"Pulse Transit Time Measured from the ECG: An Unreliable Marker of Beat-to-Beat Blood Pressure","volume":"100","author":"Payne","year":"2006","journal-title":"J. Appl. Physiol."},{"key":"ref_8","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_9","doi-asserted-by":"crossref","first-page":"891","DOI":"10.1016\/S0140-6736(00)95580-6","article-title":"Velocity of Transmission of the Pulse-Wave: And Elasticity of Arteries","volume":"199","author":"Bramwell","year":"1922","journal-title":"Lancet"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"859","DOI":"10.1109\/TBME.2016.2580904","article-title":"Cuffless Blood Pressure Estimation Algorithms for Continuous Health-Care Monitoring","volume":"64","author":"Kachuee","year":"2017","journal-title":"IEEE Trans. Biomed. Eng."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"eaap8674","DOI":"10.1126\/scitranslmed.aap8674","article-title":"Smartphone-Based Blood Pressure Monitoring via the Oscillometric Finger-Pressing Method","volume":"10","author":"Chandrasekhar","year":"2018","journal-title":"Sci. Transl. Med."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"6948","DOI":"10.1038\/s41598-017-06925-2","article-title":"Hearables: Multimodal Physiological in-Ear Sensing","volume":"7","author":"Goverdovsky","year":"2017","journal-title":"Sci. Rep."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"765","DOI":"10.1109\/TBCAS.2017.2683801","article-title":"Robust Sensing of Distal Pulse Waveforms on a Modified Weighing Scale for Ubiquitous Pulse Transit Time Measurement","volume":"11","author":"Carek","year":"2017","journal-title":"IEEE Trans. Biomed. Circuits Syst."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"120","DOI":"10.1016\/j.future.2019.02.032","article-title":"Pervasive Blood Pressure Monitoring Using Photoplethysmogram (PPG) Sensor","volume":"98","author":"Riaz","year":"2019","journal-title":"Future Gener. Comput. Syst."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"26320","DOI":"10.1109\/JSEN.2021.3064219","article-title":"Development of a PPG Sensor Array as a Wearable Device for Monitoring Cardiovascular Metrics","volume":"21","author":"Kosik","year":"2021","journal-title":"IEEE Sens. J."},{"key":"ref_16","unstructured":"IEEE Staff (2010). 2010 IEEE International Workshop on Medical Measurements and Applications, IEEE."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Miranda-Vega, J.E., Nu\u00f1ez-Patr\u00f3n, E.Y., Prieto-Avalos, G., Flores-Fuentes, W., Sergiyenko, O., Garc\u00eda-Gonz\u00e1lez, W., M\u00e1rquez-Ramirez, L.V., Castro-Contreras, R., and Ayala-Figueroa, R.I. (2025). ESP32-Powered PPG Signal Acquisition: Open-Source Hardware and Software for Research and Education. Hardware, 3.","DOI":"10.3390\/hardware3040015"},{"key":"ref_18","unstructured":"IEEE Staff (2025). 2025 IEEE Madhya Pradesh Section Conference, IEEE."},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Velchev, Y., and Laskov, L. (2025, January 16\u201318). A Wireless WebSocket-Based PPG and ECG Acquisition System. Proceedings of the 2025 34th International Scientific Conference Electronics, ET 2025\u2014Proceedings, Sozopol, Bulgaria.","DOI":"10.1109\/ET66806.2025.11204127"},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Karthikeyan, T., Prabakar, S., Abishek, M., Mounish, G., and Rajkumar, T. (2025, January 3\u20135). A Photoplethysmography (PPG)-Driven Embedded System for Detecting and Restoring Cutaneous Circulation in Immobile Patients. Proceedings of the 2025 Third International Conference on Emerging Applications of Material Science and Technology (ICEAMST), Bengaluru, India.","DOI":"10.1109\/ICEAMST67459.2025.11335844"},{"key":"ref_21","first-page":"60","article-title":"Heartbeat Detector Using ESP32 Microcontroller and Website-Based Pulse Sensor","volume":"2","author":"Imelda","year":"2025","journal-title":"J. Comput. Eng. Inf. Technol."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"68","DOI":"10.21107\/triac.v12i2.31989","article-title":"Design and Evaluation of an IoT-Enabled ESP32 Photoplethysmography Monitor for Heart Rate and SpO2","volume":"12","author":"Hardiwansyah","year":"2025","journal-title":"J. Tek. Elektro Dan Komput. TRIAC"},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Zu\u00f1iga-Aguilar, E., Ben\u00edtez-Mata, B., Reyes-Lagos, J.J., Hernandez Acosta, H.Y., Botello Arredondo, A.I., Bayareh Mancilla, R., V\u00e1zquez de la Rosa, J.F., and Guti\u00e9rrez Valenzuela, C.A. (2026). PPG Behind the Ear: An Inclusive Method of Measure. Proceedings of the XLVIII Mexican Conference on Biomedical Engineering, Springer Nature.","DOI":"10.1007\/978-3-032-13722-7"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/26\/4\/1269\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,2,16]],"date-time":"2026-02-16T08:41:29Z","timestamp":1771231289000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/26\/4\/1269"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,2,15]]},"references-count":23,"journal-issue":{"issue":"4","published-online":{"date-parts":[[2026,2]]}},"alternative-id":["s26041269"],"URL":"https:\/\/doi.org\/10.3390\/s26041269","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,2,15]]}}}