{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,5,1]],"date-time":"2026-05-01T11:21:38Z","timestamp":1777634498451,"version":"3.51.4"},"reference-count":37,"publisher":"MDPI AG","issue":"20","license":[{"start":{"date-parts":[[2024,10,21]],"date-time":"2024-10-21T00:00:00Z","timestamp":1729468800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Italian Ministry of University and Research"},{"name":"Fondazione Cassa di Risparmio di Firenze, Italy"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Virtual reality systems may benefit from wearable (fingertip-mounted) haptic displays capable of rendering the softness of virtual objects. According to neurophysiological evidence, the easiest reliable way to render a virtual softness is to generate purely tactile (as opposed to kinaesthetic) feedback to be delivered via a finger-pulp-interfaced deformable surface. Moreover, it is necessary to control not only the skin indentation depth by applying quasi-static (non-vibratory) contact pressures, but also the skin contact area. This is typically impossible with available devices, even with those that can vary the contact area, because the latter cannot be controlled due to the complexity of sensing it at high resolutions. This causes indetermination on an important tactile cue to render softness. Here, we present a technology that allows the contact area to be open-loop controlled via personalised optical calibrations. We demonstrate the solution on a modified, pneumatic wearable tactile display of softness previously described by us, consisting of a small chamber containing a transparent membrane inflated against the finger pulp. A window on the device allowed for monitoring the skin contact area with a camera from an external unit to generate a calibration curve by processing photos of the skin membrane interface at different pressures. The solution was validated by comparisons with an ink-stain-based method. Moreover, to avoid manual calibrations, a preliminary automated procedure was developed. This calibration strategy may be applied also to other kinds of displays where finger pulps are in contact with transparent deformable structures.<\/jats:p>","DOI":"10.3390\/s24206770","type":"journal-article","created":{"date-parts":[[2024,10,21]],"date-time":"2024-10-21T10:49:22Z","timestamp":1729507762000},"page":"6770","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["How to Make the Skin Contact Area Controllable by Optical Calibration in Wearable Tactile Displays of Softness"],"prefix":"10.3390","volume":"24","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7514-2906","authenticated-orcid":false,"given":"Gabriele","family":"Frediani","sequence":"first","affiliation":[{"name":"Biomedical Engineering Unit, Department of Industrial Engineering, University of Florence, 50121 Florence, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8496-5085","authenticated-orcid":false,"given":"Federico","family":"Carpi","sequence":"additional","affiliation":[{"name":"Biomedical Engineering Unit, Department of Industrial Engineering, University of Florence, 50121 Florence, Italy"},{"name":"IRCCS Fondazione don Carlo Gnocchi ONLUS, 50143 Florence, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2024,10,21]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"443","DOI":"10.1097\/SLA.0000000000000254","article-title":"Individualized deliberate practice on a virtual reality simulator improves technical performance of surgical novices in the operating room: A randomized controlled trial","volume":"259","author":"Palter","year":"2014","journal-title":"Ann. Surg."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"252","DOI":"10.1109\/TOH.2012.20","article-title":"A high performance tactile feedback display and its integration in teleoperation","volume":"5","author":"Sarakoglou","year":"2012","journal-title":"IEEE Trans. Haptics"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"1447","DOI":"10.1016\/j.cad.2005.02.015","article-title":"Manipulation of CAD surface models with haptics based on shape control functions","volume":"37","author":"Liu","year":"2005","journal-title":"Comput. Aided Des."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"5","DOI":"10.1007\/s10055-009-0153-y","article-title":"Virtual reality for assembly methods prototyping: A review","volume":"15","author":"Seth","year":"2011","journal-title":"Virtual Real."},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"Oh, S., Bailenson, J.N., and Welch, G.F. (2018). A systematic review of social presence: Definition, antecedents, and implications. Front. Robot. AI, 5.","DOI":"10.3389\/frobt.2018.00114"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"580","DOI":"10.1109\/TOH.2017.2689006","article-title":"Wearable haptic systems for the fingertip and the hand: Taxonomy, review, and perspectives","volume":"10","author":"Pacchierotti","year":"2017","journal-title":"IEEE Trans. Haptics"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"88","DOI":"10.1152\/jn.1995.73.1.88","article-title":"Tactual discrimination of softness","volume":"73","author":"Srinivasan","year":"1995","journal-title":"J. Neurophysiol."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"496","DOI":"10.1109\/70.880800","article-title":"Haptic discrimination of softness in teleoperation: The role of the contact area spread rate","volume":"16","author":"Bicchi","year":"2000","journal-title":"IEEE Trans. Rob. Autom."},{"key":"ref_9","unstructured":"Fujita, K., and Ohmori, H. (2001, January 22\u201325). A new softness display interface by dynamic fingertip contact area control. Proceedings of the 5th World Multiconference on Systemics, Cybernetics and Informatics, Orlando, FL, USA."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"189","DOI":"10.1109\/TOH.2009.16","article-title":"Cues for haptic perception of compliance","volume":"2","author":"Tiest","year":"2009","journal-title":"IEEE Trans. Haptics"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"1159","DOI":"10.1016\/j.cub.2016.02.052","article-title":"The change in fingertip contact area as a novel proprioceptive cue","volume":"26","author":"Moscatelli","year":"2016","journal-title":"Curr. Biol."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"610","DOI":"10.1109\/TIE.2019.2920602","article-title":"Multimodal haptic display for virtual reality: A survey","volume":"67","author":"Wang","year":"2020","journal-title":"IEEE Trans. Ind. Electron."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"eaaw8845","DOI":"10.1126\/sciadv.aaw8845","article-title":"Role of indentation depth and contact area on human perception of softness for haptic interfaces","volume":"5","author":"Dhong","year":"2019","journal-title":"Sci. Adv."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"2343","DOI":"10.1109\/TMECH.2016.2567453","article-title":"Haptic interface for displaying softness at multiple fingers: Combining a side-faced-type multifingered haptic interface robot and improved softness-display devices","volume":"21","author":"Endo","year":"2016","journal-title":"IEEE ASME Trans. Mech."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"304","DOI":"10.1109\/TOH.2017.2708717","article-title":"W-FYD: A wearable fabric-based display for haptic multi-cue delivery and tactile augmented reality","volume":"11","author":"Fani","year":"2018","journal-title":"IEEE Trans. Haptics"},{"key":"ref_16","unstructured":"Carpi, F., De Rossi, D., Kornbluh, R., Pelrine, R., and Sommer-Larsen, P. (2008). Dielectric Elastomers as Electromechanical Transducers. Fundamentals, Materials, Devices, Models and Applications of an Emerging Electroactive Polymer Technology, Elsevier."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"15","DOI":"10.1109\/TOH.2018.2805901","article-title":"Electro-active polymer based soft tactile interface for wearable devices","volume":"11","author":"Mun","year":"2018","journal-title":"IEEE Trans. Haptics"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"549","DOI":"10.1109\/TRO.2008.921561","article-title":"Development of soft-actuator-based wearable tactile display","volume":"24","author":"Koo","year":"2008","journal-title":"IEEE Trans. Robot."},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Frediani, G., Mazzei, D., De Rossi, D., and Carpi, F. (2014). Wearable wireless tactile display for virtual interactions with soft bodies. Front. Bioeng. Biotech., 2.","DOI":"10.3389\/fbioe.2014.00031"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"191","DOI":"10.1016\/j.sna.2013.11.009","article-title":"Design analysis and fab-rication of arrayed tactile display based on dielectric elastomer actuator","volume":"205","author":"Lee","year":"2014","journal-title":"Sens. Act. A"},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Boys, H., Frediani, G., Ghilardi, M., Poslad, S., Busfield, J., and Carpi, F. (2018, January 24\u201328). Soft wearable non-vibratory tactile displays. Proceedings of the 2018 IEEE International Conference on Soft Robotics (RoboSoft 2018), Livorno, Italy.","DOI":"10.1109\/ROBOSOFT.2018.8404931"},{"key":"ref_22","doi-asserted-by":"crossref","unstructured":"Song, K., Kim, S.H., Jin, S., Kim, S., Lee, S., Kim, J.S., Park, J.M., and Cha, Y. (2019). Pneumatic actuator and flexible piezoelectric sensor for soft virtual reality glove system. Sci. Rep., 9.","DOI":"10.1038\/s41598-019-45422-6"},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"309","DOI":"10.5739\/isfp.2002.309","article-title":"Wearable haptic display for immersive virtual environment","volume":"2022","author":"Tanaka","year":"2002","journal-title":"Proc. JFPS Intl. Symp. Fluid Power"},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Kim, Y., Oakley, I., and Ryu, J. (2006, January 18\u201321). Design and psychophysical evaluation of pneumatic tactile display. Proceedings of the 2006 SICE-ICASE International Joint Conference, Busan, Republic of Korea.","DOI":"10.1109\/SICE.2006.315347"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"041006","DOI":"10.1115\/1.2996593","article-title":"Fabrication and characterization of a balloon actuator array for haptic feedback in robotic surgery","volume":"2","author":"King","year":"2008","journal-title":"J. Med. Dev."},{"key":"ref_26","unstructured":"Moy, G., Wagner, C., and Fearing, R.S. (2000, January 24\u201328). A compliant tactile display for teletaction. Proceedings of the IEEE International Conference on Robotics and Automation ICRA 2000, San Francisco, CA, USA."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"183","DOI":"10.1016\/j.neulet.2006.01.025","article-title":"A portable and low-cost fMRI compatible pneumatic system for the investigation of the somatosensensory system in clinical and research environments","volume":"398","author":"Wienbruch","year":"2006","journal-title":"Neurosci. Lett."},{"key":"ref_28","doi-asserted-by":"crossref","unstructured":"Feng, Y.L., Peiris, R.L., Fernando, C.L., and Minamizawa, K. (2018). 3D printed haptics: Creating pneumatic haptic display based on 3D printed airbags. Haptics: Science, Technology, and Applications, Proceedings of the 11th International Conference EuroHaptics 2018, Pisa, Italy, 13\u201316 June 2018, Springer.","DOI":"10.1007\/978-3-319-93399-3_17"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"22","DOI":"10.1089\/soro.2019.0013","article-title":"Closed-loop haptic feedback control using a self-sensing soft pneumatic actuator skin","volume":"7","author":"Sonar","year":"2020","journal-title":"Soft Rob."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"e2314901121","DOI":"10.1073\/pnas.2314901121","article-title":"SORI: A softness-rendering interface to unravel the nature of softness perception","volume":"121","author":"Mete","year":"2024","journal-title":"Proc. Natl. Acad. Sci. USA"},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Frediani, G., and Carpi, F. (2020). Tactile display of softness on fingertip. Sci. Rep., 10.","DOI":"10.1038\/s41598-020-77591-0"},{"key":"ref_32","doi-asserted-by":"crossref","unstructured":"Carpi, F., Valles, M.C., Frediani, G., Toci, T., and Grippo, A. (2023). EEG Investigation on the tactile perceptual performance of a pneumatic wearable display of softness. Actuators, 12.","DOI":"10.3390\/act12120431"},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"015012","DOI":"10.1088\/0964-1726\/25\/1\/015012","article-title":"Stretch not flex: Programmable rubber keyboard","volume":"25","author":"Xu","year":"2016","journal-title":"Smart Mat. Struct."},{"key":"ref_34","doi-asserted-by":"crossref","unstructured":"Shimonomura, K. (2019). Tactile image sensors employing camera: A review. Sensors, 19.","DOI":"10.3390\/s19183933"},{"key":"ref_35","doi-asserted-by":"crossref","unstructured":"Sferrazza, C., and D\u2019Andrea, R. (2019). Design, motivation and evaluation of a full-resolution optical tactile sensor. Sensors, 19.","DOI":"10.3390\/s19040928"},{"key":"ref_36","doi-asserted-by":"crossref","unstructured":"Yuan, W., Dong, S., and Adelson, E.H. (2017). Gelsight: High-resolution robot tactile sensors for estimating geometry and force. Sensors, 17.","DOI":"10.3390\/s17122762"},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"5582","DOI":"10.1523\/JNEUROSCI.15-08-05582.1995","article-title":"Tactile resolution: Peripheral neural mechanisms underlying the human capacity to determine positions of objects contacting the fingerpad","volume":"15","author":"Wheat","year":"1995","journal-title":"J. Neurosci."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/24\/20\/6770\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T16:17:40Z","timestamp":1760113060000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/24\/20\/6770"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2024,10,21]]},"references-count":37,"journal-issue":{"issue":"20","published-online":{"date-parts":[[2024,10]]}},"alternative-id":["s24206770"],"URL":"https:\/\/doi.org\/10.3390\/s24206770","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2024,10,21]]}}}