Hierarchical Synergistic Engineering for Machine Learning-Assisted Gesture Recognition and Integrated Thermal Management

Abstract

Flexible strain sensors are revolutionizing human–machine interactions and next-generation health care by enabling real-time monitoring of human motion and precision medical treatment. However, developing lightweight flexible strain sensors that combine high sensitivity with a broad monitoring range remains a significant challenge. To address this challenge, an advanced structural engineering strategy based on the sodium chloride (NaCl) template sacrificial method is employed to simultaneously increase sensitivity and mechanical robustness. By leveraging a NaCl template sacrificial method, a hierarchical synergistic conductive network is constructed within the thermoplastic polyurethane (TPU) matrix formed via in situ growth. This design enables ultra-high sensitivity across a broad strain range, offering promising potential for wearable sensing applications. The resulting sensor exhibits exceptional performance characteristics, including a low detection limit (0.176%), high sensitivity (gage factor, GF = 331.7), wide sensing range (up to 230.1%), rapid response/recovery times (133 ms/133 ms), and remarkable durability exceeding 4000 cycles. Furthermore, the sensor demonstrated excellent electrothermal conversion performance with a positive temperature coefficient of 0.00207 °C⁻¹ and an achievable saturation temperature of 54.2 °C (1.0 A). Finally, the sensor was successfully integrated into a smart wearable system, enabling precise recognition and classification of multiple gestures through machine learning algorithms while also exhibiting significant potential for inflammation hyperthermia therapy.

Graphical Abstract

Keywords

Foam-shaped strain sensor / Multifunctionalities / Flexible electronics / Gesture recognition / Hyperthermia / Information and Computing Sciences / Artificial Intelligence and Image Processing / Engineering / Electrical and Electronic Engineering

References

[1] ZhengS, WuX, HuangY, XuZ, YangW, LiuZ, HuangS, XieB, YangM. Highly sensitive and multifunctional piezoresistive sensor based on polyaniline foam for wearable human-activity monitoring. Compos Part A-Appl S, 2019, 121: 510.

[2] ChenZ, QianD, XieD, GaoC, ShiJ, MorikawaH, ZhuC. Structure and wiring optimized TT/MT double-helical fiber sensors: fabrication and applications in human motion monitoring and gesture recognition. Adv Sci, 2025, 12: 2416564.

[3] LiuT, ZhaoZ, LiangR, HeH, LiuY, YuK, ChiM, LuoB, WangJ, ZhangS, CaiC, WangS, NieS. Tough and elastic anisotropic triboelectric materials enabled by layer-by-layer assembly. Adv Funct Mater, 2025.

[4] SencadasV, TawkC, AliciC. Highly sensitive soft foam sensors to empower robotic systems. Adv Mater Technol, 2019, 4: 1900423.

[5] CaiC, LiuT, MengX, LuoB, ChiM, WangJ, LiuY, ZhangS, GaoC, BaiY, WangS, NieS. Lightweight and mechanically robust cellulosic triboelectric materials for wearable self-powered rehabilitation training. ACS Nano, 2025, 19: 396.

[6] XuX, YanB. Bioinspired luminescent HOF-based foam as ultrafast and ultrasensitive pressure and acoustic bimodal sensor for human-machine interactive object and information recognition. Adv Mater, 2023, 35: 2303410.

[7] ChenZ, GaoQ, XieD, QianD, ShiJ, MorikawaH, ZhuC. Enhanced multifunctional sensing in human motion with speed-controlled coaxial wet-spun hollow MWCNT-TPU/TPU smart fibers. Chem Eng J, 2024, 498. 155045

[8] AmjadiM, KyungK, ParkI, SittiM. Stretchable, skin-mountable, and wearable strain sensors and their potential applications: a review. Adv Funct Mater, 2016, 26: 1678.

[9] ZhangD, ZhangK, WangY, WangY, YangY. Thermoelectric effect induced electricity in stretchable graphene-polymer nanocomposites for ultrasensitive self-powered strain sensor system. Nano Energy, 2019, 56: 25.

[10] ZhaoW, TrungV, LiH, NatsukiJ, TanJ, YangW, NatsukiT. Enhanced functionalization of nonwoven fabric by spray coating AgNPs/CNTs solution prepared by a one-step method. Chem Eng J, 2024, 494. 153101

[11] WangJ, LiuY, WeiZ, LiuT, LiY, HeB, LuoB, CaiC, ZhangS, ChiM, ShiC, WangS, NieS. Janus asymmetric cellulosic triboelectric materials enabled by gradient nano-doping strategy. Adv Funct Mater, 2025.

[12] ZhaoW, NatsukiJ, TrungV, LiH, TanJ, YangW, NatsukiT. AgNPs/CNTs modified nonwoven fabric for PET-based flexible interdigitated electrodes in pressure sensor applications. Chem Eng J, 2024, 499. 156252

[13] TrungV, LeP, NatsukiJ, ZhaoW, PhungT, TanJ, YangW, NatsukiT. Hydrovoltaic–piezoelectric hybridized device for microdroplet-pressure dual energy harvesting and piezo sensing. Chem Eng J, 2024, 496. 154164

[14] LiuY, WangJ, LiuT, WeiZ, LuoB, ChiM, ZhangS, CaiC, GaoC, ZhaoT, WangS, NieS. Triboelectric tactile sensor for pressure and temperature sensing in high-temperature applications. Nat Commun, 2025, 16: 383.

[15] XueF, PengQ, DingR, LiP, ZhaoX, ZhengH, XuL, TangZ, ZhangX, HeX. Ultra-sensitive, highly linear, and hysteresis-free strain sensors enabled by gradient stiffness sliding strategy. Npj Flex Electron, 2024, 8: 14.

[16] ZhangW, PiaoS, LinL, YinY, GuoJ, JiangZ, ChoY, LiR, GaoJ, PangH, PiaoY. Wearable and antibacterial HPMC-anchored conductive polymer composite strain sensor with high gauge factors under small strains. Chem Eng J, 2022, 435. 135068

[17] ZhangJ, WangQ, XueX, LiM, SunX, ZhaoJ, ZhangW, LuC. Waste flame-retardant polyurethane foam/ground tire rubber/carbon nanotubes composites with hierarchical segregated structures for high efficiency electromagnetic interference shielding. Compos Part A-Appl S, 2023, 169. 107530

[18] JiaX, ShenB, ZhangL, ZhengW. Construction of compressible Polymer/MXene composite foams for high-performance absorption-dominated electromagnetic shielding with ultra-low reflectivity. Carbon, 2021, 173: 932.

[19] LiC, LiH, TanJ, XueY, LiuQ, YangY, WangC, ZhangQ. Constructing segregated thermoset composite via Pickering emulsion and dynamic polythiourethanes. Compos Sci Technol, 2022, 218. 109215

[20] ZhengY, LiY, DaiK, LiuM, ZhouK, ZhengG, LiuC, ShenC. Conductive thermoplastic polyurethane composites with tunable piezoresistivity by modulating the filler dimensionality for flexible strain sensors. Compos Part A-Appl S, 2017, 101: 41.

[21] CastroM, LuJ, BruzaudS, KumarB, FellerJ. Carbon nanotubes/poly (ε-caprolactone) composite vapour sensors. Carbon, 2009, 47: 1930.

[22] ZhaoS, LiG, LiuH, DaiK, ZhengG, YanX, LiuC, ChenJ, ShenC, GuoZ. Positive temperature coefficient (PTC) evolution of segregated structural conductive polypropylene nanocomposites with visually traceable carbon black conductive network. Adv Mater Interfaces, 2017, 4: 1700265.

[23] WangM, DuanX, XuY, DuanX. Functional three-dimensional graphene/polymer composites. ACS Nano, 2016, 10: 7231.

[24] YanD, PangH, LiB, VajtaiR, XuL, RenP, WangJ, LiZ. Structured reduced graphene oxide/polymer composites for ultra-efficient electromagnetic interference shielding. Adv Funct Mater, 2015, 25: 559.

[25] KimY, RyuT, OkK, KwakM, ParkS, ParkN, HanC, KimB, KoM, SonH. Inverted layer-by-layer fabrication of an ultraflexible and transparent Ag nanowire/conductive polymer composite electrode for use in high-performance organic solar cells. Adv Funct Mater, 2015, 25: 4580.

[26] ZhaoZ, LiuJ, DuX, WangZ, ZhangC, MingS. Fabrication of silver nanoparticles/copper nanoparticles jointly decorated nitride flakes to improve the thermal conductivity of polymer composites. Colloids Surf A, 2022, 635. 128104

[27] QinL, TaoQ, LiuX, FahlmanM, HalimJ, PerssonaP, RosenJ, ZhangF. Polymer-MXene composite films formed by MXene-facilitated electrochemical polymerization for flexible solid-state microsupercapacitors. Nano Energy, 2019, 60: 734.

[28] ChenX, LiR, NiuG, XinM, XuG, ChengH, YangL. Porous graphene foam composite-based dual-mode sensors for underwater temperature and subtle motion detection. Chem Eng J, 2022, 444. 136631

[29] XueF, ZhengH, PengQ, HuY, ZhaoX, XuL, LiP, ZhuY, LiuZ, HeX. An ultra-broad-range pressure sensor based on a gradient stiffness design. Mater Horiz, 2021, 8: 2260.

[30] LiuH, XuT, CaiC, LiuK, LiuW, ZhangM, DuH, SiC, ZhangK. Multifunctional superelastic, superhydrophilic, and ultralight nanocellulose-based composite carbon aerogels for compressive supercapacitor and strain sensor. Adv Funct Mater, 2022, 32: 2113082.

[31] XueF, PengQ, LiuZ, GuoX, YanQ, ZhaoX, HeX. Micro structural design and macro synthesis of aerogel networks with gradient stiffness properties. Compos Part A-Appl S, 2024, 178. 107978

[32] LiG, LiC, LiG, YuD, SongZ, WangH, LiuX, LiuH, LiuW. Development of conductive hydrogels for fabricating flexible strain sensors. Small, 2022, 18. e2101518

[33] AzadiS, PengS, MoshiziS, AsadniaM, XuJ, ParkI, WangC, WuS. Biocompatible and highly stretchable PVA/AgNWs hydrogel strain sensors for human motion detection. Adv Mater Technol, 2020, 5: 2000426.

[34] ChenQ, GaoQ, WangX, SchubertD, LiuX. Flexible, conductive, and anisotropic thermoplastic polyurethane/polydopamine/MXene foam for piezoresistive sensors and motion monitoring. Compos Part A-Appl S, 2022, 155. 106838

[35] XuR, LuY, JiangC, ChenJ, MaoP, GaoG, ZhangL, WuS. Facile fabrication of three-dimensional graphene foam/poly(dimethylsiloxane) composites and their potential application as strain sensor. ACS Appl Mater Interfaces, 2014, 6: 13455.

[36] ZhangH, LiuN, ShiY, LiuW, YueY, WangS, MaY, WenL, LiL, LongF, ZouZ, GaoY. Piezoresistive sensor with high elasticity based on 3D Hybrid network of Sponge@CNTs@Ag NPs. ACS Appl Mater Interfaces, 2016, 8: 22374.

[37] ZhaoW, ZhengY, QianJ, ZhaoF, JinZ, QiuH, ZhuC, HongX. AgNWs/MXene derived multifunctional knitted fabric capable of high electrothermal conversion efficiency, large strain and temperature sensing, and EMI shielding. J Alloys Compd, 2022, 923. 166471

[38] LiR, XiaL, LyuP, ZhangJ, WangY, DengB, ZhangC, LiuX, XuW. High content filling, toughness, and conductive performance of thermoplastic polyurethane/carbon nanotubes composites prepared by constructing the compact interface. Compos Commun, 2021, 28. 100948

[39] XuN, LiY, ZhengT, XiaoL, LiuY, ChenS, ZhangD. A mussel-inspired strategy for CNT/carbon fiber reinforced epoxy composite by hierarchical surface modification. Colloids Surf A, 2022, 635. 128085

[40] XiaoR, YuG, XuBB, WangN, LiuX. Fiber surface/interfacial engineering on wearable electronics. Small, 2021, 17. e2102903

[41] PugaF, NavíoJ, HidalgoM. Enhanced UV and visible light photocatalytic properties of synthesized AgBr/SnO composites. Sep Purif Technol, 2021, 257. 117948

[42] MiH, SalickM, JingX, CroneW, PengX, TurngL. Electrospinning of unidirectionally and orthogonally aligned thermoplastic polyurethane nanofibers: fiber orientation and cell migration. J Biomed Mater Res A, 2015, 103: 593.

[43] BaiM, XuC, HuangX, YinH, WanJ. Preparation of CuSe-PDA/gC 3 N 4 and its visible-light photocatalytic performance to dye degradation. Environ Sci Pollut Res, 2021, 28: 3465.

[44] BarickA, TripathyD. Effect of nanofiber on material properties of vapor-grown carbon nanofiber reinforced thermoplastic polyurethane (TPU/CNF) nanocomposites prepared by melt compounding. Compos Part A-Appl S, 2010, 41: 1471.

[45] KumarS, GuptaT, VaradarajanK. Strong, stretchable and ultrasensitive MWCNT/TPU nanocomposites for piezoresistive strain sensing. Compos Part B-eng, 2019, 177. 107285

[46] MaP, TangB, KimJ. Effect of CNT decoration with silver nanoparticles on electrical conductivity of CNT-polymer composites. Carbon, 2008, 46: 1497.

[47] ChenJ, LiuS, WuH, SowadeE, BaumannR, WangY, GuF, LiuC, FengZ. Structural regulation of silver nanowires and their application in flexible electronic thin films. Mater Design, 2018, 154: 266.

[48] HuS, HanT, LinC, XiangW, ZhaoY, GaoP, DuF, LiX, SunY. Enhanced electrocatalysis via 3D graphene aerogel engineered with a silver nanowire network for ultrahigh-rate zinc-air batteries. Adv Funct Mater, 2017, 27: 1700041.

[49] AntunesE, LoboA, CoratE, Trava-AiroldiV, MartinA, VeríssimoC. Comparative study of first- and second-order Raman spectra of MWCNT at visible and infrared laser excitation. Carbon, 2006, 44: 2202.

[50] VollebregtS, IshiharaR, TichelaarF, HouY, BeenakkerC. Influence of the growth temperature on the first and second-order Raman band ratios and widths of carbon nanotubes and fibers. Carbon, 2012, 50: 3542.

[51] AntunesE, LoboA, CoratE, Trava-AiroldiV. Influence of diameter in the Raman spectra of aligned multi-walled carbon nanotubes. Carbon, 2007, 45: 913.

[52] BarickA, TripathyD. Preparation, characterization and properties of acid functionalized multi-walled carbon nanotube reinforced thermoplastic polyurethane nanocomposites. Mat Sci Eng B-Adv, 2011, 176: 1435.

[53] KasprzykP, DattaJ. Novel bio-based thermoplastic poly(ether-urethane)s. Correlations between the structure, processing and properties. Polymer, 2019, 160: 1-10.

[54] NituicaM, OpreaO, StelescuM, SonmezM, GeorgescuM, AlexandrescuL, MotelicaL. Polymeric biocomposite based on thermoplastic polyurethane (TPU) and protein and elastomeric waste mixture. Materials, 2023, 16: 5279.

[55] MitraA, MahapatraA, MallickA, ChakrabartiP. Enhanced microwave absorption and magnetic phase transitions of nanoparticles of multiferroic LaFeO incorporated in multiwalled carbon nanotubes (MWCNTs). J Magn Magn Mater, 2017, 435: 117.

[56] DongL, WangM, WuJ, ZhangC, ShiJ, OhK, YaoL, ZhuC, MorikawaH. Fully biofriendly, biodegradable and recyclable hydrogels based on covalent-like hydrogen bond engineering towards multimodal transient electronics. Chem Eng J, 2023, 457: 141276.

[57] DongL, WangM, WuJ, ZhuC, ShiJ, MorikawaH. Deformable textile-structured triboelectric nanogenerator knitted with multifunctional sensing fibers for biomechanical energy harvesting. Adv Fiber Mater, 2022, 4: 1486.

[58] LiuR, ChenK, LiuH, LiuY, CongR, GuoJ, TianY. High performance conductive hydrogel for strain sensing applications and digital image mapping. ACS Appl Mater Interfaces, 2022, 14: 51341.

[59] BiD, QuN, ShengW, LinT, HuangS, WangL, LiRJAAM. Tough and strain-sensitive organohydrogels based on MXene and PEDOT/PSS and their effects on mechanical properties and strain-sensing performance. Interfaces, 2024, 16: 11914

[60] ZhouY, WanC, YangY, YangH, WangS, DaiZ, JiK, JiangH, ChenX, LongY. Highly stretchable, elastic, and ionic conductive hydrogel for artificial soft electronics. Adv Funct Mater, 2019, 29: 1806220.

[61] GuJ, HuangJ, ChenG, HouL, ZhangJ, ZhangX, YangX, GuanL, JiangX, LiuH. Multifunctional poly(vinyl alcohol) nanocomposite organohydrogel for flexible strain and temperature sensor. ACS Appl Mater Interfaces, 2020, 12: 40815.

[62] ChenH, HuangJ, LiuJ, GuJ, ZhuJ, HuangB, BaiJ, GuoJ, YangX, GuanL. High toughness multifunctional organic hydrogels for flexible strain and temperature sensor. J Mater Chem A, 2021, 9: 23243.

[63] LeeS, JangJ, ParkT, ParkY, ParkJ, KimY, LeeH, JeonE, LeeD, AhnB, ChungC. Electrodeposited silver nanowire transparent conducting electrodes for thin-film solar cells. ACS Appl Mater Interfaces, 2020, 12: 6169.

[64] ChortosA, BaoZ. Skin-inspired electronic devices. Mater Today, 2014, 17: 321.

[65] RongJ, ZhouJ, ZhouY, HuC, LiL, GuoW. 3D single-layer-dominated graphene foam for high-resolution strain sensing and self-monitoring shape memory composite. Small, 2022, 18: 2205301.

[66] ChuZ, JiaoW, LiJ, GuoH, ZhengY, WangR, HeX. A novel wrinkle-gradient strain sensor with anti-water interference and high sensing performance. Chem Eng J, 2021, 421. 129873

[67] BuY, ShenT, YangW, YangS, ZhaoY, LiuH, ZhengY, LiuC, ShenC. Ultrasensitive strain sensor based on superhydrophobic microcracked conductive Ti(3)C(2)T(x) MXene/paper for human-motion monitoring and E-skin. Sci Bull, 2021, 66: 1849.

[68] DingY, XueC, FanQ, ZhaoL, TianQ, GuoX, ZhangJ, JiaS, AnQ. Fabrication of superhydrophobic conductive film at air/water interface for flexible and wearable sensors. Chem Eng J, 2021, 404. 126489

[69] YaoZ, FengH, ShangK, DengX, YangT. Skin-like strain sensors based on multiwalled carbon nanotube/polydimethylsiloxane composite films. Acs Applied Nano Materials, 2023, 6: 6550.

[70] HuT, ShengB. A highly sensitive strain sensor with wide linear sensing range prepared on a hybrid-structured CNT/Ecoflex film via local regulation of strain distribution. ACS Appl Mater Interfaces, 2024, 16: 21061

[71] YuanZ, LiH, DuanZ, HuangQ, ZhangM, ZhangH, GuoJ, JiangY, TaiH. High sensitivity, wide range and waterproof strain sensor with inner surface sensing layer for motion detection and gesture reconstruction. Sens Actuators A-Phys, 2024, 369. 115202

[72] HongW, GuoX, LiX, ZhangT, ZhuX, HeJ, ZhangR, YangS, ShaoY, FangY, YinZ, SunX, HuJ, HongQ, ZhaoY. Fishbone and nettle fiber inspired stretchable strain sensor with high sensitivity and wide sensing range for wearable electronics. Chem Eng J, 2024, 492. 152281

[73] HongW, GuoX, ZhangT, ZhuX, SuZ, MengY, ZhaoY, XuD, PanJ, HuangY, WangH, XuK, DongH, ZhangC, LiY, YanX, HuangX. Dual bionic-inspired stretchable strain sensor based on graphene/ multi-walled carbon nanotubes/polymer composites for electronic skin. Compos Part A-Appl S, 2024, 179. 108043

[74] XingL, LiL, LiuY, RenJ, GuoG, ChenY, ZhengY, SunB. Highly stretchable GR/TPU strain sensor based on one-step electrospun fibrous yarns for wearable devices. J Mater Chem A, 2024, 12: 10905.

[75] LomovS, AkmanovI, LiuQ, WuQ, AbaimovS. Negative temperature coefficient of resistance in aligned CNT networks: influence of the underlying phenomena. Polymers, 2023, 15: 678.

[76] ZhaoY, WuZ, BaiS. Study on thermal properties of graphene foam/graphene sheets filled polymer composites. Compos Part A-Appl S, 2015, 72: 200.

[77] ChenX, HuX, JiaP, XieZ, LiuJ. Tunable anisotropic thermal transport in porous carbon foams: The role of phonon coupling. Int J Mech Sci, 2021, 206. 106576

[78] PettesM, JiH, RuoffR, ShiL. Thermal transport in three-dimensional foam architectures of few-layer graphene and ultrathin graphite. Nano Lett, 2012, 12: 2959.

Funding

Foundation for Promotion of Material Science and Technology of Japan(JP23H01299)

RIGHTS & PERMISSIONS

Donghua University, Shanghai, China