Open research questions in Advanced Sensor and Energy Harvesting Materials

What the literature leaves open

• This review has summarized the recent advances driving the convergence of soft electronics and artificial intelligence, spanning from materials to intelligent systems. Deployable performance is shaped by the coupled behavior across physical transduction, integration density, and learning pipelines under realistic wear, rather than by isolated improvements in any single layer. Stable, low-noise coupling to the body and deformation-tolerant conduction remain prerequisites for extracting meaningful information. Simultaneously, scalable integration and interconnect architectures define what can be measured, how densely it can be sampled, and how reliably it can be routed, thereby shaping the data streams that learning systems must interpret. Recent progress in soft materials and interfaces has broadened the design space beyond filler-based composite percolation concepts to include intrinsically stretchable conductors, conducting polymers, and platforms that support conformal epidermal coupling and low-impedance interfacing. In long-duration wear, comfort is inseparable from signal quality. Breathable substrates and skin-compatible interfaces influence moisture management and contact stability, and these effects propagate directly to drift, noise, and artifact susceptibility at the inference input. As systems expand from individual sensors toward wearable platforms, the importance of integration and manufacturability becomes central to system design. Scalable fabrication and multilayer interconnect strategies enable higher channel counts and multifunctional stacks, but they also increase the dimensionality and variability of the acquired data streams. As multimodal and arrayed configurations become more common, data interpretation becomes a primary design constraint, given that on-body signal statistics shift with placement, motion, and environment. Battery-free and energy-harvesting approaches can extend operation time, but they tie duty cycle, sensing modality, and compute budget together, reinforcing the value of co-design between data acquisition and on-device processing. To bridge the gap between laboratory prototypes and commercially viable intelligent soft systems, the field requires tighter coupling between scalable manufacturing, standardized validation, and drift-tolerant artificial intelligence. Future progress depends not only on producing larger or denser devices, but also on ensuring that batch-level reproducibility, longitudinal stability, and cross-user robustness are quantified with shared benchmarks. Particularly for wearable systems intended for prolonged real-world operation, standardized reporting of fabrication windows, deviceto-device variation, aging behavior, recalibration frequency, and out-of-distribution performance will be essential for meaningful comparison across studies.

  Convergence of Soft Electronics and Artificial Intelligence: From Materials to Intelligent Systems · 2026 · DOI

• [29-31] The effect of chemical surface modification of the fibers to increase charge density could also be investigated, and the possibility of fabricating the device on real textile substrates to achieve superior wearability and comfort could be explored.

  Synergistic Enhancement of Piezoelectric and Triboelectric Outputs in Barium Titanate/Nylon-11 (BaTiO₃/Nylon-11) Nanofibrous Hybrid Nanogenerators for Self-Powered Wearable Electronics · 2026 · DOI

• LLM API Latency and Cost: GPT-4 inference introduces 2–8 second latency per ticket and incurs per-token costs that may be prohibitive for repositories exceeding 500 tickets/day. Caching of common root cause patterns partially mitigates this. Cold-Start Problem: The RAG knowledge base requires a minimum corpus of ~500 resolved tickets before retrieval quality becomes effective. Newly created repositories will produce lower-quality Q&A responses during the initial deployment period. Hallucination Risk: Despite RAG grounding, LLM-generated code suggestions may contain subtle logical errors not caught by syntax validation. All generated patches are clearly labelled as AI-suggestions requiring human review and are never auto-merged. Language and Domain Coverage: The DistilBERT classifier is fine-tuned primarily on English-language issues from mainstream technology repositories. Performance may degrade for non-English issues or highly specialised domain repositories. Retraining Trigger Sensitivity: The drift detection threshold (200 flagged samples) is a heuristic requiring per-repository calibration to balance retraining frequency against model stability. 11. FUTURE WORK Future development will focus on five primary directions: (1) multi-repository and cross-project knowledge transfer to improve resolution quality for projects with sparse issue history; (2) automated PR generation as an opt-in feature for high-confidence bug fixes, creating draft pull requests for maintainer review; (3) fine-grained priority classification adding severity levels (critical, high, medium, low) as an additional classification head; (4) multi-language support by incorporating mBERT and XLM-R variants trained on non-English issue datasets; and (5) comprehensive multi-centre evaluation across ten or more active repositories spanning different domains, languages and organisation sizes. 12.

  GitHub IntelliSolve: AI-Powered GitHub Ticket Analyzer & Automated Resolver · 2026 · DOI

• Future development will focus on five primary directions: (1) multi-repository and cross-project knowledge transfer to improve resolution quality for projects with sparse issue history; (2) automated PR generation as an opt-in feature for high-confidence bug fixes, creating draft pull requests for maintainer review; (3) fine-grained priority classification adding severity levels (critical, high, medium, low) as an additional classification head; (4) multi-language support by incorporating mBERT and XLM-R variants trained on non-English issue datasets; and (5) comprehensive multi-centre evaluation across ten or more active repositories spanning different domains, languages and organisation sizes. 12.

  GitHub IntelliSolve: AI-Powered GitHub Ticket Analyzer & Automated Resolver · 2026 · DOI

• Because reconstruction quality improves progressively with measurement count, SPTS can rapidly localize contact from sparse data and refine tactile images over time, enabling scalable, responsive tactile sensing for physical interaction and control.

  Single-Pixel Tactile Skin via compressive sampling · 2026 · DOI

• Poly(L-lactic acid) (PLLA) nanofiber-based piezoelectric membranes hold promise for self-powered health monitoring and tissue repair, yet their intrinsic piezoelectric performance remains insufficient for practical applications.

  Topologically Structured PLLA Fibers With Stress Concentration Effects for Health Monitoring · 2026 · DOI

• Finally, current challenges—including the conductivity–mechanics coupling bottleneck, insufficient long-term stability, biosafety concerns for skin-contact deployment, the lack of standardized evaluation protocols, and device-integration barriers—are identified, and future directions for this field are outlined.

  Metal-Ion-Coordinated Conductive Hydrogels for Strain Sensing from Coordination Design to Wearable Applications · 2026 · DOI

• By comparatively analyzing seven representative metal-ion systems within a unified framework, this work aims to clarify how the choice of metal ion governs the interplay among conductivity, mechanical robustness, self-healing, and strain sensitivity—a perspective that has not yet been systematically addressed in prior reviews.

  Metal-Ion-Coordinated Conductive Hydrogels for Strain Sensing from Coordination Design to Wearable Applications · 2026 · DOI

• ABSTRACT Levodopa (L‐Dopa) pharmacokinetic monitoring is crucial for optimizing therapy in Parkinson's disease (PD); however, existing sweat sensors are still limited by their reliance on active perspiration and poor specificity in complex biofluids.

  A Wearable Paper‐Based Patch Integrating Osmotic Sweat Extraction and Differential Sensing for Continuous L‐Dopa Monitoring in Parkinson's Disease · 2026 · DOI

• However, their sensitivity is often limited by the agglomeration and inefficient conductive network formation induced by the single‐component nanofillers.

  Highly Sensitive Conductive Hydrogel Enabled by Nanoscale Synergy of Carboxylated MXene and Polypyrrole for Multifunctional Applications · 2026 · DOI

• This review highlights recent breakthroughs in 3D-printed hy- drogel bioelectronics, from innovative material designs to their transformative applications. We explored how hydrogel inks can be engineered to balance printability with functionality, em- phasizing the interplay between rheology, conductivity, adhe- sion, and biocompatibility. We also examined the performance of implantable and wearable systems fabricated via DIW and re- lated techniques, showing their potential in electrophysiologi- cal recording, precise stimulation, and multimodal biosensing. Despite these substantial advances, several key directions re- quire further investigation. (1) Integrated multifunctional printing: current devices of- ten focus on single functions; however, physiological process- es are inherently multidimensional. A key goal is to develop multi-material, multi-scale 3D printing techniques that en- able the seamless integration of sensing, actuation, and ther- apeutic modules. Achieving this will require advances in in- terfacial fusion, such as optimizing the ink rheology and poly- merization, to ensure robust bonding between dissimilar ma- terials. Hybrid printing methods could bridge the gap be- tween microscale biomimicry and macroscale mechanical conformity, while spatiotemporal programming could create "smart" systems that adapt to physiological cues in real time. (2) Intelligent manufacturing. The reliance on artisanal ex- pertise limits the standardization and scalability of 3D-print- ed bioelectronics. Data-driven intelligent manufacturing frameworks must be developed to address this issue. Such systems would combine comprehensive material and pro- cess databases with machine learning to predict and opti- mize the device performance. In situ monitoring and closed- loop feedback could further enhance quality control, en- abling real-time adjustments and defect correction during fabrication. (3) Personalized applications. The future of 3D-printed bio- electronics lies in personalized applications, moving beyond anatomical fitting to functional customization. Advanced manufacturing can produce electrode arrays with intricate 3D geometries tailored to individual neuroanatomical and elec- trophysiological profiles. By co-integrating heterogeneous el- ements, 3D printing can enable multimodal sensing plat- forms for targeted health monitoring, accelerating the clini- cal translation of personalized diagnostics and therapies. BIOGRAPHY Ji Liu is currently an Associate Professor in the Department of Mechanical and Energy Engineering, Southern University of Sci- ence and Technology. He obtained his Ph.D. from the University of Liege (Belgium) and University of Bordeaux (France). Prior to joining SUSTech, he conducted post-doc research in the Univer- sity of Cambridge, Massachusetts Institute of Technology and Harvard Medical School. His research interests include the de- sign and fabrication of functional hydrogels and hydrogel- based electronics for biointerfaces.

  Direct Ink Writing 3D Printing of Hydrogel Bioelectronics · 2026 · DOI

• Conclusions Polymer-based flexible wireless sensors are catalyzing a fundamental shift in physiological data acquisition, moving health monitoring from sporadic, clinic-bound measurements to uninterrupted, human-integrated observation. Convergent advances in deformable electronics, molecularly tailored materials, sub-milliwatt radios, and adaptive signal fusion now permit thin, skinlike devices to continuously translate cardiovascular, respiratory, metabolic, wound-healing, and kinematic information into secure digital streams. These platforms eliminate the sampling gaps inherent in conventional spot checks while preserving natural mobility, thereby establishing a non-invasive, comfort-centric route to longitudinal health analytics. This review systematically surveys recent advances in polymer-based flexible wireless sensors for health monitoring from the perspective of coordinated sensing response mechanisms and wireless system integration. Centered on the requirements of physiological signal acquisition and wireless transmission, an integrated analytical framework encompassing response mechanisms, system functionalities, and application scenarios is established, highlighting the synergistic interactions among different technological components under practical monitoring conditions. First, from the standpoint of sensing response mechanisms, this review summarizes the roles and characteristics of various physical and chemical transduction modes in physiological signal detection, and analyzes how response speed, dynamic range, and long-term stability influence the overall performance of wireless health-monitoring systems. On this basis, common wireless communication schemes and energy management strategies are systematically reviewed with respect to wireless transmission and power supply, and their suitability is discussed in terms of power consumption control, link stability, and systemlevel integration. Considering the operational requirements of wireless flexible sensing systems, the review further examines the application of data preprocessing techniques and lightweight machine learning approaches in physiological signal analysis, outlining their roles in enhancing signal reliability and analytical efficiency. Subsequently, fabrication methods for flexible devices are summarized, with emphasis on the characteristics of different processes in terms of structural uniformity, scalability, and compatibility with system integration. Building upon these analyses, the applications of different materials in wireless flexible health monitoring are comparatively reviewed, with particular attention to their performance in sensitivity, mechanical compliance, signal stability, and suitability for long-term monitoring.

  Polymer-Based Flexible Wireless Sensors for Health Monitoring · 2026 · DOI

• Superhydrophobic wearable strain sensors have emerged as a pivotal category of wearable electronics, with the core goal of reconciling high sensing performance—encompass- ing sensitivity, stretchability, and response speed—with robust environmental adaptability. This review systemati- cally summarizes recent advances in this field, covering design principles, material innovations, structural engi- neering, and functional integration. The synergy between flexible substrates and conductive components lays the foundation for flexible conductive sensors [44]. Carbon- based materials like graphene and MXene stand out due to their robustness, flexibility, and piezoresistive properties [135], while conductive polymers [204] and metallic nano- materials [130] offer high conductivity and processability. Structural designs—such as micro-/nanohierarchical rough surfaces and biomimetic architectures [421]—combined with self-healing systems enhance superhydrophobic- ity, mechanical strength, and durability [418]. Functional integration further enables antifouling [322], underwater monitoring [335], and antibacterial performance [345], allowing reliable operation in complex environments such as sweat, oil, and biofluids. https://doi.org/10.1007/s40820-026-02220-w © The authors Nano-Micro Lett. (2026) 18:392 Page 49 of 68 392 Notably, substantial progress has been made in address- ing three core stability challenges: Chemical robustness is enhanced via corrosion-resistant coatings as well as UV/ thermal shielding components [362, 370]; mechanical robustness is improved through interfacial modification like PDA, structural reinforcement including wrinkled and buck- led structures, and self-healing mechanisms [398, 402, 405, 415]; state robustness is achieved by structural optimization such as porous and interlocked networks, environmentally responsive surfaces including photothermal and electrother- mal deicing, and antiadhesion designs like superamphipho- bicity and antibacterial components [347, 387, 424]. Despite these advances, critical bottlenecks remain: insufficient long- term stability under multifactor coupled harsh environments such as cyclic mechanical stress combined with chemical corrosion and biological contamination, lack of standardized evaluation systems for multidimensional robustness cover- ing chemical, mechanical, and state aspects, and practical hurdles in low-cost scalable fabrication of high-uniformity sensors. To address these bottlenecks, future research must pivot toward a holistic strategy for environmental resilience. This demands a fundamental shift in material design from single-function solutions to multi-mechanism architec- tures that synergistically integrate self-healing chemistry, mechanically durable frameworks, and stable surface repel- lency. Building upon this material foundation, the field must establish standardized, multifactor evaluation protocols that accurately replicate the coupled chemical–mechanical–bio- logical stresses of real-world scenarios. Furthermore, the transition from laboratory concept to reliable technology hinges on developing scalable manufacturing processes capable of faithfully replicating these critical micro–nano- textures at commercial scale. Ultimately, the success of this integrated approach must be validated through demonstra- tion in demanding application scenarios, such as prolonged underwater monitoring, harsh weather tracking, and durable human–machine interfaces for extreme environments. Acknowledgements This work was supported by the Guangdong Basic and Applied Basic Research Foundation (2024A1515240037), and the National Natural Science Founda- tion of China (52571071). Author Contributions HS was involved in writing—original draft, visualization, conceptualization, investigation, and writing— reviewing and editing. YL conducted formal analysis, investiga- tion, and visualization. GZ performed investigation, data curation, and visualization. KL and XH carried out investigation and formal analysis. KS did investigation, formal analysis, and validation. CL took part in supervision and conceptualization. YQ participated in supervision, conceptualization, funding acquisition, writing— reviewing and editing, and project administration.

  Superhydrophobic Wearable Strain Sensors: From Strategic Design to Robustness Paradigm · 2026 · DOI

• In order to fill in the final gap between laboratory inno- vation and clinical efficacy, any future research should focus on two key areas: first, the creation of dynamic mathematical models of real time physiological time- lag compensation and second, the completion of rigor- ous age stratified pediatric clinical validation studies.

  Non-enzymatic sweat sensors for pediatric continuous glucose monitoring: a systematic review of engineering readiness and the cost-accuracy trade-off · 2026 · DOI

• This review summarizes recent advances in the material systems, ion transport mechanisms, signal transduction pathways, and representative applications of HBIS (Figure 9). At the materials level, polyelectrolyte hydrogels, ionic liquids, ionogels/eutectogels, and nanocomposite hydrogels provide complementary advantages in fixed-charge regulation, environmental stability, and coupled mechanical-electrical enhancement, collectively driving the field beyond single-parameter optimization toward integrated materials design that balances conductivity, mechanical robustness, stability, bandwidth, and interfacial adaptability [12]. Figure 9. Conclusion and Perspective of HBIS. At the mechanistic level, the macroscopic response of HBIS is governed not by an isolated process, but by the coupled interplay of diffusion, electromigration, electro-osmotic flow/convection, electric-double-layer reorganization, and adsorption/desorption processes across both bulk and interface. Incorporation of these processes into PNP-based analysis and EIS-supported quantitative frameworks provides a mechanistic basis for linking structure, transport, and function, and thus offers practical design handles for tuning sensitivity, response speed, operational bandwidth, hysteresis, and long-term drift [12]. Furthermore, two significant obstacles to long-term use are stability and biointerface behavior. Thermal drift, swelling, and environmental drying all happen at the same time. They alter the hydrogel’s ion mobility and interfacial polarization. Thus, low-frequency noise rises. In the meantime, biointerface problems directly affect the safety and dependability of devices. The contact may be harmed by a biomechanical mismatch. To guarantee long-term stability, these components (including moisture-barrier encapsulation, ionic liquid or organic solvent substitution, interfacial buffer layers, and electrode material optimization) must be co-designed. The readout strategy must be supported by these material selections. Signal-to-noise ratio can be enhanced by lock-in detection. https://doi.org/10.53941/sen.2026.100006 28 of 36 Li et al. Sustain. Eng. Novit 2026, 2(2), 1 When combined, they aid in maintaining signal constancy in complicated media and under long-term cyclic loading [12]. Intelligent HBIS devices have gained prominence due to the quick development of artificial intelligence (AI) technologies. As research advances, HBIS has progressed past a phase focused on structural improvement and performance tweaking. Its potential for next-generation wearable technology is further enhanced by the fact that it is about to enter an era of integration and intelligence. Simultaneously, raw electrical outputs can be converted into useful information using AI-driven techniques. They allow for the delicate decoding of minute variations in the resistive, capacitive, or triboelectric signals that hydrogels produce. In systems connected with mechanical metamaterials that incorporate programmed deformation modes, for example, this advantage is maintained even in situations when the mechanical response is extremely complicated. Beyond these challenge-driven directions, hybrid modeling and AI-assisted design are expected to further accelerate the field. Multiphysics models that integrate deformation, electric field, diffusion, thermal, and fluidic effects can improve predictive capability and reduce reliance on empirical trial-and-error optimization. In parallel, data-driven analysis and generative design may enable more efficient optimization of polymer composition, crosslinking architecture, and device geometry. However, the value of these approaches lies not merely in computational sophistication, but in their ability to improve interpretability, standardization, long-term reliability, and translational practicality. Overall, the next stage of HBIS development will likely be defined by the convergence of mechanism-guided material design, standardized evaluation, and manufacturable system integration. With continued advances along these directions, HBIS is expected to evolve from laboratory-scale demonstrations into robust, intelligent, and application- ready iontronic platforms for biomedical monitoring, environmental sensing, and human-machine interfacing.

  Hydrogel-Based Iontronic Sensing · 2026 · DOI

• Water-immersion aging tests were conducted for predefied durations up to 15 days under ambient conditions; extended immersion periods and varied environmental conditions (temperature, pH, humidity) are not characterized.

  Eco-sustainable magnetoresistive sensors towards disposable magnetoelectronics · 2026 · DOI

• Simulations consider only core radii from 50 nm onwards; behavior of very small core radii below 55 nm with equilibrium states in 3D onion configuration is excluded from gain factor analysis.

  Eco-sustainable magnetoresistive sensors towards disposable magnetoelectronics · 2026 · DOI

• Advanced machine learning classifiers beyond ELM (such as deep learning approaches) could be explored to better handle overlapping feature distributions and improve gesture recognition robustness.

  Hydrogel-based electrodes for high-fidelity sEMG acquisition and robotic hand control · 2026 · DOI

• The paper demonstrates proof-of-concept with a small patterned device displaying 'AME', but scalability to larger-area displays and practical wearable device integration remains unexplored.

  Intrinsically stretchable organic light-emitting-diode with high brightness and stretchability via elastic-microphase-engineered emitter and dual-embedded electrode · 2026 · DOI

• The elastic-microphase-engineering strategy and stretchable electrode technology are stated to be applicable to other thin-film LED systems, but actual demonstration with other LED types beyond the green fluorescent polymer system is not provided.

  Intrinsically stretchable organic light-emitting-diode with high brightness and stretchability via elastic-microphase-engineered emitter and dual-embedded electrode · 2026 · DOI

• Cyclic testing was conducted at 40% strain in ambient atmosphere showing 67.65% luminance retention after 100 cycles, but long-term durability testing under more extreme environmental conditions (temperature, humidity) is not reported.

  Intrinsically stretchable organic light-emitting-diode with high brightness and stretchability via elastic-microphase-engineered emitter and dual-embedded electrode · 2026 · DOI

• Device failure at 120% strain is attributed to combined effects of resistance increase and charge transport pathway severance, but the exact mechanisms and relative contributions of each factor are not fully elucidated.

  Intrinsically stretchable organic light-emitting-diode with high brightness and stretchability via elastic-microphase-engineered emitter and dual-embedded electrode · 2026 · DOI

• Long-term durability was assessed up to 1000 bending cycles and 50 minutes of continuous operation; extended lifetime testing under real-world conditions over weeks or months is needed.

  Bioinspired Auxetic Metastructures Enable Biomechanically Adaptive, Machine Learning-Enhanced Self-Powered Sensing with Ultrahigh Efficiency · 2026 · DOI

• However, the emission efficiency of ML materials reported so far still fails to meet the growing application requirements due to the insufficiently understood mechano-to-photon conversion mechanism.

  Quantifying the interfacial triboelectricity in inorganic-organic composite mechanoluminescent materials · 2024 · DOI

• For wide deployment, new materials and simpler fabrication methods suitable for mass production will need to be investigated to reduce the cost and complexity asso- ciated with the current fabrication approach, which relies on the microelectromechanical systems (MEMS) process.

  Skin preparation–free, stretchable microneedle adhesive patches for reliable electrophysiological sensing and exoskeleton robot control · 2024 · DOI

Most-cited papers in Advanced Sensor and Energy Harvesting Materials

• Solution-processable, soft, self-adhesive, and conductive polymer composites for soft electronics · Nature Communications · 2022 · 370 citations
• A three-dimensional liquid diode for soft, integrated permeable electronics · Nature · 2024 · 370 citations
• A physicochemical-sensing electronic skin for stress response monitoring · Nature Electronics · 2024 · 356 citations
• A three-dimensionally architected electronic skin mimicking human mechanosensation · Science · 2024 · 344 citations
• Skin-inspired soft bioelectronic materials, devices and systems · Nature Reviews Bioengineering · 2024 · 330 citations
• Hybrid multimodal wearable sensors for comprehensive health monitoring · Nature Electronics · 2024 · 305 citations
• Age of Flexible Electronics: Emerging Trends in Soft Multifunctional Sensors · Advanced Materials · 2024 · 278 citations
• A Highly Stretchable, Conductive, and Transparent Bioadhesive Hydrogel as a Flexible Sensor for Enhanced Real‐Time Human Health Monitoring · Advanced Materials · 2024 · 236 citations
• Single body-coupled fiber enables chipless textile electronics · Science · 2024 · 231 citations
• A 10-micrometer-thick nanomesh-reinforced gas-permeable hydrogel skin sensor for long-term electrophysiological monitoring · Science Advances · 2024 · 212 citations

Most recent work

• Ion‐Driven Interfacial Engineering of MXene–Polyacrylamide Hydrogels for Advanced Wearable Electrocardiography and AI‐Driven Blood Pressure Monitoring · Small Science · 2026
• Microstructurally engineered MXene/Metal organic framework based hybrid nanogenerator for water energy harvesting and self-powered gas sensor · Journal of Power Sources · 2026
• Super-adhesive sensor based on amylopectin-polyacrylic acid hydrogel for deep learning-assisted sign language recognition · Journal of Colloid and Interface Science · 2026
• A silent speech interface with machine learning recognition model using microneedle array electrodes and polymer-based strain sensors · Sensors and Actuators Reports · 2026
• Enhanced Droplet Triboelectric Nanogenerator via Interfacial Engineering for Raindrop Energy Harvesting and pH Monitoring · Langmuir · 2026
• Hydrogel-based electrodes for high-fidelity sEMG acquisition and robotic hand control · Microsystems & Nanoengineering · 2026
• In situ self-layering bilayer alginate-gelatin hydrogels enabling synergistic adhesion and sensing for pressure distribution recognition · Carbohydrate Polymers · 2026
• Filament sensing tension: A bionic artificial tendon for self–force-regulated artificial muscle–driven wearable robotics · Science Advances · 2026
• A time-stamping tactile sensor enabled by pseudoconductive interface design at dielectric heterojunctions · Science Advances · 2026
• Liquid metal nano-gyroid stretchable transparent conductor for ultra-resilient optoelectronics and electroluminescence · npj Flexible Electronics · 2026

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