@article{wu_hong_zhao_yin_zhu_2023, title={Caterpillar-inspired soft crawling robot with distributed programmable thermal actuation}, volume={9}, ISSN={["2375-2548"]}, DOI={10.1126/sciadv.adf8014}, abstractNote={Many inspirations for soft robotics are from the natural world, such as octopuses, snakes, and caterpillars. Here, we report a caterpillar-inspired, energy-efficient crawling robot with multiple crawling modes, enabled by joule heating of a patterned soft heater consisting of silver nanowire networks in a liquid crystal elastomer (LCE)-based thermal bimorph actuator. With patterned and distributed heaters and programmable heating, different temperature and hence curvature distribution along the body of the robot are achieved, enabling bidirectional locomotion as a result of the friction competition between the front and rear end with the ground. The thermal bimorph behavior is studied to predict and optimize the local curvature of the robot under thermal stimuli. The bidirectional actuation modes with the crawling speeds are investigated. The capability of passing through obstacles with limited spacing are demonstrated. The strategy of distributed and programmable heating and actuation with thermal responsive materials offers unprecedented capabilities for smart and multifunctional soft robots.}, number={12}, journal={SCIENCE ADVANCES}, author={Wu, Shuang and Hong, Yaoye and Zhao, Yao and Yin, Jie and Zhu, Yong}, year={2023}, month={Mar} }
 @article{hong_chi_wu_li_zhu_yin_2022, title={Boundary curvature guided programmable shape-morphing kirigami sheets}, volume={13}, ISSN={["2041-1723"]}, DOI={10.1038/s41467-022-28187-x}, abstractNote={Kirigami, a traditional paper cutting art, offers a promising strategy for 2D-to-3D shape morphing through cut-guided deformation. Existing kirigami designs for target 3D curved shapes rely on intricate cut patterns in thin sheets, making the inverse design challenging. Motivated by the Gauss-Bonnet theorem that correlates the geodesic curvature along the boundary with the Gaussian curvature, here, we exploit programming the curvature of cut boundaries rather than the complex cut patterns in kirigami sheets for target 3D curved morphologies through both forward and inverse designs. The strategy largely simplifies the inverse design. Leveraging this strategy, we demonstrate its potential applications as a universal and nondestructive gripper for delicate objects, including live fish, raw egg yolk, and a human hair, as well as dynamically conformable heaters for human knees. This study opens a new avenue to encode boundary curvatures for shape-programing materials with potential applications in soft robotics and wearable devices.}, number={1}, journal={NATURE COMMUNICATIONS}, author={Hong, Yaoye and Chi, Yinding and Wu, Shuang and Li, Yanbin and Zhu, Yong and Yin, Jie}, year={2022}, month={Jan} }
 @article{wu_moody_kollipara_zhu_2022, title={Highly Sensitive, Stretchable, and Robust Strain Sensor Based on Crack Propagation and Opening}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.2c16741}, abstractNote={Soft and stretchable strain sensors have been attracting significant attention. However, the trade-off between the sensitivity (gauge factor) and the sensing range has been a major challenge. In this work, we report a soft stretchable resistive strain sensor with an unusual combination of high sensitivity, large sensing range, and high robustness. The sensor is made of a silver nanowire network embedded below the surface of an elastomeric matrix (e.g., poly(dimethylsiloxane)). Periodic mechanical cuts are applied to the top surface of the sensor, changing the current flow from uniformly across the sensor to along the conducting path defined by the open cracks. Both experiment and finite element analysis are conducted to study the effect of the slit depth, slit length, and pitch between the slits. The stretchable strain sensor can be integrated into wearable systems for monitoring physiological functions and body motions associated with different levels of strain, such as blood pressure and lower back health. Finally, a soft three-dimensional (3D) touch sensor that tracks both normal and shear stresses is developed for human-machine interfaces and tactile sensing for robotics.}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Wu, Shuang and Moody, Katherine and Kollipara, Abhiroop and Zhu, Yong}, year={2022}, month={Dec} }
 @article{yao_zhou_hinson_dong_wu_ives_hu_huang_zhu_2022, title={Ultrasoft Porous 3D Conductive Dry Electrodes for Electrophysiological Sensing and Myoelectric Control}, volume={5}, ISSN={["2365-709X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85132598682&partnerID=MN8TOARS}, DOI={10.1002/admt.202101637}, abstractNote={Abstract Biopotential electrodes have found broad applications in health monitoring, human–machine interactions, and rehabilitation. This article reports the fabrication and applications of ultrasoft breathable dry electrodes that can address several challenges for their long‐term wearable applications—skin compatibility, wearability, and long‐term stability. The proposed electrodes rely on porous and conductive silver nanowire‐based nanocomposites as the robust mechanical and electrical interface. The highly conductive and conformable structure eliminates the necessity of conductive gel while establishing a sufficiently low electrode–skin impedance for high‐fidelity electrophysiological sensing. The introduction of gas‐permeable structures via a simple and scalable method based on sacrificial templates improves breathability and skin compatibility for applications requiring long‐term skin contact. Such conformable and breathable dry electrodes allow for efficient and unobtrusive monitoring of heart, muscle, and brain activities. In addition, based on the muscle activities captured by the electrodes and a musculoskeletal model, electromyogram‐based neural–machine interfaces are realized, illustrating the great potential for prosthesis control, neurorehabilitation, and virtual reality.}, number={10}, journal={ADVANCED MATERIALS TECHNOLOGIES}, author={Yao, Shanshan and Zhou, Weixin and Hinson, Robert and Dong, Penghao and Wu, Shuang and Ives, Jasmine and Hu, Xiaogang and Huang, He and Zhu, Yong}, year={2022}, month={May} }
 @article{wu_cui_baker_mahendran_xie_zhu_2021, title={A Biaxially Stretchable and Self-Sensing Textile Heater Using Silver Nanowire Composite}, volume={13}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.1c17651}, abstractNote={Wearable heaters have garnered significant attention from academia and industry for their great potential in thermotherapy. Silver nanowire (AgNW) is a promising conductive material for flexible and stretchable electrodes. Here, a resistive, biaxially stretchable heater based on AgNW composite is reported for the first time, where a AgNW percolation network is encased in a thin polyimide (PI) film and integrated with a highly stretchable textile. AgNW/PI is patterned with a 2D Kirigami structure, which enables constant resistance under a large tensile strain (up to uniaxial 100% strain and 50% biaxial strain). The heater can achieve a high temperature of ∼140 °C with a low current of 0.125 A, fast heating and cooling rates of ∼16.5 and ∼14.1 °C s-1, respectively, and stable performance over 400 heating cycles. A feedback control system is developed to provide constant heating temperature under a temperature change of the surrounding environment. Demonstrated applications in applying thermotherapy at the curvilinear surface of the knee using the stretchable heater illustrate its promising potential for wearable applications.}, number={49}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Wu, Shuang and Cui, Zheng and Baker, G. Langston and Mahendran, Siddarth and Xie, Ziyang and Zhu, Yong}, year={2021}, month={Dec}, pages={59085–59091} }
 @article{wu_baker_yin_zhu_2021, title={Fast Thermal Actuators for Soft Robotics}, ISSN={["2169-5180"]}, DOI={10.1089/soro.2021.0080}, abstractNote={Thermal actuation is a common actuation method for soft robots. However, a major limitation is the relatively slow actuation speed. Here we report significant increase in the actuation speed of a bimorph thermal actuator by harnessing the snap-through instability. The actuator is made of silver nanowire/polydimethylsiloxane composite. The snap-through instability is enabled by simply applying an offset displacement to part of the actuator structure. The effects of thermal conductivity of the composite, offset displacement, and actuation frequency on the actuator speed are investigated using both experiments and finite element analysis. The actuator yields a bending speed as high as 28.7 cm−1/s, 10 times that without the snap-through instability. A fast crawling robot with locomotion speed of 1.04 body length per second and a biomimetic Venus flytrap were demonstrated to illustrate the promising potential of the fast bimorph thermal actuators for soft robotic applications.}, journal={SOFT ROBOTICS}, author={Wu, Shuang and Baker, Gregory Langston and Yin, Jie and Zhu, Yong}, year={2021}, month={Dec} }
 @article{li_liu_hossain_paul_yao_wu_ristaino_zhu_wei_2021, title={Real-time monitoring of plant stresses via chemiresistive profiling of leaf volatiles by a wearable sensor}, volume={4}, ISSN={["2590-2385"]}, DOI={10.1016/j.matt.2021.06.009}, abstractNote={•We report a leaf-attachable VOC sensor for real-time profiling of plant volatiles•The sensor patch detects plant VOCs at low-ppm concentrations•13 individual plant VOCs were differentiated with >97% accuracy by the sensor•It has been used to detect tomato late blight and mechanical damage noninvasively Developing a noninvasive sensing technique that enables continuous plant volatile organic compound (VOC) analysis in their natural habitat is essential to capture the VOC flux from plants for accurate monitoring of plant diseases and stresses. Several wearable sensor platforms have recently been developed that can be attached to living plants for continuous monitoring. However, detecting and discrimination of plant chemical cues such as VOCs have rarely been reported by using a wearable sensor platform. Here, a real-time VOC-profiling sensor device on a stretchable substrate has been developed for instant monitoring of plant host responses for early disease diagnosis and rapid stress identification of living plants. Determination of plant stresses such as infections by plant pathogens is currently dependent on time-consuming and complicated analytical technologies. Here, we report a leaf-attachable chemiresistive sensor array for real-time fingerprinting of volatile organic compounds (VOCs) that permits noninvasive and early diagnosis of plant diseases, such as late blight caused by Phytophthora infestans. The imperceptible sensor patch integrates an array of graphene-based sensing materials and flexible silver nanowire electrodes on a kirigami-inspired stretchable substrate, which can minimize strain interference. The sensor patch has been mounted on live tomato plants to profile key plant volatiles at low-ppm concentrations with fast response (<20 s). The multiplexed sensor array allows for accurate detection and classification of 13 individual plant volatiles with >97% classification accuracy. The wearable sensor patch was used to diagnose tomato late blight as early as 4 days post inoculation and abiotic stresses such as mechanical damage within 1 h. Determination of plant stresses such as infections by plant pathogens is currently dependent on time-consuming and complicated analytical technologies. Here, we report a leaf-attachable chemiresistive sensor array for real-time fingerprinting of volatile organic compounds (VOCs) that permits noninvasive and early diagnosis of plant diseases, such as late blight caused by Phytophthora infestans. The imperceptible sensor patch integrates an array of graphene-based sensing materials and flexible silver nanowire electrodes on a kirigami-inspired stretchable substrate, which can minimize strain interference. The sensor patch has been mounted on live tomato plants to profile key plant volatiles at low-ppm concentrations with fast response (<20 s). The multiplexed sensor array allows for accurate detection and classification of 13 individual plant volatiles with >97% classification accuracy. The wearable sensor patch was used to diagnose tomato late blight as early as 4 days post inoculation and abiotic stresses such as mechanical damage within 1 h. Phytophthora infestans is the causal agent of late blight and one of the most destructive diseases of economically important crops such as tomato and potato.1Ristaino J.B. Cooke D.E.L. Acuña I. Muñoz M. The threat of late blight to global food security.in: Ristaino J.B. Records A. Emerging Plant Diseases and Global Food Security. 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On the other hand, detection and discrimination of chemical or biological signals has not been fully explored by using a plant-wearable sensor. Lee et al. reported a carbon nanotube-based wearable field-effect transistor device for gas sensing on the surface of plant leaves.53Lee K. Park J. Lee M.S. Kim J. Hyun B.G. Kang D.J. Na K. Lee C.Y. Bien F. Park J.U. In-situ synthesis of carbon nanotube-graphite electronic devices and their integrations onto surfaces of live plants and insects.Nano Let. 2014; 14: 2647-2654Crossref PubMed Scopus (72) Google Scholar However, only simulant gas molecules such as dimethyl methylphosphonate but no actual plant VOCs has been analyzed using the wearable sensor. As such, a wearable sensor platform that can measure VOC emissions from plants in real time is urgently needed to ensure the early detection of plant diseases or stresses. In this work, we demonstrate a wearable sensor platform for real-time profiling of plant VOC markers based on a chemiresistive sensor array made of reduced graphene oxide (rGO) functionalized with various ligands (Figure 1). This multiplexed sensor array includes two different types of rGO sensors that can form reversible interactions with various plant VOCs via either hydrogen or halogen bonding. The rGO nanosheets are modified either with functionalized gold nanoparticles (AuNPs) or directly with chemical ligands containing different recognition groups (Figure S1B) to selectively capture oxygen- or nitrogen-containing organic compounds (e.g., aldehydes, ketones, alcohols) from a wide variety of plant VOCs (Figure S1A).26Weis J.G. Ravnsbæk J.B. Mirica K.A. Swager T.M. Employing halogen bonding interactions in chemiresistive gas sensors.ACS Sens. 2015; 1: 115-119Crossref Scopus (39) Google Scholar,54Frazier K.M. Swager T.M. Robust cyclohexanone selective chemiresistors based on single-walled carbon nanotubes.Anal. Chem. 2013; 85: 7154-7158Crossref PubMed Scopus (36) Google Scholar,55Jaini A.K.A. Hughes L.B. Kitimet M.M. Ulep K.J. Leopold M.C. Parish C.A. Halogen bonding interactions for aromatic and nonaromatic explosive detection.ACS Sens. 2019; 4: 389-397Crossref PubMed Scopus (16) Google Scholar The optimized sensor array composed of 4–8 rGO sensors exhibits excellent discrimination results among different individual plant VOCs, such as green leaf volatiles (GLVs) and phytohormones under ambient conditions. The sensor patch was insensitive to common mechanical perturbations such as wind blowing and hand touching as a result of the stretchable kirigami-based substrate. The environmental perturbations from temperature and humidity were also investigated. Using this sensor platform, early detection of P. infestans infection (within 4 days of inoculation) and abiotic stresses such as mechanical injury on different parts of tomato plants were achieved by in situ monitoring of plant VOC emission in real-time fashion. A large number of conductive and durable nanomaterials have been employed as flexible sensing materials.56Kamyshny A. Magdassi S. Conductive nanomaterials for 2D and 3D printed flexible electronics.Chem. Soc. Rev. 2019; 48: 1740Crossref Google Scholar Carbon-based materials possess low density, exceptional optical transparency, and superior chemical or electrochemical stability, and therefore are frequently selected as the sensing elements.18Laothawornkitkul J. Jansen R.M.C. Smid H.M. Bouwmeester H.J. Muller J. van Bruggen A.H.C. Volatile organic compounds as a diagnostic marker of late blight infected potato plants: a pilot study.Crop Prot. 2010; 29: 872Crossref Scopus (26) Google Scholar,57Angione M.D. Pilolli R. Cotrone S. Magliulo M. Mallardi A. Palazzo G. Sabbatini L. Fine D. Dodabalapur A. Cioffi N. Torsi L. Carbon based materials for electronic bio-sensing.Mater. Today. 2011; 14: 424-433Crossref Scopus (127) Google Scholar, 58Nekoueian K. Amiri M. Sillanpaa M. Marken F. Boukherroub R. Szunerits S. Carbon-based quantum particles: an electroanalytical and biomedical perspective.Chem. Soc. Rev. 2019; 48: 4281-4316Crossref PubMed Google Scholar, 59Yang L. Yi N. Zhu J. Cheng Z. Yin X. Zhang X. Zhu H. Cheng H. Novel gas sensing platform based on a stretchable laser-induced graphene pattern with self-heating capabilities.J. Mater. Chem. A. 2020; 8: 6487-6500Crossref Google Scholar Among various carbon-based materials, graphene has a larger surface-area-to-volume ratio and improved conductivity than other carbon nanomaterials such as carbon dots and carbon nanotubes. We tested four potential carbon substrates in response to 10 ppm (E)-2-hexenal, a C6 GLV known as a VOC biomarker for late blight.18Laothawornkitkul J. Jansen R.M.C. Smid H.M. Bouwmeester H.J. Muller J. van Bruggen A.H.C. Volatile organic compounds as a diagnostic marker of late blight infected potato plants: a pilot study.Crop Prot. 2010; 29: 872Crossref Scopus (26) Google Scholar All carbon nanomaterials were functionalized with 1,3-dis[3,5-bis(trifluoromethyl)phenyl]thiourea (or thiourea for short), a chemical selector reported to form strong hydrogen bonds with electronegative elements such as carbonyl oxygen (Figure 1B).54Frazier K.M. Swager T.M. Robust cyclohexanone selective chemiresistors based on single-walled carbon nanotubes.Anal. Chem. 2013; 85: 7154-7158Crossref PubMed Scopus (36) Google Scholar Sensor elements were placed in a three-dimensional (3D)-printed gas chamber (4 × 3 × 1.2 cm) for sensing performance characterization using a three-way gas-mixing setup (Figure S2). We demonstrated that rGO showed the optimal and most reproducible sensor responses compared with graphene oxide (GO) and single-walled or multiwalled carbon nanotubes (SWCNTs or MWCNTs, respectively; Figure S3). Trends in the sensor response are consistent with the chemical and physical properties of different carbon materials: compared with GO and MWCNTs, rGO and SWCNTs have higher conductivity, larger surface areas, and less discrepancy in surface chemistry (higher product purity). As a comparison, pristine rGO without any surface functionalization showed nearly no response to the gas analyte (Figure S4). This result confirms that the specificity of the rGO sensors truly comes from the functional chemical ligands and therefore reduces the possible interference from the rGO itself. The mass ratio between thiourea ligand and rGO (mthiourea:mrGO) also affects the sensing performance. We found that 16.7 wt % of thiourea in the [email protected] hybrids produced the largest signal response (Figure S5), which was used for all subsequent studies. We then examined the sensing performance of the [email protected] sensor toward four model plant VOCs, namely (E)-2-hexenal, 1-hexenal, methyl jasmonate, and 2-phenylethanol, which have been reported as potential VOC diagnostic markers of P. infestans-infected tomato plants.18Laothawornkitkul J. Jansen R.M.C. Smid H.M. Bouwmeester H.J. Muller J. van Bruggen A.H.C. Volatile organic compounds as a diagnostic marker of late blight infected potato plants: a pilot study.Crop Prot. 2010; 29: 872Crossref Scopus (26) Google Scholar The four model VOCs differ in multiple physical and chemical properties such as polarity, hydrophilicity, and nucleophilicity of particular atoms or functional groups, which reflects the complicated nature of emitted plant VOC mixtures under different stresses. The [email protected] sensor displayed a quick response within the first 20 s of exposure (i.e., >90% of equilibrated response), and the positive electrical responses were totally reversible after purging with N2 (Figure 2A). The electric resistance of [email protected] sensor presented a nearly linear dependence on VOC concentrations (Figure S6A), with a limit of detection (LOD) ranging from ∼0.13 ppm to 1.4 ppm (Figure S6B). Compared with our previously reported colorimetric sensor array,34Li Z. Paul R. Ba Tis T. Saville A.C. Hansel J.C. Yu T. Ristaino J.B. Wei Q. Non-invasive plant disease diagnostics enabled by smartphone-based fingerprinting of leaf volatiles.Nat. Plants. 2019; 5: 856-866Crossref PubMed Scopus (87) Google Scholar the LODs of the wearable VOC sensors were improved by 2- to 10-fold for all tested VOCs except for (E)-2-hexenal (Figure S6B). To enhance the chemical diversity of the sensor array, we also integrated [email protected] sensors based on halogen or hydrogen interactions with plant VOCs (Figure 1B). Six probe molecules (Figure S1B), including four halothiophenols (ITP [iodothiophenol], BPT [bromothiophenol], CTP [chlorothiophenol], and FTP [fluorothiophenol]), a nitrothiophenol (NTP; hydrogen interaction), and a methoxythiophenol (MTP; control) were first attached to the surface of AuNPs (Figure 1B), and then mixed thoroughly with rGO substrates using the ball-milling technique. It has been reported that halogen-bonding interactions can be formed between electropositive aryl halide-based selectors and electron donors such as pyridine or pyrrole, which induces negative changes in resistance as opposed to positive changes obtained by the [email protected] sensor.26Weis J.G. Ravnsbæk J.B. Mirica K.A. Swager T.M. Employing halogen bonding interactions in chemiresistive gas sensors.ACS Sens. 2015; 1: 115-119Crossref Scopus (39) Google Scholar Since most plant VOCs are abundant in nitrogen- or oxygen-containing functional groups, those hydrogen or halogen-bonding mechanisms could possibly be applied for the recognition of multiple structurally similar plant VOCs. The mixing ratio of [email protected] with the rGO substrate again has been optimized, which was determined to be 9.1 wt % of capped AuNPs f}, number={7}, journal={MATTER}, author={Li, Zheng and Liu, Yuxuan and Hossain, Oindrila and Paul, Rajesh and Yao, Shanshan and Wu, Shuang and Ristaino, Jean B. and Zhu, Yong and Wei, Qingshan}, year={2021}, month={Jul}, pages={2553–2570} }
 @article{songkakul_wu_ahmmed_reynolds_zhu_bozkurt_2021, title={Wearable Bioimpedance Hydration Monitoring System using Conformable AgNW Electrodes}, ISSN={["1930-0395"]}, url={http://dx.doi.org/10.1109/sensors47087.2021.9639469}, DOI={10.1109/SENSORS47087.2021.9639469}, abstractNote={Monitoring hydration level could be vital for maintaining physiological and cognitive performance during physical exertion and thermal stress. We present a custom miniaturized wearable bioimpedance spectroscopy (BIS) system consisting of a Bluetooth-enabled system-on-a-chip and an analog front-end circuit integrated with conformable, flexible, and stretchable silver-nanowire electrodes. This system is capable of performing four-electrode BIS at a range of frequencies between 5 kHz and 195 kHz, transmitting the data wirelessly to a data aggregator, and configuring the front-end circuit parameters over-the-air when needed. A 150 mAh lithium polymer battery can power the system for 18 hours. In this study, proof-of-concept in-vitro validation of the system generated promising results.}, journal={2021 IEEE SENSORS}, publisher={IEEE}, author={Songkakul, Tanner and Wu, Shuang and Ahmmed, Parvez and Reynolds, William D., Jr. and Zhu, Yong and Bozkurt, Alper}, year={2021} }
 @article{wu_yao_liu_hu_huang_zhu_2020, title={Buckle-Delamination-Enabled Stretchable Silver Nanowire Conductors}, volume={12}, ISSN={["1944-8252"]}, url={http://dx.doi.org/10.1021/acsami.0c09775}, DOI={10.1021/acsami.0c09775}, abstractNote={Controlled buckling and delamination of thin films on a compliant substrate has attracted much attention for applications ranging from micro/nanofabrication to flexible and stretchable electronics to bioengineering. Here, a highly conductive and stretchable conductor is fabricated by attaching a polymer composite film (with a thin layer of silver nanowires embedded below the surface of the polymer matrix) on top of a prestretched elastomer substrate followed with releasing the prestrain. A partially delaminated wavy geometry of the polymer film is created. During the evolution of the buckle-delamination, the blisters pop-up randomly but self-adjust into a uniform distribution, which effectively reduces the local strain in the silver nanowires. The resistance change of the conductor is less than 3% with the applied strain up to 100%. A theoretical model on the buckle-delamination structure is developed to predict the geometrical evolution, which agrees well with experimental observation. Finally, an integrated silver nanowire/elastomer sensing module and a stretchable thermochromic device are developed to demonstrate the utility of the stretchable conductor. This work highlights the important relevance of mechanics-based design in nanomaterial-enabled stretchable devices.}, number={37}, journal={ACS APPLIED MATERIALS & INTERFACES}, publisher={American Chemical Society (ACS)}, author={Wu, Shuang and Yao, Shanshan and Liu, Yuxuan and Hu, Xiaogang and Huang, He Helen and Zhu, Yong}, year={2020}, month={Sep}, pages={41696–41703} }