@article{moon_xue_ganesh_shukla_kreager_cai_wu_zhu_sharma_jiang_2024, title={Ultrasound-Compatible Electrode for Functional Electrical Stimulation}, volume={12}, ISSN={["2227-9059"]}, url={https://doi.org/10.3390/biomedicines12081741}, DOI={10.3390/biomedicines12081741}, abstractNote={Functional electrical stimulation (FES) is a vital method in neurorehabilitation used to reanimate paralyzed muscles, enhance the size and strength of atrophied muscles, and reduce spasticity. FES often leads to increased muscle fatigue, necessitating careful monitoring of the patient's response. Ultrasound (US) imaging has been utilized to provide valuable insights into FES-induced fatigue by assessing changes in muscle thickness, stiffness, and strain. Current commercial FES electrodes lack sufficient US transparency, hindering the observation of muscle activity beneath the skin where the electrodes are placed. US-compatible electrodes are essential for accurate imaging and optimal FES performance, especially given the spatial constraints of conventional US probes and the need to monitor muscle areas directly beneath the electrodes. This study introduces specially designed body-conforming US-compatible FES (US-FES) electrodes constructed with a silver nanowire/polydimethylsiloxane (AgNW/PDMS) composite. We compared the performance of our body-conforming US-FES electrode with a commercial hydrogel electrode. The findings revealed that our US-FES electrode exhibited comparable conductivity and performance to the commercial one. Furthermore, US compatibility was investigated through phantom and in vivo tests, showing significant compatibility even during FES, unlike the commercial electrode. The results indicated that US-FES electrodes hold significant promise for the real-time monitoring of muscle activity during FES in clinical rehabilitative applications.}, number={8}, journal={BIOMEDICINES}, author={Moon, Sunho and Xue, Xiangming and Ganesh, Vidisha and Shukla, Darpan and Kreager, Benjamin C. and Cai, Qianqian and Wu, Huaiyu and Zhu, Yong and Sharma, Nitin and Jiang, Xiaoning}, year={2024}, month={Aug} }
 @article{xue_zhang_moon_xu_huang_sharma_jiang_2023, title={Development of a Wearable Ultrasound Transducer for Sensing Muscle Activities in Assistive Robotics Applications}, volume={13}, ISSN={["2079-6374"]}, url={https://doi.org/10.3390/bios13010134}, DOI={10.3390/bios13010134}, abstractNote={Robotic prostheses and powered exoskeletons are novel assistive robotic devices for modern medicine. Muscle activity sensing plays an important role in controlling assistive robotics devices. Most devices measure the surface electromyography (sEMG) signal for myoelectric control. However, sEMG is an integrated signal from muscle activities. It is difficult to sense muscle movements in specific small regions, particularly at different depths. Alternatively, traditional ultrasound imaging has recently been proposed to monitor muscle activity due to its ability to directly visualize superficial and at-depth muscles. Despite their advantages, traditional ultrasound probes lack wearability. In this paper, a wearable ultrasound (US) transducer, based on lead zirconate titanate (PZT) and a polyimide substrate, was developed for a muscle activity sensing demonstration. The fabricated PZT-5A elements were arranged into a 4 × 4 array and then packaged in polydimethylsiloxane (PDMS). In vitro porcine tissue experiments were carried out by generating the muscle activities artificially, and the muscle movements were detected by the proposed wearable US transducer via muscle movement imaging. Experimental results showed that all 16 elements had very similar acoustic behaviors: the averaged central frequency, −6 dB bandwidth, and electrical impedance in water were 10.59 MHz, 37.69%, and 78.41 Ω, respectively. The in vitro study successfully demonstrated the capability of monitoring local muscle activity using the prototyped wearable transducer. The findings indicate that ultrasonic sensing may be an alternative to standardize myoelectric control for assistive robotics applications.}, number={1}, journal={BIOSENSORS-BASEL}, author={Xue, Xiangming and Zhang, Bohua and Moon, Sunho and Xu, Guo-Xuan and Huang, Chih-Chung and Sharma, Nitin and Jiang, Xiaoning}, year={2023}, month={Jan} }
 @article{xue_wu_cai_chen_moon_huang_kim_peng_feng_sharma_et al._2024, title={Flexible Ultrasonic Transducers for Wearable Biomedical Applications: A Review on Advanced Materials, Structural Designs, and Future Prospects}, volume={71}, ISSN={["1525-8955"]}, url={https://doi.org/10.1109/TUFFC.2023.3333318}, DOI={10.1109/TUFFC.2023.3333318}, abstractNote={Due to the rapid developments in materials science and fabrication techniques, wearable devices have recently received increased attention for biomedical applications, particularly in medical ultrasound imaging, sensing, and therapy. Ultrasound is ubiquitous in biomedical applications because of its non-invasive nature, nonionic radiating, high precision, and real-time capabilities. While conventional ultrasound transducers are rigid and bulky, flexible transducers can be conformed to curved body areas for continuous sensing without restricting tissue movement or transducer shifting. This article comprehensively reviews the application of flexible ultrasound transducers in the field of biomedical imaging, sensing, and therapy. First, we review the background of flexible ultrasound transducers. Following that, we discuss advanced materials and fabrication techniques for flexible ultrasound transducers and their enabling technology status. Lastly, we highlight and summarize some promising preliminary data with recent applications of flexible ultrasound transducers in biomedical imaging, sensing, and therapy. We also provide technical barriers, challenges, and future perspectives for further research and development.}, number={7}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, author={Xue, Xiangming and Wu, Huaiyu and Cai, Qianqian and Chen, Mengyue and Moon, Sunho and Huang, Ziping and Kim, Taeyang and Peng, Chang and Feng, Wuwei and Sharma, Nitin and et al.}, year={2024}, month={Jul}, pages={786–810} }
 @article{xue_iyer_sharma_2023, title={Koopman-based Data-driven Model Predictive Control of Limb Tremor Dynamics with Online Model Updating: A Theoretical Modeling and Simulation Approach}, ISSN={["2378-5861"]}, DOI={10.23919/ACC55779.2023.10156240}, abstractNote={Patients suffering from tremors have difficulty performing activities of daily living. The development of a model of a limb with tremors can pave the way for non-surgical tremor suppression control techniques. Nevertheless, nonlinearity and actuator saturation make it difficult to develop an accurate model and a tremor suppression control method. Towards addressing this issue, this paper describes a Koopman-based method for system identification and its application to the design of a model predictive control (MPC) scheme to suppress tremors. Since model prediction accuracy is critical to the performance of an MPC, it is essential to update the model online if the predictions are not sufficiently accurate. We propose a recursive least squares (RLS) algorithm to improve control performance with low computational complexity. Finally, for the first time, stability analysis and recursive feasibility of the Koopman-based MPC (KMPC) closed-loop updated system are presented. The proposed modeling and control approach have been validated by experimental data and simulation results.}, journal={2023 AMERICAN CONTROL CONFERENCE, ACC}, author={Xue, Xiangming and Iyer, Ashwin and Sharma, Nitin}, year={2023}, pages={2873–2878} }
 @article{xue_iyer_roque_sharma_2023, title={Nonlinear System Identification of Tremors Dynamics: A Data-driven Approximation Using Koopman Operator Theory}, ISSN={["1948-3546"]}, DOI={10.1109/NER52421.2023.10123909}, abstractNote={People who suffer from tremors have difficulty performing activities of daily living. Efforts in developing a model of a limb with tremors can pave the way for non-surgical tremor suppression techniques. However, due to the nonlinearity, developing an accurate model of tremors is challenging. This paper implements a data-driven method for approximating the Koopman operator, which is capable of presenting nonlinear dynamics in a linear framework and is promising for predicting the nonlinear system. A dynamic model of tremors is developed with ultrasound (US) image data collected from a patient with essential tremor as they grasp objects. The method is applied to predict the patient's tremor dynamics and is compared with the nonlinear Hammerstein-Wiener system identification technique.}, journal={2023 11TH INTERNATIONAL IEEE/EMBS CONFERENCE ON NEURAL ENGINEERING, NER}, author={Xue, Xiangming and Iyer, Ashwin and Roque, Daniel and Sharma, Nitin}, year={2023} }
 @article{sharma_xue_iyer_jiang_roque_2023, title={Towards ultrasound imaging-based closed-loop peripheral nerve stimulation for tremor suppression}, volume={28}, ISSN={["2468-4511"]}, url={https://doi.org/10.1016/j.cobme.2023.100484}, DOI={10.1016/j.cobme.2023.100484}, abstractNote={Despite several decades of research investigating the use of peripheral electrical stimulation (ES) for tremor suppression in an upper limb, ES design for effective tremor suppression remains elusive. The article reviews sensing approaches to measure limb tremors and existing musculoskeletal models of tremor and their use in closed-loop suppression control. We also motivate a case for incorporating ultrasound (US) imaging into the closed-loop control for increased tremor suppression efficacy. When combined with wearable US transducers, the novel approach could be a promising technique to advance musculoskeletal models that investigate tremor mechanisms and new ES closed-loop techniques with personalized stimulation parameters for tremor suppression.}, journal={CURRENT OPINION IN BIOMEDICAL ENGINEERING}, author={Sharma, Nitin and Xue, Xiangming and Iyer, Ashwin and Jiang, Xiaoning and Roque, Daniel}, year={2023}, month={Dec} }
 @article{xue_zhang_moon_xu_huang_sharma_jiang_2022, title={Development of a wearable ultrasound transducer for sensing muscle activities in assistive robotics applications: In vivo study}, ISSN={["1948-5719"]}, DOI={10.1109/IUS54386.2022.9958535}, abstractNote={People who suffer from the amputation of limbs or with mobility impairment due to methodological disorder sometimes require assistive robotics (AR), such as robotic prostheses and exoskeletons, to function satisfactorily and productively in daily life. Dynamic measurements of muscle voluntary activities are widely used to control AR, and sensors used to control AR should be non-invasive, effective, and wearable. Ultrasound (US) imaging is an effective method for measuring muscle activity. Nevertheless, conventional US transducers are cumbersome and inflexible, making them inconvenient for continuous monitoring of muscle activity for AR control. In light of no report available about using a flexible transducer for detecting muscle activities for AR, this work aims to develop a novel wearable US device for detecting muscle activities. In specific, a 16-element 10 MHz flexible sparse array was designed, fabricated, and characterized. The feasibility of monitoring muscle activity in different regions was demonstrated by an in vivo human experiment.}, journal={2022 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IEEE IUS)}, author={Xue, Xiangming and Zhang, Bohua and Moon, Sunho and Xu, Guo-Xuan and Huang, Chih-Chung and Sharma, Nitin and Jiang, Xiaoning}, year={2022} }