Developing Wound Moisture Sensors: Opportunities and Challenges for Laser-Induced Graphene-Based Materials

Recent advances in polymer composites have led to new, multifunctional wound dressings that can greatly improve healing processes, but assessing the moisture status of the underlying wound site still requires frequent visual inspection. Moisture is a key mediator in tissue regeneration and it has long been recognised that there is an opportunity for smart systems to provide quantitative information such that dressing selection can be optimised and nursing time prioritised. Composite technologies have a rich history in the development of moisture/humidity sensors but the challenges presented within the clinical context have been considerable. This review aims to train a spotlight on existing barriers and highlight how laser-induced graphene could lead to emerging material design strategies that could allow clinically acceptable systems to emerge

Frequency signatured directly printable humidity sensing tag using organic electronics

In this paper chipless RFID tag, capable of carrying 9-bit data is presented. The tag is optimized for several flexible substrates. With growing information and communication technology, sensor integration with data transmission has gained significant attention. Therefore, the tag with the same dimension is then optimized using paper substrate. For different values of permittivity, the relative humidity is observed. Hence, besides carrying information bits, the tag is capable of monitoring and sensing the humidity. The overall dimension of the tag comprising of 9 ring slot resonators is 7 mm. Due to its optimization on the paper substrate, the tag can be an ideal choice for deploying in various low-cost sensing application

Microwave Humidity Sensor for Early Detection of Sweat and Urine Leakage

A planar microwave sensor devoted to the detection of humidity in underwear and clothes in general is proposed. The ultimate goal of the sensor is to detect the presence of liquids in fabrics, which is of interest to aid patients who suffer from certain pathologies, such as hyperhidrosis and enuresis. The main target in the design of the sensor, considering the envisaged application, is simplicity. Thus, the sensor operates at a single frequency, and the working principle is the variation in the magnitude of the transmission coefficient of a matched line loaded with an open-ended quarter-wavelength sensing stub resonator. The stub, which must be in contact with the so-called fabric under test (FUT), generates a notch in the transmission coefficient with a resonance frequency that depends on the humidity level of the fabric. By designing the stub with a moderately high-quality factor, the variation in the resonance frequency causes a significant change in the magnitude level at the operating frequency, which is the resonance frequency when the sensing stub is loaded with the dry fabric, and the presence of liquid can be detected by means of an amplitude detector. A prototype device is proposed and experimentally validated. The measured change in the magnitude level by simply depositing one 50 μL drop of water in the FUT is roughly 25 dB

Compact readout system for chipless passive LC tags and its application for humidity monitoring

The development of a contactless readout system for High Frequency (HF) tags and its application to relative humidity monitoring is presented. The system consists of a Colpitts oscillator circuit whose frequency response is determined by a built-in logic counter of a microcontroller unit. The novel readout strategy is based on the frequency response change due to the inductive coupling between the coil of the Colpitts oscillator and the load impedance of a parallel LC resonator tag, as a result of the variation of the humidity sensing capacitor. The frequency is monitored with a low cost microcontroller, resulting in a simple readout circuit. This passive LC tag has been directly screen-printed on a humidity-sensitive flexible substrate. The readout circuit experimental uncertainty as frequency meter was 4 kHz in the HF band. A linear temperature drift of (-1.52 ± 0.17) kHz/⁰C was obtained, which can be used to apply thermal compensation if required. The readout system has been validated as a proof of concept for humidity measurement, obtaining a significant change of about 260 kHz in the resonance frequency of the Colpitts oscillator when relative humidity varies from 10% to 90%, with a maximum uncertainty of ±3% (±2 SD). Therefore, the proposed readout system stands as a compact, low-cost, contactless solution for chipless HF tags that avoids the use of bulky and costly equipment for the analog reading of wireless passive LC sensors.This work was supported by project CTQ2016-78754-C2-1-R from the Spanish Ministry of Economics and Competitivity. P. Escobedo wants to thank the Spanish Ministry of Education, Culture and Sport (MECD) for a pre-doctoral grant (FPU13/05032)

Directly Printed Nanomaterial Sensor for Strain and Vibration Measurement

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 기계공학부, 2020. 8. 안성훈.Most discussions about Industrie 4.0 tacitly assume that any such system would involve the processing and evaluation of large data volumes. Specifically, the operation of complex production processes requires stable and reliable data measurement and communication systems. However, while modern sensor technology may already be capable of collecting a wide range of machine and production data, it has been proving difficult to measure and analyse the data which is not easy to measurable and feed the results quickly back into an optimised production cycle. This is why the cost and installation of sensor, data acquisition, and transmission systems for flexible and adaptive manufacturing process have not been match the requirement of industrial demands. In this dissertation, directly printed nanomaterial sensor capable of strain and vibration measurement with high sensitivity and wide measurable range was fabricated using aerodynamically focused nanomaterial (AFN) printing system which is a direct printing technique for conductive and stretchable pattern printing onto flexible substrate. Specifically, microscale porous conductive pattern composed of silver nanoparticles (AgNPs) and multi-walled carbon nanotubes (MWCNTs) composite was printed onto polydimethylsiloxane (PDMS). Printing mechanism of AFN printing system for nanocomposite onto flexible substrate in order of mechanical crack generation, seed layer deposition, partial aggregation, and fully deposition was demonstrated and experimentally validated. The printed nanocomposite sensor exhibited gauge factor (GF) of 58.7, measurable range of 0.74, and variance in peak resistance under 0.05 during 1,000 times life cycle evaluation test. Furthermore, vibration measurement performance was evaluated according to vibration amplitude and frequency with Q-factor evaluation and statistical verification. Sensing mechanism for nanocomposite sensor was also analysed and discussed by both analytical and statistical methods. First, electron tunnelling effect among nanomaterials was analysed statistically using bivariate probit model. Since electrical property varies by the geometrical properties of nanomaterial, Monte Carlo simulation method based on Lennard-Jones (LJ) potential model and the voter model was developed for deeper understanding of the dynamics of nanomaterial by strain. By simply counting the average attachment among nanomaterials by strain, electrical conductivity was easily estimated with low simulation cost. The main objective of all processes to manufacture high-tech products is compliance with the specified ranges of permissible variation. In this perspective, all data must be recorded that might provide some evidence of status changes anywhere along the process chain. This dissertation covers the monitoring of forming and milling process. By measurement of mechanical deformation of stamp during forming process, it was possible to estimate the forming force according to various process parameters including maximum force, force gradient, and the thickness of sheet metal. Furthermore, accurate and reliable vibration monitoring was also conducted during milling process by simple and direct attachment of printed sensor to workpiece. Using frequency and power spectrum analysis of obtained data, the vibration of workpiece was measured during milling process according to process parameters including RPM, feed rate, cutting depth and width of spindle. Finally, developed sensor was applied to the digital twin of turbine blade manufacturing that vibration greatly affects the quality of product to predict the process defects in real time. To overcome the wire required data acquisition and transmission system, directly printed wireless communication sensor was also developed using chipless radio frequency identification (RFID) technology. It is one of the widely used technique for internet-of-things (IoT) devices due to low-cost, printability, and simplicity. The developed stretchable and chipless RFID sensor exhibited GF more than 0.6 and maximum measurable range more than 0.2 with high degree-of-freedom of motion. Since it showed its original characteristics of sensing in only one direction independently, sensor patch composed of various sensor with different resonance frequency was capable of measuring not only normal strains but also shear strains in all directions. Sensors in machinery and equipment can provide valuable clues as to whether or not the actual values will fall into the tolerance range. In this aspect, a real-time, accurate, and reliable process monitoring is a basic and crucial enabler of intelligent manufacturing operations and digital twin applications. In this dissertation, developed sensor was used for various manufacturing process include forming process, milling process, and wireless communication using highly sensitive and wide measuring properties with low fabrication cost. It is expected that developed sensor could be applied for the digital twin and process defects prediction in real-time.4차 산업혁명에 대한 대부분의 논의는 많은 양의 데이터를 처리하고 평가하는 시스템을 암묵적으로 가정한다. 특히, 복잡한 생산 공정을 운영하기 위해서는 안정적이고 신뢰할 수 있는 데이터 측정 및 통신 시스템이 필요하다. 하지만, 최신 센서 기술은 광범위한 기계 및 생산 공정 중 데이터를 수집하는 것이 가능하지만 측정하기 쉽지 않은 데이터를 측정하고 분석하여 그 결과를 최적화된 생산 공정에 신속하게 제공하는데 한계를 가지고 있다. 때문에, 유연하고 적응 가능한 제조 공정을 위한 센서의 가격과 설치 방법, 데이터 수집 및 전송 시스템이 실제 산업의 요구 사항에 도달하지 못하고 있다. 이 학위 논문에서는 유연 기판에 전도성 및 신축성 패턴을 직접 인쇄할 수 있는 공기역학적 나노물질 집속 인쇄 시스템을 사용하여 높은 민감도와 넓은 측정 가능 범위를 가진 변위 및 진동 센서를 개발하였다. 구체적으로, 은 나노입자와 다중 벽 탄소 나노튜브로 구성된 나노 복합재를 폴리디메틸실록산 위에 직접 인쇄하였다. 유연 기판 위에 공기역학적 나노물질 집속 인쇄 시스템을 사용한 나노 복합재 인쇄 방법의 기작이 기계적 균열 발생, 시드층 적층, 부분 응집 및 완전 증착 순으로 논의 및 실험적으로 검증되었다. 인쇄된 나노 복합재 센서는 58.7의 게이지 팩터, 0.74의 측정 가능 범위를 나타내었으며 1,000번 반복된 수명 주기 평가에서 5% 미만의 정점 저항 변화를 확인할 수 있었다. 또한 Q 인자 평가 및 통계 검증을 사용하여 진동의 진폭 및 주파수에 따른 진동 측정 성능을 평가하였다. 나노 복합재 센서에 대한 측정 기작 또한 해석적 및 통계적 방법으로 분석되었다. 먼저, 나노물질 간 터널 효과가 이변량 프로빗 모델을 통해 통계적으로 분석되었다. 센서의 전기적 물성이 나노물질의 기하학적 물성에 따라 상이하기 때문에 변위에 따른 나노물질의 동적인 이해를 위해 레너드존스 전위 및 유권자 모델을 기반으로 한 몬테카를로 시뮬레이션 방법이 개발되었다. 이를 활용하여 나노물질 간 평균 부착 수를 계산하여 낮은 비용으로 전기전도도를 추정할 수 있었다. 첨단 제품을 제조하기 위한 모든 공정의 주요 목표는 지정된 범위의 허용 가능한 변동을 준수하는 것이다. 이를 위해 공정 중 어디에서나 상태 변경의 증거를 제공할 수 있는 모든 데이터를 기록하는 것이 필수적이다. 이 학위 논문에서는 제작된 센서를 통해 성형 및 절삭 공정의 데이터를 기록함으로써 공정을 모니터링하였다. 성형 공정 동안 스탬프의 기계적 변형을 측정함으로써 최대 힘, 힘의 구배 및 판금의 두께를 포함하는 다양한 공정 변수에 따라 성형 힘을 추정할 수 있었다. 또한, 절삭 공정 중 공작물에 제작된 센서를 직접 부착하여 정확하고 안정적인 진동 모니터링을 수행하였다. 얻어진 데이터의 주파수 및 전력 스펙트럼 분석을 이용하여, 분당 회전 수, 이송 속도, 스핀들의 절삭 깊이 및 너비에 따른 공작물의 진동을 측정하였다. 마지막으로 제조된 센서를 진동이 제품 품질에 큰 영향을 미치는 터빈 동익 제조 공정의 디지털 트윈으로 적용하여 실시간으로 공정 결함을 예측하였다. 유선 데이터 수집 및 전송 시스템을 극복하기 위해 칩리스 무선 주파수 식별 기술을 사용하여 직접 인쇄된 무선 통신 센서를 개발하였다. 칩리스 무선 주파수 식별 기술은 저비용, 인쇄성 및 공정의 평이성으로 인해 사물 인터넷 장치에 널리 사용되는 기술 중 하나이다. 개발된 유연한 칩리스 센서는 0.6 이상의 게이지 팩터와 0.2 이상의 측정 가능 범위를 나타냈다. 또한 제작된 센서는 한 방향의 변위만 독립적으로 측정할 수 있는 특성을 가지고 있기 때문에, 모든 방향의 수직 및 전단 변형을 측정할 수 있는 다양한 공진 주파수로 구성된 센서 패치가 개발되었다. 기계 및 장비의 센서는 실제 값이 공차 범위에 속하는지 여부에 대한 중요한 단서를 제공할 수 있다. 이러한 측면에서, 정확하고 신뢰할 수 있는 실시간 공정 모니터링은 지능형 제조 공정 및 디지털 트윈으로의 응용을 위한 기본적이고 결정적인 요소이다. 이 학위 논문에서 개발된 센서는 낮은 제조 비용과 높은 민감도 및 신축성을 가지고 있기 때문에 성형 공정, 절삭 공정, 무선 통신을 포함한 다양한 제조 공정에서 응용되었다. 뿐만 아니라 제작된 센서는 디지털 트윈 및 공정 결함의 실시간 예측을 위해 다양하게 사용될 수 있을 것으로 예상된다.Chapter 1. Introduction 1 1.1. Toward smart manufacturing 1 1.2. Sensor in manufacturing 4 1.3. Research objective 11 Chapter 2. Background 16 2.1. Aerodynamically focused nanomaterial printing 16 2.2. Printing system envelope 26 2.3. Highly sensitive sensor printing 34 Chapter 3. Sensor fabrication and evaluation 42 3.1. Highly sensitive and wide measuring sensor printing 42 3.2. Sensing performance evaluation 59 3.3. Environmental and industrial evaluation 87 Chapter 4. Sensing mechanism analysis 97 4.1. Theoretical background 97 4.2. Statistical regression anaylsis 101 4.3. Monte Carlo simulation 104 Chapter 5. Application to process monitoring 126 5.1. Forming process monitoring 126 5.2. Milling process monitoring 133 5.3. Wireless communication monitoring 149 Chapter 6. Conclusion 185 Bibliography 192 Abstract in Korean 211Docto

Wearable sensors for respiration monitoring: a review

This paper provides an overview of flexible and wearable respiration sensors with emphasis on their significance in healthcare applications. The paper classifies these sensors based on their operating frequency distinguishing between high-frequency sensors, which operate above 10 MHz, and low-frequency sensors, which operate below this level. The operating principles of breathing sensors as well as the materials and fabrication techniques employed in their design are addressed. The existing research highlights the need for robust and flexible materials to enable the development of reliable and comfortable sensors. Finally, the paper presents potential research directions and proposes research challenges in the field of flexible and wearable respiration sensors. By identifying emerging trends and gaps in knowledge, this review can encourage further advancements and innovation in the rapidly evolving domain of flexible and wearable sensors.This work was supported by the Spanish Government (MICINN) under Projects TED2021-131209B-I00 and PID2021-124288OB-I00.Peer ReviewedPostprint (published version

Chipless RFID Tag for Touch Event Sensing and Localization.

A novel Radio Frequency Identification (RFID) based sensor supporting touch detection and localization features is proposed in this work. The formulated sensor leverages chipless variant of RFID technology for the design of a passive fully-printable frequency domain-based sensor-incorporated tag. The sensor is composed of square resonators arranged in a peculiar fashion laid down across a 3×2 grid. The proposed sensor incorporated-tag readily keeps track of human-digit position, allowing for tracking of finger-swipes which, in turn, can potentially be used for recognition of unlock patterns and security codes. Performance of the sensor is analyzed using its Radar Cross Section (RCS) response observable in the spectral domain. Each constituent resonant-element making up the sensor resonates at a single frequency represented by a distinct dip in the RCS response. The spectral dip drifts well outside of its allocated band upon occurrence of a touch event. A functional prototype of the sensor tag is fabricated on a 0.508 mm thick Rogers RT/Duroid ® 5880 laminate is scrutinized of its electromagnetic performance. The sensor possesses a compact physical footprint equal to 45 mm ×55 mm. The obtained results solidify the suitability of the proposed sensor for deployment in secure access control settings prevalent in smart cities and connected home applications

Antenna sensing for wearable applications

As wearable technologies are growing fast, there is emerging trend to increase functionality of the devices. Antennas which are primarily component in communication systems can offer attractive route forward to minimize the number of components functioning as a sensing element for wearable and flexible electronics. Toward development of flexible antenna as sensing element, this thesis investigates the development of the flexible and printed sensing NFC RFID tag. In this approach, the sensor measurement is supported by the internal sensor and analog-to-digital convertor (ADC) of the NFC transponder. Design optimisation, fabrication and characterization of the printed antenna are described. Besides, the printed antenna, NFC transponder and two simple resistive sensors are integrated to form a fully flexible sensing RFID tag demonstrating applicability in food and health monitoring. This thesis also presents development of two antenna sensors by using functional materials: (i) An inductor-capacitor (LC) resonant tank based wireless pressure sensor on electrospun Poly-L-lactide (PLLA) nanofibers-based substrate. The screen-printed resonant tank (resonant frequency of ~13.56 MHz) consists of a planar inductor connected in parallel with an interdigitated capacitor. Since the substrates is piezoelectric, the capacitance of the interdigitated capacitor varies in response to the applied pressure. To demonstrate a potential application of developed pressure sensor, it was integrated on a compression bandage to monitor sub-bandage pressure. (ii) To investigate the realization of sensing antenna as temperature sensor simple loop antenna is designed and in this study unlike the first study that the sensing element was the substrate, the conductive body of the antenna itself is considered as a functional material. In this case, a small part of a loop antenna which originally was printed using silver paste is replaced by Poly(3,4-ethylenedioxythiophene): polystyrene (PEDOT: PSS). The sensing mechanism is based on the resonant frequency shift by varying temperature. While using functional materials is useful for realization of antenna sensor, another approach also is presented by developing stretchable textile-based microstrip antennas on deformable substrate which can measure joint angles of a human limb. The EM characteristics of the meshed patch antenna were compared with its metallic counterpart fabricated with lithography technique. Moreover, the concept of stretchable UHF RFID-based strain sensor is touched in the final part of this thesis

Microwave sensors based on resonant elements

This paper highlights interest in the implementation of microwave sensors based on resonant elements, the subject of a special issue in the journal. A classification of these sensors on the basis of the operating principle is presented, and the advantages and limitations of the different sensor types are pointed out. Finally, the paper summarizes the different contributions to the special issue

Textile UHF-RFID antenna sensor for measurements of sucrose solutions in different levels of concentration

With the trend of textile antennas development, ultra high frequency (UHF, 865–868 MHz) radio frequency identification (RFID) devices using textile materials are expected to be developed in many areas for replacing or simplifying some complex RFID devices on a PCB. In this paper, we present a textile UHF-RFID tag with two sensing positions ('radiation parts' and 'loop part') for exploring the feasibility of sucrose solutions measurements and the relationship between variables of the proposed design and the sucrose solutions. The simulated and measured resonance curves of the designs both match well (-20 dB in the simulation and -15.8 dB in the measurement at 868 MHz) and the read range measured by the RFID reader (M6e kit) is 1.71 m in air. Before the tests by the solutions on the proposed designs, a test board is developed as a preparation work for confirming the relative dielectric constants of the sensing substrate area in the real measurements. Compare the results of the simulation and the real tests, the proposed design shows good feasibility by comparing the simulated and measured results in confirmed relative dielectric constants. Moreover, its two sensing positions have different sensing features. The sensing 'radiation parts' position shows a stable frequency operation performance but sensing range is from 1.71 m (dry) to 2.3 m, while the sensing 'loop part' position has a wide sensing range from 0.4 m to 1.71 m (dry) but a lower frequency operation performance.This work was supported by Spanish Government-MINECO under Project TEC2016- 79465-R and China Scholarship Council No.201908440233.Peer ReviewedPostprint (author's final draft