Biomimetic Soft Polymer Microstructures and Piezoresistive Graphene MEMS Sensors using Sacrificial Metal 3D Printing

Recent advances in 3D printing technology have enabled unprecedented design freedom across an ever-expanding portfolio of materials. However, direct 3D printing of soft polymeric materials such as polydimethylsiloxane (PDMS) is challenging, especially for structural complexities such as high-aspect ratio (>20) structures, 3D microfluidic channels (∼150 μm diameter), and biomimetic microstructures. This work presents a novel processing method entailing 3D printing of a thin-walled sacrificial metallic mold, soft polymer casting, and acidic etching of the mold. The proposed workflow enables the facile fabrication of various complex, bioinspired PDMS structures (e.g., 3D double helical microfluidic channels embedded inside high-aspect ratio pillars) that are difficult or impossible to fabricate using currently available techniques. The microfluidic channels are further infused with conductive graphene nanoplatelet ink to realize two flexible piezoresistive microelectromechanical (MEMS) sensors (a bioinspired flow/tactile sensor and a dome-like force sensor) with embedded sensing elements. The MEMS force sensor is integrated into a Philips 9000 series electric shaver to demonstrate its application in "smart"consumer products in the future. Aided by current trends in industrialization and miniaturization in metal 3D printing, the proposed workflow shows promise as a low-temperature, scalable, and cleanroom-free technique of fabricating complex, soft polymeric, biomimetic structures, and embedded MEMS sensors

Creating underwater vision through wavy whiskers:A review of the flow sensing mechanisms and biomimetic potential of seal whiskers

Seals are known to use their highly-sensitive whiskers to precisely follow the hydrodynamic trail left behind by prey. Studies estimate that a seal can track a herring that is swimming as far as 180 m away, indicating an incredible detection apparatus on par with the echolocation system of dolphins and porpoises. This remarkable sensing capability is enabled by the unique undulating structural morphology of the whisker that suppresses vortex-induced vibrations (VIV) and thus increases the signal-to-noise ratio of the flow sensing whiskers. In other words, the whiskers vibrate minimally due to the seal’s swimming motion, eliminating most of the self-induced noise and making them ultra-sensitive to the vortices in the wake of escaping prey. Due to this impressive ability, the seal whisker has attracted much attention in the scientific community, encompassing multiple fields of sensory biology, fluid mechanics, biomimetic flow sensing, and soft robotics. This article presents a comprehensive review of the seal whisker literature, covering the behavioral experiments on real seals, VIV suppression capabilities enabled by the undulating geometry, wake vortex-sensing mechanisms, morphology and material properties, and finally engineering applications inspired by the shape and functionality of seal whiskers. Promising directions for future research are proposed

A Biologically Inspired Controllable Stiffness Multimodal Whisker Follicle

This thesis takes a soft robotics approach to understand the computational role of a soft whisker follicle with mechanisms to control the stiffness of the whisker. In particular, the thesis explores the role of the controllable stiffness whisker follicle to selectively favour low frequency geometric features of an object or the high frequency texture features of the object.Tactile sensing is one of the most essential and complex sensory systems for most living beings. To acquire tactile information and explore the environment, animals use various biological mechanisms and transducing techniques. Whiskers, or vibrissae are a form of mammalian hair, found on almost all mammals other than homo sapiens. For many mammals, and especially rodents, these whiskers are essential as a means of tactile sensing.The mammalian whisker follicle contains multiple sensory receptors strategically organised to capture tactile sensory stimuli of different frequencies via the vibrissal system. Nocturnal mammals such as rats heavily depend on whisker based tactile perception to find their way through burrows and identify objects. There is diversity in the whiskers in terms of the physical structure and nervous innervation. The robotics community has developed many different whisker sensors inspired by this biological basis. They take diverse mechanical, electronic, and computational approaches to use whiskers to identify the geometry, mechanical properties, and objects' texture. Some work addresses specific object identification features and others address multiple features such as texture and shape etc. Therefore, it is vital to have a comprehensive discussion of the literature and to understand the merits of bio-inspired and pure-engineered approaches to whisker-based tactile perception.The most important contribution is the design and use of a novel soft whisker follicle comprising two different frequency-dependent data capturing modules to derive more profound insights into the biological basis of tactile perception in the mammalian whisker follicle. The new insights into the biological basis of tactile perception using whiskers provide new design guidelines to develop efficient robotic whiskers

액상 중합체 및 금속염을 활용한 변형 센서의 제작 및 응용

학위논문(박사) -- 서울대학교대학원 : 공과대학 기계항공공학부(멀티스케일 기계설계전공), 2022. 8. 이정훈.A variety of stretchable strain sensors have been developed for various applications in diverse fields. Based on their core function represented by the conversion of mechanical deformations into electrical signals, numerous fabrication techniques combined with miscellaneous combinations of materials have been suggested and applied for different purposes. Recently, a series of innovations in agriculture in the name of smart farming have been achieved to meet increasing needs for high-quality crops. As part of the collection of essential information for plant growth, it becomes indispensable to measure axial dimensions of trees or fruits. Although certain kinds of apparatuses were reported to show precise size measurement for the trunk of a tree or the diameter of a fruit, improved instruments categorized as dendrometers had been awaited to overcome current limitations such as bulkiness, complexities in working mechanisms, dependence on users, expensiveness, etc. In this study, I proposed a liquid polymer/metallic salt-based stretchable strain sensor. Compared to conventional strain sensors often used as wearable sensors for instant motion detection, the newly developed sensor included conductive liquid made of silver nitrate and polyethylene glycol (PEG). The introduction of this liquid polymer brought high viscosity and chemical stability while the addition of silver nitrate supplied electrolytes in the conductive liquid. The formation of the structure of the stretchable strain sensor was finalized with a mixture of distinct elastomers called polydimethylsiloxane (PDMS) and Ecoflex. After multiple experiments, the optimal mixing ratio (20:80) of these elastomers was found to reach the equilibrium between strain, stress and stickiness, which was essential to the effective monitoring of fruit growth. The performance of the stretchable strain sensor was analyzed, showing highly linear relationships between strain and resistance as well as good repeatability. The fruit monitoring test demonstrated the stability of the stretchable strain sensor at least for two weeks with increasing ratios of 1.7 to 3.9 kΩ/mm. As an alternative instrument for fruit growth measurement, this tunable band-shaped sensor would be able to show industrial potential in terms of simple fabrication, reliable measurement, and long-term evaluation. The use of the composite of silver nitrate and PEG also led to the development of an antenna-shaped biomimetic tactile sensor. The conductive liquid was selected to imitate the aqueous cavity of the hair of insects while wires connecting the conductive liquid and the measurement system of the sensor were installed to realize the tubular body, whose role is to transmit mechanical deformation-driven electric signals to the central nervous system of insects. This bio-inspired tactile sensor was designed to compensate the malfunction of visual sensors exposed to dark areas. The performance test of the tactile sensor through wall scanning experiments proved its ability to detect various geographical features expressed on three dimensional (3D)-printed walls with repeatable and linear relationships between bending angle and resistance. The working mechanism established with the conductive liquid and wires revealed that the resistance of the tactile sensor would be decided by the positioning of wires in the composite of silver nitrate and PEG. When the distance between a wall and the tactile sensor was fixed during the scanning, the bio-inspired tactile sensor could offer reliable resistance data enough to reconstruct surrounding geographical features with high accuracy. This antenna-shaped biomimetic tactile sensor was characterized by the use of novel materials compared to existing tactile sensors, the adoption of a simple fabrication process, the investigation of an alternative working mechanism, the establishment of high repeatability based on bending angle and resistance, and the presentation of a perspective of being studied further for 3D image reconstruction.다양한 분야에서 사용 목적에 따라 수많은 종류의 변형 센서들이 개발되어 왔다. 기계적 변형을 전기적 신호로 바꾸어 주는 본연의 기능에 기초하여 여러 물질들을 활용한 변형 센서 제작 방법이 소개되었다. 최근 스마트 농장이라는 이름으로 농업에 일련의 혁신이 일어나고 있는 가운데 고품질 작물을 얻기 위한 수요가 꾸준히 증가하고 있다. 식물 생장에 필수적인 정보를 획득하기 위한 방편 중 하나로 나무 줄기나 과실 크기를 측정하는 것이 중요해지고 있다. 이러한 목적의 도구들이 이미 보고된 바 있으나 기존 제품에 비하여 발전된 형태의 측수기(測樹器)에 대한 기대에 못 미친 것이 사실이다. 즉, 크기를 줄이고 작동 원리를 단순화하며 사용자 숙련도에 대한 의존성을 줄이는 동시에 가격적으로 경쟁력 있는 제품에 대한 필요성이 제기되어 왔다. 본 연구에서 액상 중합체 및 금속염(金屬鹽) 기반 신축성 변형 센서를 제안하였다. 신체 등의 즉각적인 움직임을 감지하기 위한 착용형(着用型) 센서로 쓰이는 종래의 변형 센서들에 비하여, 새로이 개발된 센서에는 전도성(傳導性) 액체로 질산은과 폴리에틸렌글리콜(PEG)의 혼합물이 적용되었다. 이 액상 중합체를 통하여 높은 점도와 화학적 안정성을 도모하면서 질산은을 이용하여 전해질을 전도성 액체에 공급할 수 있다. 신축성 변형 센서의 골격은 탄성 중합체인 폴리디메틸실록산(PDMS)과 에코플렉스(Ecoflex)로 만들었다. 반복적인 실험을 거쳐 최적의 배합 비율(PDMS 20:80 Ecoflex)을 찾음으로써 효과적인 과실 생장 관찰에 필요한 변수인 변형, 응력(應力), 마찰의 적절한 타협점을 알아낼 수 있었다. 신축성 변형 센서의 성능 분석 결과, 변형과 저항 사이에 높은 선형성을 보이는 것이 확인되었다. 과실 생장 관찰을 통하여 2주 동안 약 1.7 ~ 3.9 kΩ/mm의 범위 내에서 신축성 변형 센서가 안정적으로 작동하는 것을 증명하였다. 과실 생장 측정을 위한 대안적인 도구로서, 이번에 개발된 조정 가능한 밴드형 센서는 제작 공정이 단순하고 신뢰성 있는 측정이 가능하며 장기간 평가에 적합하다는 면에서 우수한 잠재성을 가지고 있다고 말할 수 있을 것이다. 질산은과 PEG의 조합을 이용하여 더듬이 형태의 생체모방 촉각 센서도 선보였다. 이 전도성 액체는 곤충의 더듬이 내에서 림프로 이루어진 부분(lymph space)을, 금속 전선은 관 모양의 구조물(tubular body)을 구현하는 데 활용되었다. 특히 이 관 모양의 구조물은 곤충의 촉각에서 기계적 변형을 전기적 신호로 바꾸어 중추 신경계로 전달하는 역할을 한다. 이 생체모방 촉각 센서는 어두운 공간에 노출된 시각 센서의 오작동에서 비롯되는 문제를 보완하기 위하여 고안되었다. 3D 프린터로 제작된 요철이 있는 벽면을 가지고 스캔 실험을 진행한 결과 촉각 센서가 굽히는 각도와 저항 사이에서 반복적이면서 선형적인 관계가 형성된다는 것을 증명하였다. 아울러 금속 전선이 전도성 액체에 놓이는 위치에 따라 촉각 센서의 저항이 결정된다는 사실도 밝혔다. 그리고 촉각 센서와 벽면 사이 거리가 고정되어 있다는 가정하에 스캔 실험을 실시한 결과 벽면의 요철을 높은 정확도로 재구성할 만큼 신뢰성 있는 값을 제공한다는 것을 알 수 있었다. 이 더듬이 형태의 생체모방 촉각 센서는 기존의 촉각 센서와 비교하였을 때 새로운 물질이 적용되었고 상대적으로 단순한 제작 공정을 통하여 제작되었다. 기존 촉각 센서와 다른 작동 원리에 대한 이해를 높이는 동시에 센서가 굽히는 각도와 저항 사이의 높은 반복성을 확립함으로써 후속 연구를 통하여 동일 센서를 활용한 3차원 이미지 재구성에 대한 가능성도 엿볼 수 있었다.Chapter 1. Introduction 1 1.1 Conventional strain sensors 1 1.2 Strain sensors in agricultural engineering 2 1.3 Current tactile sensors 3 1.4 Tactile sensors in military industries 4 1.5 Research objectives and contributions 5 Chapter 2. Fabrication of a stretchable strain sensor 7 2.1 Synthesis of polyethylene glycol (PEG)/silver nitrate composites 7 2.2 Fabrication of a strain sensor with the liquid composites 8 2.3 Preparation of a flexible band for the incorporation of the strain sensor 9 2.4 Encapsulation of the strain sensor into the flexible band 9 Chapter 3. Methods for the stretchable strain sensor 10 3.1 Measurement of strain and resistance 10 3.2 Tensile strength measurement 10 3.3 Ultraviolet-visible (UV-Vis) spectroscopy 11 3.4 Field emission-scanning electron microscopy (FE-SEM) and elemental analysis 11 3.5 Fruit model simulation 11 3.6 Performance test as a dendrometer using real fruits 12 Chapter 4. Analysis of the stretchable strain sensor 13 4.1 Formation of PEG/silver nitrate composites 13 4.2 Sealing process of the strain sensor 15 4.3 Correlation between strain and resistance 17 4.4 Comparison between theoretical calculations and experiments 21 4.5 Optimization of elasticity for large strain, low stress and high sensitivity 24 4.6 Characterization of silver nanoparticles in the PEG/silver nitrate composites 27 4.7 Reliability test of the strain sensor through fruit model simulation 30 4.8 Continuous monitoring of real fruits with the strain sensor 32 Chapter 5. Fabrication of a bio-inspired tactile sensor and its methods 35 5.1 Fabrication of a bio-inspired tactile sensor 35 5.2 Compatibility test using PEG, silver nitrate and sodium chloride 36 5.3 Fourier transform infrared (FTIR) spectrum analysis 37 5.4 Measurement of silver particles in the liquid composites 37 5.5 Simulations for the bio-inspired tactile sensor 38 5.6 Wall scanning tests with bio-inspired tactile sensors 38 Chapter 6. Analysis of the bio-inspired tactile sensor 39 6.1 Compatibility test between PEG and silver nitrate 39 6.2 Chemical properties of PEG/silver nitrate composites 41 6.3 Simulations of mechanical and electrical properties 43 6.4 Wall scanning test with the bio-inspired tactile sensor 45 6.5 Wall scanning test with multiple walls and multiple sensors 48 6.6 Wall scanning test with a single wall and multiple sensors 49 6.7 Geographical reconstruction 50 Chapter 7. Discussion 53 7.1 Temperature compensation of the stretchable strain sensor 53 7.2 Portable power supplier for the stretchable strain sensor and the bio-inspired tactile sensor 54 7.3 Future work 55 Chapter 8. Conclusion 57 Bibliography 59 Appendix 67 A. Statistical analysis of the relationship between strain and resistance 67 B. Dendrometer requirements and specifications 68 Abstract in Korean 71박

Bioinspired PDMS-graphene cantilever flow sensors using 3D printing and replica moulding

Flow sensors found in animals often feature soft and slender structures (e.g. fish neuromasts, insect hairs, mammalian stereociliary bundles, etc.) that bend in response to the slightest flow disturbances in their surroundings and heighten the animal's vigilance with respect to prey and/or predators. However, fabrication of bioinspired flow sensors that mimic the material properties (e.g. low elastic modulus) and geometries (e.g. high-aspect ratio structures) of their biological counterparts remains a challenge. In this work, we develop a facile and low-cost method of fabricating high-aspect ratio (HAR) cantilever flow sensors inspired by the mechanotransductory flow sensing principles found in nature. The proposed workflow entails high-resolution 3D printing to fabricate the master mould, replica moulding to create HAR polydimethylsiloxane (PDMS) cantilevers (thickness = 0.5 – 1 mm, width = 3 mm, aspect ratio = 20) with microfluidic channel (150 µm wide × 90 µm deep) imprints, and finally graphene nanoplatelet ink drop-casting into the microfluidic channels to create a piezoresistive strain gauge near the cantilever's fixed end. The piezoresistive flow sensors were tested in controlled airflow (0 – 9 m/s) inside a wind tunnel where they displayed high sensitivities of up to 5.8 kΩ/ms-1, low hysteresis (11% of full-scale deflection), and good repeatability. The sensor output showed a second order dependence on airflow velocity and agreed well with analytical and finite element model predictions. Further, the sensor was also excited inside a water tank using an oscillating dipole where it was able to sense oscillatory flow velocities as low as 16 – 30 µm/s at an excitation frequency of 15 Hz. The methods presented in this work can enable facile and rapid prototyping of flexible HAR structures that can find applications as functional biomimetic flow sensors and/or physical models which can be used to explain biological phenomena

Cupula-Inspired Hyaluronic Acid-Based Hydrogel Encapsulation to Form Biomimetic MEMS Flow Sensors

Blind cavefishes are known to detect objects through hydrodynamic vision enabled by arrays of biological flow sensors called neuromasts. This work demonstrates the development of a MEMS artificial neuromast sensor that features a 3D polymer hair cell that extends into the ambient flow. The hair cell is monolithically fabricated at the center of a 2 µm thick silicon membrane that is photo-patterned with a full-bridge bias circuit. Ambient flow variations exert a drag force on the hair cell, which causes a displacement of the sensing membrane. This in turn leads to the resistance imbalance in the bridge circuit generating a voltage output. Inspired by the biological neuromast, a biomimetic synthetic hydrogel cupula is incorporated on the hair cell. The morphology, swelling behavior, porosity and mechanical properties of the hyaluronic acid hydrogel are characterized through rheology and nanoindentation techniques. The sensitivity enhancement in the sensor output due to the material and mechanical contributions of the micro-porous hydrogel cupula is investigated through experiments.Singapore. National Research Foundation (Campus for Research Excellence and Technological Enterprise programme

A Deep-Learning Model for Underwater Position Sensing of a Wake's Source Using Artificial Seal Whiskers

Various marine animals possess the ability to track their preys and navigate dark aquatic environments using hydrodynamic sensing of the surrounding flow. In the present study, a deep-learning model is applied to a biomimetic sensor for underwater position detection of a wake-generating body. The sensor is composed of a bundle of spatially-distributed optical fibers that act as artificial seal-like whiskers and interact with the body's wake in the form of time-variant (bending) deflections. Supervised learning is employed to relate the vibrations of the artificial whiskers to the position of an upstream cylinder. The labeled training data are prepared based on the processing and reduction of the recorded bending responses of the artificial whiskers while the cylinder is placed at various locations. An iterative training algorithm is performed on two neural-network models while using the 10-fold cross-validation technique. The models are able to predict the coordinates of the cylinder in the two-dimensional (2D) space with a high degree of accuracy. The current implementation of the sensor can passively sense the wake generated by the cylinder at Re ≃ 6000 and estimate its position with an average error smaller than the characteristic diameter D of the cylinder and for inter-distances (in the water tunnel) up to 25-times D