Highly elastic conductive polymeric MEMS

The Role of Interdigitated Electrodes in Printed and Flexible Electronics

Flexible electronics, also referred to as printable electronics, represent an interesting technology for implementing electronic circuits via depositing electronic devices onto flexible substrates, boosting their possible applications. Among all flexible electronics, interdigitated electrodes (IDEs) are currently being used for different sensor applications since they offer significant benefits beyond their functionality as capacitors, like the generation of high output voltage, fewer fabrication steps, convenience of application of sensitive coatings, material imaging capability and a potential of spectroscopy measurements via electrical excitation frequency variation. This review examines the role of IDEs in printed and flexible electronics since they are progressively being incorporated into a myriad of applications, envisaging that the growth pattern will continue in the next generations of flexible circuits to come.

Internal Rotation Measurement of the Knee with Polymer-Based Capacitive Strain Gauges versus Mechanical Rotation Measurement Taking Gender Differences into Account: A Comparative Analysis

With the conventional mechanical rotation measurement of joints, only static measurements are possible with the patient at rest. In the future, it would be interesting to carry out dynamic rotation measurements, for example, when walking or participating in sports. Therefore, a measurement method with an elastic polymer-based capacitive measuring system was developed and validated. In our system, the measurement setup was comprised of a capacitive strain gauge made from a polymer, which was connected to a flexible printed circuit board. The electronics integrated into the printed circuit board allowed data acquisition and transmission. As the sensor strip was elongated, it caused a change in the spacing between the strain gauge's electrodes, leading to a modification in capacitance. Consequently, this alteration in capacitance enabled the measurement of strain. The measurement system was affixed to the knee by adhering the sensor to the skin in alignment with the anterolateral ligament (ALL), allowing the lower part of the sensor (made of silicone) and the circuit board to be in direct contact with the knee's surface. It is important to note that the sensor should be attached without any prior stretching. To validate the system, an in vivo test was conducted on 10 healthy volunteers. The dorsiflexion of the ankle was set at 2 Nm using a torque meter to eliminate any rotational laxity in the ankle. A strain gauge sensor was affixed to the Gerdii's tubercle along the course of the anterolateral ligament, just beneath the lateral epicondyle of the thigh. In three successive measurements, the internal rotation of the foot and, consequently, the lower leg was quantified with a 2 Nm torque. The alteration in the stretch mark's length was then compared to the measured internal rotation angle using the static measuring device. A statistically significant difference between genders emerged in the internal rotation range of the knee (p = 0.003), with female participants displaying a greater range of rotation compared to their male counterparts. The polymer-based capacitive strain gauge exhibited consistent linearity across all measurements, remaining within the sensor's initial 20% strain range. The comparison between length change and the knee's internal rotation angle revealed a positive correlation (r = 1, p < 0.01). The current study shows that elastic polymer-based capacitive strain gauges are a reliable instrument for the internal rotation measurement of the knee. This will allow dynamic measurements in the future under many different settings. In addition, significant gender differences in the internal rotation angle were seen.

Investigation into the effects of foaming variables on the cellular structure and expansion ratio of foamed TPU using response surface methodology

Thermoplastic polyurethane elastomer (TPU) foams were prepared using the high-pressure autoclave with supercritical fluid carbon dioxide (SC-CO2). The effects of foaming variables (i.e. saturation temperature, saturation pressure, and depressurization rate) on cellular structure and expansion ratio were investigated. The model between expansion ratio and foaming variables was constructed using the Box-Behnken design (BBD) of response surface methodology (RSM), and analysis of variance (ANOVA) was conducted to evaluate the validity and significance of the model. Finally, the interactive effects of foaming variables on the expansion ratio were investigated, and the expansion ratios of maximum and center point from numerical model were verified by experiment. The result showed higher saturation pressure and depressurization rate resulted in the more uniform cellular structure and higher cell density, however the higher saturation temperature resulted in the bigger cell and nonuniform structure. The ranges of average cell diameter and cell density were 15.26–45.4 μm and 0.32 × 108 to 6.24 × 108 cells/cm3, respectively. The model obtained using BBD of RSM was valid to predict the expansion ratio in the design window. The saturation temperature was the most important factor influencing the expansion ratio. With the increase of saturation temperature, the expansion ratio always increases in the design window. The maximum expansion ratio from numerical optimization was 4.91, which was located at saturation temperature 190°C, saturation pressure 12.51 MPa, and depressurization rate 5 MPa/s, and the corresponding experiment value was 4.56. The error between them was 7.13%.

Batch Fabrication of a Polydimethylsiloxane Based Stretchable Capacitive Strain Gauge Sensor for Orthopedics

Polymer-based capacitive strain gauges are a novel and promising concept for measuring large displacements and strains in various applications. These novel sensors allow for high strain, well above the maximum values achieved with state-of-the-art strain gauges (Typ. 1%). In recent years, a lot of interest in this technology has existed in orthopedics, where the sensors have been used to measure knee laxity caused by a tear of the anterior cruciate ligament (ACL), and for other ligament injuries. The validation of this technology in the field has a very low level of maturity, as no fast, reproducible, and reliable manufacturing process which allows mass production of sensors with low cost exists. For this reason, in this paper, a new approach for the fabrication of polymer-based capacitive strain gauges is proposed, using polydimethylsiloxane (PDMS) as base material. It allows (1) the fast manufacturing of sensor batches with reproducible geometry, (2) includes a fabrication step for embedding rigid electrical contacts on the sensors, and (3) is designed to produce sensor batches in which the size, the number, and the position of the sensors can be adapted to the patient’s anatomy. In the paper, the process repeatability and the robustness of the design are successfully proven. After 1000 large-strain elongation cycles, in the form of accelerated testing caused much higher strains than in the above-mentioned clinical scenario, the sensor’s electrical contacts remained in place and the functionalities were unaltered. Moreover, the prototype of a patient customizable patch, embedding multiple sensors, was produced.

A MEMS-Based Drug Delivery Device With Integrated Microneedle Array-Design and Simulation

One of the most effective treatments for type 1 and 2 diabetes is the administration of Insulin. Single needle mechanical insulin pumps are heavy and painful. Microneedle-based MEMS drug delivery devices can be an excellent solution for insulin dosing. The stackable structure provides minimum dimensions and the final product can be in the form of a patch that can be applied to any flat area of human skin. The use of microneedle array provides a safe, painless, and robust injection application. The design of positive volumetric insulin pump is a Multiphysics problem where the volumetric changes of the main pump chamber and the pumped fluid are directly coupled. We use a Multiphysics simulation system to investigate the performance of a MEMS-based insulin micropump with a piezoelectric actuator pumping a viscous Newtonian fluid. The model captures the accumulated out-flow, the netflow, or flow fluctuations based on deflection of piezoelectric diaphragm actuator. Different input voltages and different excitation frequencies cause movement of piezoelectric actuator, which moves the diaphragm disk in positive-negative directions thereby inducing discharge pressures at the microneedle array. In this study, we address various aspects of design and simulation of a MEMS-based piezoelectric insulin micropump including polydimethylsiloxane microvalves and microneedle array. We investigate the micropump performance at human skin interfacial pressure to match minimum to maximum delivery targets/requirements for total range of diabetic patient's expected operating parameters. comsolmultiphysics is used for this study.

Production of elastomer-based highly conductive hybrid nanocomposites and treatment with sulfuric acid

As an elastomer, poly(dimethylsiloxane) (PDMS) is used in various applications such as wearable technology and sealants, and is especially preferred in microelectromechanical device production due to its advantage in fabrication of microstructures. However, some of its applications such as sensor-based or electrode-based are limited due to its insulator aspect. Various conductive nanomaterials such as carbon nanotubes (CNTs), graphene, graphite, carbon black, and silver nanoparticles were incorporated into the PDMS matrix for the production of conductive nanocomposites. In this study, we produced highly conductive PDMS nanocomposites by addition of multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) in a three-dimensional network. Due to the synergistic effect between CNTs and GNPs inside a polymeric matrix, we expected to obtain PDMS nanocomposites more conductive than nanocomposites with only CNTs. Additionally, we investigated the effect of sulfuric acid treatment on the electrical conductivity and surface composition of prepared PDMS/MWCNT/GNP nanocomposites. Results indicated that the electrical conductivity in sulfuric acid-treated samples was significantly higher than in untreated samples. Levels of conductivity in the range of 270.7–1074.8 S/m were achieved; the higher ones were the samples treated with sulfuric acid solution.

Advancements in Microfabricated Gas Sensors and Microanalytical Tools for the Sensitive and Selective Detection of Odors

In recent years, advancements in micromachining techniques and nanomaterials have enabled the fabrication of highly sensitive devices for the detection of odorous species. Recent efforts done in the miniaturization of gas sensors have contributed to obtain increasingly compact and portable devices. Besides, the implementation of new nanomaterials in the active layer of these devices is helping to optimize their performance and increase their sensitivity close to humans' olfactory system. Nonetheless, a common concern of general-purpose gas sensors is their lack of selectivity towards multiple analytes. In recent years, advancements in microfabrication techniques and microfluidics have contributed to create new microanalytical tools, which represent a very good alternative to conventional analytical devices and sensor-array systems for the selective detection of odors. Hence, this paper presents a general overview of the recent advancements in microfabricated gas sensors and microanalytical devices for the sensitive and selective detection of volatile organic compounds (VOCs). The working principle of these devices, design requirements, implementation techniques, and the key parameters to optimize their performance are evaluated in this paper. The authors of this work intend to show the potential of combining both solutions in the creation of highly compact, low-cost, and easy-to-deploy platforms for odor monitoring.

Vascular Pressure-Flow Measurement Using CB-PDMS Flexible Strain Sensor

Regular monitoring of blood flow and pressure in vascular reconstructions or grafts would provide early warning of graft failure and improve salvage procedures. Based on biocompatible materials, we have developed a new type of thin, flexible pulsation sensor (FPS) which is wrapped around a graft to monitor blood pressure and flow. The FPS uses carbon black (CB) nanoparticles dispersed in polydimethylsiloxane (PDMS) as a piezoresistive sensor layer, which was encapsulated within structural PDMS layers and connected to stainless steel interconnect leads. Because the FPS is more flexible than natural arteries, veins, and synthetic vascular grafts, it can be wrapped around target conduits at the time of surgery and remain implanted for long-term monitoring. In this study, we analyze strain transduction from a blood vessel and characterize the electrical and mechanical response of CB-PDMS from 0-50% strain. An optimum concentration of 14% CB-PDMS was used to fabricate 300-μm thick FPS devices with elastic modulus under 500 kPa, strain range of over 50%, and gauge factor greater than 5. Sensors were tested in vitro on vascular grafts with flows of 0-1,100 mL/min. In vitro testing showed linear output to pulsatile flows and pressures. Cyclic testing demonstrated robust operation over hundreds of cardiac cycles, with ±2.6 mmHg variation in pressure readout. CB-PDMS composite material showed excellent potential in biologic strain sensing applications where a flexible sensor with large maximum strain range is needed.

Conductive silicone elastomers electrodes processable by screen printing

Conductive inks consisting of graphene and carbon black conductive fillers into a polydimethylsiloxane (PDMS) matrix, which can be processed into thin films by screen printing are developed. The influence of filler composition and content on mechanical and electrical properties of the conductive composites is investigated. The best composites were evaluated as electrode material for dielectric elastomer actuators and for piezoelectric sensors. With increasing filler content, the electrical properties of the resulting composites of graphite nanoplates (GNPs) or a binary mixture of GNPs and carbon black (CB) with PDMS (Mw = 139 kg/mol) are enhanced. Hence, PDMS composites filled with GNPs (42 wt.%) or a binary mixture of GNPs/CB (300/150 ratio, 30 wt.% of total filler loading) exhibited constant contact resistance values of 0.5 and 5 Ω determined in life-cycle test, respectively, thus rendering them suitable as electrode materials for piezosensors. On the other hand, dielectric elastomer actuators require more flexible electrode materials, which could be tuned by varying the polymer molecular weight and by reducing the filler content. Therefore, a composite consisting of PDMS (Mw = 692 kg/mol) and a binary filler mixture of GNPs/CB (150/75 ratio, 18 wt.% of total filler loading) was used for producing the electrodes of dielectric elastomer transducers (DETs). The produced DETs with different electrode thicknesses were characterized in terms of their performance. The negligible hysteresis of the electrode materials is favorable for sensor and actuator applications.

Monolithic 3D printing of embeddable and highly stretchable strain sensors using conductive ionogels

Medical training simulations that utilize 3D-printed, patient-specific tissue models improve practitioner and patient understanding of individualized procedures and capacitate pre-operative, patient-specific rehearsals. The impact of these novel constructs in medical training and pre-procedure rehearsals has been limited, however, by the lack of effectively embedded sensors that detect the location, direction, and amplitude of strains applied by the practitioner on the simulated structures. The monolithic fabrication of strain sensors embedded into lifelike tissue models with customizable orientation and placement could address this limitation. The demonstration of 3D printing of an ionogel as a stretchable, piezoresistive strain sensor embedded in an elastomer is presented as a proof-of-concept of this integrated fabrication for the first time. The significant hysteresis and drift inherent to solid-phase piezoresistive composites and the dimensional instability of low-hysteresis piezoresistive liquids inspired the adoption of a 3D-printable piezoresistive ionogel composed of reduced graphene oxide and an ionic liquid. The shear-thinning rheology of the ionogel obviates the need to fabricate additional structures that define or contain the geometry of the sensing channel. Sensors are printed on and subsequently encapsulated in polydimethylsiloxane (PDMS), a thermoset elastomer commonly used for analog tissue models, to demonstrate seamless fabrication. Strain sensors demonstrate geometry- and strain-dependent gauge factors of 0.54-2.41, a high dynamic strain range of 350% that surpasses the failure strain of most dermal and viscus tissue, low hysteresis (<3.5% degree of hysteresis up to 300% strain) and baseline drift, a single-value response, and excellent fatigue stability (5000 stretching cycles). In addition, we fabricate sensors with stencil-printed silver/PDMS electrodes in place of wires to highlight the potential of seamless integration with printed electrodes. The compositional tunability of ionic liquid/graphene-based composites and the shear-thinning rheology of this class of conductive gels endows an expansive combination of customized sensor geometry and performance that can be tailored to patient-specific, high-fidelity, monolithically fabricated tissue models.

Drug delivery across length scales

Over the last century, there has been a dramatic change in the nature of therapeutic, biologically active molecules available to treat disease. Therapies have evolved from extracted natural products towards rationally designed biomolecules, including small molecules, engineered proteins and nucleic acids. The use of potent drugs which target specific organs, cells or biochemical pathways, necessitates new tools which can enable controlled delivery and dosing of these therapeutics to their biological targets. Here, we review the miniaturisation of drug delivery systems from the macro to nano-scale, focussing on controlled dosing and controlled targeting as two key parameters in drug delivery device design. We describe how the miniaturisation of these devices enables the move from repeated, systemic dosing, to on-demand, targeted delivery of therapeutic drugs and highlight areas of focus for the future.

Polydimethylsiloxane pressure sensors for force analysis in tension band wiring of the olecranon

BACKGROUND

Several different surgical techniques are used in the treatment of olecranon fractures. Tension band wiring is one of the most preferred options by surgeons worldwide. The concept of this technique is to transform a tensile force into a compression force that adjoins two surfaces of a fractured bone. Currently, little is known about the resulting compression force within a fracture.

OBJECTIVE

Sensor devices are needed that directly transduce the compression force into a measurement quality. This allows the comparison of different surgical techniques. Ideally the sensor devices ought to be placed in the gap between the fractured segments.

METHODS

The design, development and characterization of miniaturized pressure sensors fabricated entirely from polydimethylsiloxane (PDMS) for a placement within a fracture is presented. The pressure sensors presented in this work are tested, calibrated and used in an experimental in vitro study.

RESULTS

The pressure sensors are highly sensitive with an accuracy of approximately 3 kPa. A flexible fabrication process for various possible applications is described. The first in vitro study shows that using a single-twist or double-twist technique in tension band wiring of the olecranon has no significant effect on the resulting compression forces.

CONCLUSIONS

The in vitro study shows the feasibility of the proposed measurement technique and the results of a first exemplary study.

Highly stretchable conductive polypyrrole film on a three dimensional porous polydimethylsiloxane surface fabricated by a simple soft lithography process

We create an elastic porous polydimethylsiloxane highly stretchable conductive substrate. The surface is fabricated by a simple soft lithography process that replicates the 3D corrugated porous microstructures from a low-cost commercially available abrasive paper.

Length Changes of the Anterolateral Ligament During Passive Knee Motion: A Human Cadaveric Study

BACKGROUND

Persistent rotatory instability after anterior cruciate ligament (ACL) reconstruction may be a result of unaddressed insufficiency of the anterolateral structures. Recent publications about the anatomy of the anterolateral ligament (ALL) have led to a renewed interest in lateral extra-articular procedures, and several authors have proposed ALL reconstruction to supplement intra-articular ACL reconstruction. However, only limited knowledge about the biomechanical characteristics of the ALL exists.

PURPOSE/HYPOTHESIS

The purpose of this study was to analyze length changes of the ALL during passive knee motion. The study hypothesis was that the ALL lengthens with knee flexion and internal tibial rotation.

STUDY DESIGN

Controlled laboratory study.

METHODS

The ALL of 6 cadaveric knees was dissected. Specimens were mounted in a specifically designed test rig that allowed unconstrained passive flexion/extension movement between 0° and 90° as well as external/internal tibial rotation of 25° at various flexion angles. Highly elastic, capacitive polydimethylsiloxane strain gauges were attached to the insertion sites of the ALL. Length changes were recorded continuously at flexion angles between 0° and 90° and during internal/external tibial rotation at 0°, 15°, 30°, 45°, 60°, 75°, and 90°. All measurements were calculated as the relative length change (%) of the ALL compared with 0° of flexion and neutral rotation.

RESULTS

The mean relative length of the ALL significantly increased with increasing knee flexion (P < .001), with an estimated mean length change of +0.15% per degree. Both internal and external tibial rotation were independent determinants for length change; internal rotation significantly increased the length of the ALL (P < .001), whereas external rotation significantly decreased its length (P < .001). The mean length change with internal rotation increased with knee flexion, with a significantly greater length change at 90° compared with 0° (P = .048), 15° (P = .033), and 30° (P = .015). The maximum mean length change was +33.77% ± 9.62%, which was observed at 25° of internal rotation and 90° of flexion.

CONCLUSION

The ALL is a nonisometric structure that tensions with knee flexion and internal tibial rotation. Length changes with internal rotation were greater at higher flexion angles, with the greatest length change of the ALL observed at 90° of flexion.

CLINICAL RELEVANCE

The ALL can be considered a stabilizer against internal tibial rotation, especially at deep flexion angles. With regard to ALL reconstruction procedures, tensioning and fixation of the graft should be performed near 90° of flexion because graft tensioning near extension may cause excessive ligament strain with increasing knee flexion.

Novel approach to dynamic knee laxity measurement using capacitive strain gauges

PURPOSE

Knee joint laxities are observed in patients after severe trauma to the joint, resulting in ligament tears. Specifically, injuries to the anterior cruciate ligament may cause a significant instability. The degree of these laxities is essential in diagnostics and may affect which treatment option is suggested.

METHODS

Polydimethylsiloxane (PDMS) strain gauges are proposed as a non-invasive, highly accurate and easy-to-use measurement method to quantify anterolateral and rotational laxities of the knee joint during active and passive motion. In this work, proof-of-concept measurements and a prototype of the proposed device are displayed. The measurements were taken using a knee test rig, which has specifically been designed for this purpose. This apparatus allows the simulation of isolated knee joint instabilities with a motor-controlled model of a human knee.

RESULTS

The absolute sensitivity [Formula: see text] of an exemplary sensor was determined to be 2.038 [Formula: see text]; the relative sensitivity [Formula: see text] was 1.121 [Formula: see text]. Optimal positions of sensors to capture bone-to-bone displacement as projected displacement on the skin were identified.

CONCLUSION

PDMS strain gauges are capable of measuring bone-to-bone displacements on the skin. We present an experimental in vitro study using an artificial knee test rig to simulate knee joint laxities and display the feasibility of our novel measurement approach.