Full elastic constitutive relation of non-isotropic aligned-CNT/PDMS flexible nanocomposites

Solvent-Free and Cost-Efficient Fabrication of a High-Performance Nanocomposite Sensor for Recording of Electrophysiological Signals

Carbon nanotube (CNT)-based nanocomposites have found applications in making sensors for various types of physiological sensing. However, the sensors' fabrication process is usually complex, multistep, and requires longtime mixing and hazardous solvents that can be harmful to the environment. Here, we report a flexible dry silver (Ag)/CNT/polydimethylsiloxane (PDMS) nanocomposite-based sensor made by a solvent-free, low-temperature, time-effective, and simple approach for electrophysiological recording. By mechanical compression and thermal treatment of Ag/CNT, a connected conductive network of the fillers was formed, after which the PDMS was added as a polymer matrix. The CNTs make a continuous network for electrons transport, endowing the nanocomposite with high electrical conductivity, mechanical strength, and durability. This process is solvent-free and does not require a high temperature or complex mixing procedure. The sensor shows high flexibility and good conductivity. High-quality electroencephalography (EEG) and electrooculography (EOG) were performed using fabricated dry sensors. Our results show that the Ag/CNT/PDMS sensor has comparable skin-sensor interface impedance with commercial Ag/AgCl-coated dry electrodes, better performance for noninvasive electrophysiological signal recording, and a higher signal-to-noise ratio (SNR) even after 8 months of storage. The SNR of electrophysiological signal recording was measured to be 26.83 dB for our developed sensors versus 25.23 dB for commercial Ag/AgCl-coated dry electrodes. Our process of compress-heating the functional fillers provides a universal approach to fabricate various types of nanocomposites with different nanofillers and desired electrical and mechanical properties.

Enhancement of the mechanical properties in ultra-low weight SWCNT sandwiched PDMS composites using a novel stacked architecture

This study focuses on enhancing the mechanical properties of thin, soft, free-standing films via a layer-by-layer (LBL) fabrication process called LBL-FP. Soft polymer nanocomposite (PNC) thin films, combining polydimethylsiloxane (PDMS) and single-walled carbon nanotubes (SWCNT) at ultra-low loadings using a unique bottom-up LBL-FP, are examined. Two different structures of layered composites, (i) LBL PNCs- Layered composites with alternating layers of PDMS and SWCNT, (ii) Bulk PNCs- Layered composites with SWCNT dispersed in the bulk of PDMS, are comparatively investigated for their structural and mechanical properties. Silane-functionalized SWCNT strengthens the chemical bonding with PDMS, improving adhesion and dispersion. Mechanical analysis using nanoindentation, delamination, and dynamic analysis highlights the advantages of LBL PNCs with alternating layers of PDMS and SWCNT. Notably, LBL PNC (0.5 wt%) exhibits significant improvements, such as 2.6X increased nanoindentation resistance, 3X improved viscoelasticity, and (2-5)X enhanced tensile properties in comparison with neat PDMS. Due to this, LBL PNCs offer potential for soft, lightweight applications like wearables, electromagnetic interference shielding materials, and strain sensors while advancing composite thin film mechanics. The study emphasizes using a stacked architecture to produce PDMS-SWCNT multilayered PNCs with improved mechanics utilizing ultra-low concentrations of SWCNT. This first-of-its-kind stack design facilitates possibilities for lightweight composites utilizing less fillers. The LBL assembly involves the stacking of alternating layers of different materials, each contributing specific properties to enhance the overall strength and toughness of the structure.

Computational Micromechanics Investigation of Percolation and Effective Electro-Mechanical Properties of Carbon Nanotube/Polymer Nanocomposites using Stochastically Generated Realizations: Effects of Orientation and Waviness

The electrical and mechanical properties of carbon nanotube/polymer nanocomposites depend strongly upon several factors such as CNT volume fraction, CNT alignment, CNT dispersion and CNT waviness among others. This work focuses on obtaining estimates and distribution for the effective electrical conductivity, elastic constants and piezoresistive properties as a function of these factors using a stochastic approach with numerous CNT/polymer realizations coupled with parallel computation. Additionally, electrical percolation volume fraction and percolation transitional behavior is also studied. The effective estimates and percolation values were found to be in good agreement with experimental works in the literature. It was found that with increasing CNT volume fraction, the mechanical properties improved. However, due to the interaction of CNTs with one another through electrical tunneling, the conductivity and piezoresistivity properties evolved in a more complex manner. While the degree of alignment played a strong role in the effective properties making them anisotropic, the effect of waviness was found to be insubstantial.

Tip-scan high-speed atomic force microscopy with a uniaxial substrate stretching device for studying dynamics of biomolecules under mechanical stress

High-speed atomic force microscopy (HS-AFM) is a powerful tool for studying the dynamics of biomolecules in vitro because of its high temporal and spatial resolution. However, multi-functionalization, such as combination with complementary measurement methods, environment control, and large-scale mechanical manipulation of samples, is still a complex endeavor due to the inherent design and the compact sample scanning stage. Emerging tip-scan HS-AFM overcame this design hindrance and opened a door for additional functionalities. In this study, we designed a motor-driven stretching device to manipulate elastic substrates for HS-AFM imaging of biomolecules under controllable mechanical stimulation. To demonstrate the applicability of the substrate stretching device, we observed a microtubule buckling by straining the substrate and actin filaments linked by α-actinin on a curved surface. In addition, a BAR domain protein BIN1 that senses substrate curvature was observed while dynamically controlling the surface curvature. Our results clearly prove that large-scale mechanical manipulation can be coupled with nanometer-scale imaging to observe biophysical effects otherwise obscured.

Large-area flexible MWCNT/PDMS pressure sensor for ergonomic design with aid of deep learning learning

The achievement of well-performing pressure sensors with low pressure detection, high sensitivity, large-scale integration, and effective analysis of the subsequent data remains a major challenge in the development of flexible piezoresistive sensors. In this study, a simple and extendable sensor preparation strategy was proposed to fabricate flexible sensors on the basis of multiwalled carbon nanotube/polydimethylsiloxane (MWCNT/PDMS) composites. A dispersant of tetrahydrofuran (THF) was added to solve the agglomeration of MWCNTs in PDMS, and the resistance of the obtained MWCNT/PDMS conductive unit with 7.5 wt.% MWCNTs were as low as 180 Ω/hemisphere. Sensitivity (0.004 kPa^-1), excellent response stability, fast response time (36 ms), and excellent electromechanical properties were demonstrated within the pressure range from 0 to 100 kPa. A large-area flexible sensor with 8 × 10 pixels was successfully adopted to detect the pressure distribution on the human back and to verify its applicability. Combining the sensor array with deep learning, inclination of human sitting was easily recognized with high accuracy, indicating that the combined technology can be used to guide ergonomic design.

Fabrication of Magnetically Inorganic/Organic Superhydrophobic Fabrics and Their Applications

In order to solve the problem caused by oil spills and organic solvent contamination, novel magnetically inorganic/organic superhydrophobic fabrics are fabricated via a facile method. Cotton fabrics are immersed in a mixture of functionalized Co_0.2Mg_0.8Fe₂O₄ (FCMFO) nanoparticles, vinyl-terminated polydimethylsiloxane (VPDMS), trimethylolpropane triacrylate, and 2-hydroxy-2-methylpropiophenone before UV irradiation for 100 s to obtain the multifunctional superhydrophobic fabrics with magnetic property. The coated fabrics show excellent superhydrophobicity, and the water contact angle is 157.1° when the mass ratio of FCMFO nanoparticles to VPDMS is 0.3. These superhydrophobic fabrics have high oil/water separation efficiency (98.7% for dichloromethane/water) and high oil flux (71,506 L·m^-2·h^-1 for dichloromethane/water). Even after 20 separation cycles, oil/water separation efficiency and oil flux maintain 96.4% and 64,012 L·m^-2·h^-1, respectively. Furthermore, the magnetic property of these superhydrophobic fabrics could be used in the separation of oil from water. Moreover, the superhydrophobic fabrics possess exceptional self-cleaning performance, mechanical durability, chemical stability, and flame retardancy. These multifunctional superhydrophobic fabrics are potential for wide applications.

One-Rupee Ultrasensitive Wearable Flexible Low-Pressure Sensor

A capacitive flexible pressure sensor with polydimethylsiloxane (PDMS) as an elastomeric dielectric layer sandwiched between two flexible conducting electrodes is developed. The porosity/flexibility of PDMS has been increased by altering its microstructure by incorporating a thin layer of a scrubber into the dielectric layer to improve the pressure sensitivity. The fabricated sensor with a porous PDMS-scrubber composite (microstructured/porous PDMS layer) shows 0.058-25.84% relative capacitance change with varying static pressure from 4.4 Pa to 216 kPa. The sensor device with a porous PDMS layer showed a significantly high sensitivity (%) of 0.0083 Pa-1 in a low-pressure range (less than 0.022 kPa) and has a fast response, long-life, and ultralow pressure detection limit. The sensitivity associated with the device was found to vary with the effective area of the device. The pressure sensitivity associated with devices having an effective area of 324 mm2 was 10 times more than that of the sensor having an effective area of 36 mm2. Due to the high sensitivity of the sensor in a wide pressure range, low manufacturing cost, and simple and convenient way of fabrication, this flexible pressure sensor shows potential for pressure and wearable applications.

Recent Progress in 3D Printed Mold-Based Sensors

The paper presents a review of some of the significant research done on 3D printed mold-based sensors performed in recent times. The utilization of the master molds to fabricate the different parts of the sensing prototypes have been followed for quite some time due to certain distinct advantages. Some of them are easy template preparation, easy customization of the developed products, quick fabrication, and minimized electronic waste. The paper explains the different kinds of sensors and actuators that have been developed using this technique, based on their varied structural dimensions, processed raw materials, designing, and product testing. These differences in the attributes were based on their individualistic application. Furthermore, some of the challenges related to the existing sensors and their possible respective solutions have also been mentioned in the paper. Finally, a market survey has been provided, stating the estimated increase in the annual growth of 3D printed sensors. It also states the type of 3D printing that has been preferred over the years, along with the range of sensors, and their related applications.

Shape programming of polymeric based electrothermal actuator (ETA) via artificially induced stress relaxation

Electrothermal actuators (ETAs) are a new generation of active materials that can produce different motions from thermal expansion induced by Joule heating. It is well-known that the degree of deformation is determined by the amount of Joule heating and the coefficient of thermal expansion (CTE) of the material. Previous works on polymeric ETAs are strongly focused on increasing electrical conductivity by utilizing super-aligned carbon nanotube (CNT) sheets. This allows greater deformation for the same drive voltage. Despite these accomplishments with low-voltage actuation, many of the ETAs were constructed to have basic geometries such as a simple cantilever shape. In this paper, it was discovered that shape of polymeric ETA can be programmed into a desired configuration by applying an induced stress relaxation mechanism and post secondary curing. By utilizing such effects, an ETA can be programmed into a curled resting state which allows the actuator to achieve an active bending angle over 540°, a value far greater than any previous studies. This shape programming feature also allows for tailoring the actuator configuration to a specific application. This is demonstrated here by fabricating a small crawling soft robot similar to mimic an inchworm motion.

Low-cost sensor-integrated 3D-printed personalized prosthetic hands for children with amniotic band syndrome: A case study in sensing pressure distribution on an anatomical human-machine interface (AHMI) using 3D-printed conformal electrode arrays

Interfacing anatomically conformal electronic components, such as sensors, with biology is central to the creation of next-generation wearable systems for health care and human augmentation applications. Thus, there is a need to establish computer-aided design and manufacturing methods for producing personalized anatomically conformal systems, such as wearable devices and human-machine interfaces (HMIs). Here, we show that a three-dimensional (3D) scanning and 3D printing process enabled the design and fabrication of a sensor-integrated anatomical human-machine interface (AHMI) in the form of personalized prosthetic hands that contain anatomically conformal electrode arrays for children affected by amniotic band syndrome, a common birth defect. A methodology for identifying optimal scanning parameters was identified based on local and global metrics of registered point cloud data quality. This method identified an optimal rotational angle step size between adjacent 3D scans. The sensitivity of the optimization process to variations in organic shape (i.e., geometry) was examined by testing other anatomical structures, including a foot, an ear, and a porcine kidney. We found that personalization of the prosthetic interface increased the tissue-prosthesis contact area by 408% relative to the non-personalized devices. Conformal 3D printing of carbon nanotube-based polymer inks across the personalized AHMI facilitated the integration of electronic components, specifically, conformal sensor arrays for measuring the pressure distribution across the AHMI (i.e., the tissue-prosthesis interface). We found that the pressure across the AHMI exhibited a non-uniform distribution and became redistributed upon activation of the prosthetic hand's grasping action. Overall, this work shows that the integration of 3D scanning and 3D printing processes offers the ability to design and fabricate wearable systems that contain sensor-integrated AHMIs.

Robust nanocoatings based on ionic silicones

Two oppositely charged water-soluble oligosiloxanes with the same main chain were synthesized and used for the formation of multilayer nanocoatings. In spite of low molecular weight of the components, due to entropic reasons, linearly growing and robust films with a hydrophilic surface were formed for the first time. The multilayer films were found to be resistant to high temperature water treatment undergoing only reversible swelling and no surface recovery was observed after prolonged exposure to air indicating permanent water wettability of these silicone-based coatings. High flexibility of the silicone chains resulted in low glass transition temperature (ca. 27 °C) of both dry polyplexes and films as determined using calorimetry and spectroscopic ellipsometry, respectively. Moreover, the thin coating was applied on plasma-treated poly(dimethylsiloxane) preventing surface reconstruction in air and leading to long-lasting hydrophilization of the surface (water contact angles around 65°). Such water-borne systems may be used in common applications of silicones providing high flexibility and at the same time water wettability of the coatings.

Photoactuated Pens for Molecular Printing

The photoactuation of pen arrays made of polydimethylsiloxane carbon nanotube composites is explored, and the first demonstration of photoactuated pens for molecular printing is reported. Photoactuation of these composites is characterized using atomic force microscopy and found to produce microscale motion in response to modest illumination, with an actuation efficiency as high as 200 nm mW^-1 on the sub-1 s time scale. Arrays of composite pens are synthesized and it is found that local illumination is capable of moving selected pens by more than 3 µm out of the plane, bringing them into contact to perform controllable and high quality printing while completely shutting off the nonilluminated counterparts. In light of the scalability limitations of nanolithography, this work presents an important step and paves the way for arbitrary control of individual pens in massive arrays. As an example of a scalable soft actuator, this approach can also aid progress in other fields such as soft robotics and microfluidics.

Materials, Mechanics, and Patterning Techniques for Elastomer-Based Stretchable Conductors

Stretchable electronics represent a new generation of electronics that utilize soft, deformable elastomers as the substrate or matrix instead of the traditional rigid printed circuit boards. As the most essential component of stretchable electronics, the conductors should meet the requirements for both high conductivity and the capability to maintain conductive under large deformations such as bending, twisting, stretching, and compressing. This review summarizes recent progresses in various aspects of this fascinating and challenging area, including materials for supporting elastomers and electrical conductors, unique designs and stretching mechanics, and the subtractive and additive patterning techniques. The applications are discussed along with functional devices based on these conductors. Finally, the review is concluded with the current limitations, challenges, and future directions of stretchable conductors.

Mechanics of aligned carbon nanotube polymer matrix nanocomposites simulated via stochastic three-dimensional morphology

The promise of enhanced and tailored properties motivates the study of one-dimensional nanomaterials, especially aligned carbon nanotubes (A-CNTs), for the reinforcement of polymeric materials. While CNTs have remarkable theoretical properties, previous work on aligned CNT polymer matrix nanocomposites (A-PNCs) reported mechanical properties that are orders of magnitude lower than those predicted by rule of mixtures. This large difference primarily originates from the morphology of the CNTs, because the CNTs that comprise the A-PNCs have significant local curvature commonly referred to as waviness. Here we present a simulation framework capable of analyzing 10(5) wavy CNTs with realistic three-dimensional morphologies to quantify the impact of waviness on the effective elastic modulus contribution of wavy CNTs. The simulation results show that due to the low shear modulus of the reinforcing CNT 'fibers', and large ([Formula: see text]) compliance contribution of the shear deformation mode, waviness reduces the effective stiffness contribution of the A-CNTs by two to three orders of magnitude. Also, the mechanical property predictions resulting from the simulation framework outperform those previously reported using finite element analysis since representative descriptions of the morphology are required to accurately predict properties of the A-PNCs. Further work to quantify the morphology of A-PNCs in three-dimensions, simulate their full non-isotropic constitutive relations, and predict their failure mechanisms is planned.

Bio-based graphene/sodium alginate aerogels for strain sensors

Bio-based graphene aerogels are fabricated with graphene oxide and sodium alginate, showing great potential in flexible strain sensors due to the excellent mechanical stability and high sensitivity to compression and bending deformations.

Mechanically Stretchable and Electrically Insulating Thermal Elastomer Composite by Liquid Alloy Droplet Embedment

Stretchable electronics and soft robotics have shown unsurpassed features, inheriting remarkable functions from stretchable and soft materials. Electrically conductive and mechanically stretchable materials based on composites have been widely studied for stretchable electronics as electrical conductors using various combinations of materials. However, thermally tunable and stretchable materials, which have high potential in soft and stretchable thermal devices as interface or packaging materials, have not been sufficiently studied. Here, a mechanically stretchable and electrically insulating thermal elastomer composite is demonstrated, which can be easily processed for device fabrication. A liquid alloy is embedded as liquid droplet fillers in an elastomer matrix to achieve softness and stretchability. This new elastomer composite is expected useful to enhance thermal response or efficiency of soft and stretchable thermal devices or systems. The thermal elastomer composites demonstrate advantages such as thermal interface and packaging layers with thermal shrink films in transient and steady-state cases and a stretchable temperature sensor.

Effect of random/aligned nylon-6/MWCNT fibers on dental resin composite reinforcement

The aims of this study were (1) to synthesize and characterize random and aligned nanocomposite fibers of multi-walled carbon nanotubes (MWCNT)/nylon-6 and (2) to determine their reinforcing effects on the flexural strength of a dental resin composite. Nylon-6 was dissolved in hexafluoropropanol (10 wt%), followed by the addition of MWCNT (hereafter referred to as nanotubes) at two distinct concentrations (i.e., 0.5 or 1.5 wt%). Neat nylon-6 fibers (without nanotubes) were also prepared. The solutions were electrospun using parameters under low- (120 rpm) or high-speed (6000 rpm) mandrel rotation to collect random and aligned fibers, respectively. The processed fiber mats were characterized by scanning (SEM) and transmission (TEM) electron microscopies, as well as by uni-axial tensile testing. To determine the reinforcing effects on the flexural strength of a dental resin composite, bar-shaped (20×2×2 mm(3)) resin composite specimens were prepared by first placing one increment of the composite, followed by one strip of the mat, and one last increment of composite. Non-reinforced composite specimens were used as the control. The specimens were then evaluated using flexural strength testing. SEM was done on the fractured surfaces. The data were analyzed using ANOVA and the Tukey׳s test (α=5%). Nanotubes were successfully incorporated into the nylon-6 fibers. Aligned and random fibers were obtained using high- and low-speed electrospinning, respectively, where the former were significantly (p<0.001) stronger than the latter, regardless of the nanotubes׳ presence. Indeed, the dental resin composite tested was significantly reinforced when combined with nylon-6 fibrous mats composed of aligned fibers (with or without nanotubes) or random fibers incorporated with nanotubes at 0.5 wt%.

Tuning the dielectric properties of metallic-nanoparticle/elastomer composites by strain

Tunable metal/dielectric composites are promising candidates for a large number of potential applications in electronics, sensor technologies and optical devices. Here we systematically investigate the dielectric properties of Ag-nanoparticles embedded in the highly flexible elastomer poly-dimethylsiloxane (PDMS). As tuning parameter we use uniaxial and biaxial strain applied to the composite. We demonstrate that both static variations of the filling factor and applied strain lead to the same behavior, i.e., the filling factor of the composite can be tuned by application of strain. In this way the effective static permittivity εeff of the composite can be varied over a very large range. Once the Poisson's ratio of the composite is known, the strain dependent dielectric constant can be accurately described by effective medium theory without any additional free fit parameter up to metal filling factors close to the percolation threshold. It is demonstrated that, starting above the percolation threshold in the metallic phase, applying strain provides the possibility to cross the percolation threshold into the insulating region. The change of regime from conductive phase down to insulating follows the description given by percolation theory and can be actively controlled.

Patient-specific carbon nanocomposite tracheal prosthesis

PURPOSE

Surgical removal of the trachea is the current gold standard for treating severe airway carcinoma and stenosis. Resection of 6 cm or more of the trachea requires a replacement graft due to anastomotic tension. The high failure rates of current grafts are attributed to a mismatching of mechanical properties and slow epithelium formation on the inner lumen surface. There is also a current lack of tracheal prostheses that are closely tailored to the patient's anatomy.

METHODS

We propose the development of a patient-specific, artificial trachea made of carbon nanotubes and poly-di-methyl-siloxane (CNT-PDMS) composite material. Computational simulations and finite element analysis were used to study the stress behavior of the designed implant in a patient-specific, tracheal model.

RESULTS

Finite element studies indicated that the patient-specific carbon nanocomposite prosthesis produced stress distributions that are closer to that of the natural trachea. In vitro studies conducted on the proposed material have demonstrated its biocompatibility and suitability for sustaining tracheal epithelial cell proliferation and differentiation. In vivo studies done in porcine models showed no adverse side effects or breathing difficulties, with complete regeneration of the epithelium in the prosthesis lumen within 2 weeks.

CONCLUSIONS

This paper highlights the potential of a patient-specific CNT-PDMS graft as a viable airway replacement in severe tracheal carcinoma.

Implantable flexible pressure measurement system based on inductive coupling

One of the currently available treatments for aortic aneurysms is endovascular aneurysm repair (EVAR). In spite of major advances in the operating techniques, complications still occur and lifelong surveillance is recommended. In order to reduce and even eliminate the commonly used surveillance imaging exams, as well as to reduce follow-up costs, new technological solutions are being pursued. In this paper, we describe the development, including design and performance characterization, of a flexible remote pressure measurement system based on inductive-coupling for post-EVAR monitoring purposes. The telemetry system architecture and operation are described and main performance characteristics discussed. The implantable sensor details are provided and its model is presented. Simulations with the reading circuit and the sensor's model were performed and compared with measurements carried out with air and a phantom as media, in order to characterize the telemetry system and validate the models. The transfer characteristic curve (pressure versus frequency) of the monitoring system was obtained with measurements performed with the sensor inside a controlled pressure vacuum chamber. Additional experimental results which proof the system functionality were obtained within a hydraulic test bench that emulates the aorta. Several innovative aspects, when compared to the state of the art, both in the sensor and in the telemetry system were achieved.