Toward Development of Novel Remote Ultrasound Robotic System Using Soft Robotics Technology.

This paper reports on the development of a novel soft robotic system for remote ultrasound applications. Direct contact of the ultrasound probe with the patient's body represents a safety risk and therefore control of the probe's positioning and applied force is a crucial task. The proposed robot uses a passive control system that provides safe interaction between the robot and the patient by leveraging soft robotics technology. The soft robot's structure can be considered as a nonlinear spring which can be designed to exert a safe force within the robot's workspace to guarantee the safety of human-robot interaction. The literature suggests that effective ultrasound imaging of both the heart and abdomen requires six degrees-of-freedom. These degrees-of-freedom consist of three translational motions, which are achieved using a novel hybrid soft cable-driven parallel robot, and three wrist motions, which is based on a universal joint design. The experimental results show that the robot can achieve all these six degrees-of-freedom, and its blocking force can be engineered to generate a uniform force within the workspace.

Low-cost prototyping of nitinol wires/frames using polymeric cores and sacrificial fixtures with application in individualized frames anchoring through the atrial septum.

Self-expanding frames for minimally invasive implants are typically made from nitinol wires and are heat treated to maintain the desired shapes. In the process of heat treatment, nitinol structures are placed in a high-temperature oven, while they are confined by a fixture. During this process, nitinol exerts a high amount of force. Accordingly, a fixture requires high mechanical strength and temperature resistance; this is why fixtures are typically made from metals. The use of metal fixture also increases the turnaround time and cost. However, accelerating this process is beneficial in many applications, such as rapid development of medical implants that are patient-specific. Inspired by the use of sacrificial layers in microfabrication technology, here we propose a novel method for shape setting nitinol wires using a sacrificial metal fixture. In this process, the nitinol wires are first aligned inside copper hypotubes. Next, the forming process is done using hand-held tools to shape complex geometrical structures, annealing the nitinol reinforced by copper, and then selectively etching copper hypotubes in ammonium persulfate solutions. In this process, other sacrificial cores, which are 3D printed or cast from low-cost polymers, are also used. This combination of polymeric cores and minimal use of metals enables reducing the cost and the turnaround time. As a proof of concept, we showed that this process was capable of fabricating springs with mm or sub-mm diameters. The result showed a change of less than 5% in the intended diameter of the nitinol spring with diameters ranging from ~ 0.7 to 1.9 mm, which confirms copper as a suitable sacrificial fixture to obtain the desired complex geometry for nitinol. A metric, based on the elastic strain stored in copper is suggested to predict the possible variation of the intended dimensions in this process. Finally, to demonstrate the potential of this method, as proof of concept, we fabricated NiTi wire frames designed for anchoring through the atrial septum. These frames demonstrated septal defect occluders that were designed based on a patient's cardiac image available in the public domain. This low-cost rapid fabrication technique is highly beneficial for a variety of applications in engineering and medicine with specific applications in rapid prototyping of medical implants.

Application of deep neural networks for inferring pressure in polymeric acoustic transponders/sensors.

Passive sensor-transponders have raised interest for the last few decades, due to their capability of low-cost remote monitoring without the need for energy storage. Their operating principle includes receiving a signal from a source and then reflecting the signal. While well-established transponders operate through electromagnetic antennas, those with a fully acoustic design have advantages such as lower cost and simplicity. Therefore, detection of pressures using the ultrasound signal that is backscattered from an acoustic resonator has been of interest recently. In order to infer the pressure from the backscattered signal, the established approach has been based upon the principle of detection of the shift to the frequency of resonance. Nevertheless, regression of the pressure from the signal with a small error is challenging and has been subject to research. Here in this paper, we explore an approach that employs deep learning for inferring pressure from the ultrasound reflections of polymeric resonators. We assess if neural network regressors can efficiently infer pressure reflected from a fully acoustic transponder. For this purpose, we compare the performance of several regressors such as a convolutional neural network, a network inspired by the ResNet, and a fully connected neural network. We observe that deep neural networks are advantageous in inferring pressure information with a minimal need for analyzing the signal. Our work suggests that a deep learning approach has the potential to be integrated with or replace other traditional approaches for inferring pressure from an ultrasound signal reflected from fully acoustic transponders or passive sensors.

Improved Functional Assessment of Ischemic Severity Using 3D Printed Models.

Objective: To develop a novel in vitro method for evaluating coronary artery ischemia using a combination of non-invasive coronary CT angiograms (CCTA) and 3D printing (FFR3D). Methods: Twenty eight patients with varying degrees of coronary artery disease who underwent non-invasive CCTA scans and invasive fractional flow reserve (FFR) of their epicardial coronary arteries were included in this study. Coronary arteries were segmented and reconstructed from CCTA scans using Mimics (Materialize). The segmented models were then 3D printed using a Carbon M1 3D printer with urethane methacrylate (UMA) family of rigid resins. Physiological coronary circulation was modeled in vitro as flow-dependent stenosis resistance in series with variable downstream resistance. A range of physiological flow rates (Q) were applied using a peristaltic steady flow pump and titrated with a flow sensor. The pressure drop (ΔP) and the pressure ratio (Pd/Pa) were assessed for patient-specific aortic pressure (Pa) and differing flow rates (Q) to evaluate FFR3D using the 3D printed model. Results: There was a good positive correlation (r = 0.87, p < 0.0001) between FFR3D and invasive FFR. Bland-Altman analysis revealed a good concordance between the FFR3D and invasive FFR values with a mean bias of 0.02 (limits of agreement: -0.14 to 0.18; p = 0.2). Conclusions: 3D printed patient-specific models can be used in a non-invasive in vitro environment to quantify coronary artery ischemia with good correlation and concordance to that of invasive FFR.

Rapid Manufacturing of Thin Soft Pneumatic Actuators and Robots.

This protocol describes a method for rapid manufacturing of soft pneumatic actuators and robots with an ultrathin form factor using a heat press and a laser cutter machine. The method starts with the lamination of thermoplastic polyurethane (TPU) sheets using a heat press for 10 min at the temperature of ~93 °C. Next, the parameters of the laser cutter machine are optimized to produce a rectangular balloon with maximum burst pressure. Using the optimized parameters, the soft actuators are laser cut/welded three times sequentially. Next, a dispensing needle is attached to the actuator, allowing it to be inflated. The effect of geometrical parameters on the deflection of the actuator are studied systematically by varying the channel width and length. Finally, the performance of the actuator is characterized using an optical camera and a fluid dispenser. Conventional fabrication methods of soft pneumatic actuators based on silicone molding are time consuming (several hours). They also result in strong but bulky actuators, which limits the actuator's applications. Moreover, microfabrication of thin pneumatic actuators is both time-consuming and expensive. The proposed manufacturing method in the current work resolves these issues by introducing a fast, simple, and cost-effective fabrication method of ultrathin pneumatic actuators.

Microneedle Patterning of 3D Nonplanar Surfaces on Implantable Medical Devices Using Soft Lithography.

Micropatterning is often used to engineer the surface properties of objects because it allows the enhancement or modification of specific functionalities without modification of the bulk material properties. Microneedle arrays have been explored in the past for drug delivery and enhancement of tissue anchoring; however, conventional methods are primarily limited to thick, planar substrates. Here, we demonstrate a method for the fabrication of microneedle arrays on thin flexible polyurethane substrates. These thin-film microneedle arrays can be used to fabricate balloons and other inflatable objects. In addition, these thin-filmed microneedles can be transferred, using thermal forming processes, to more complex 3D objects on which it would otherwise be difficult to directly pattern microneedles. This function is especially useful for medical devices, which require effective tissue anchorage but are a challenging target for micropatterning due to their 3D nonplanar shape, large size, and the complexity of the required micropatterns. Ultrathin flexible thermoplastic polyurethane microneedle arrays were fabricated from a polydimethylsiloxane (PDMS) mold. The technique was applied onto the nonplanar surface of rapidly prototyped soft robotic implantable polyurethane devices. We found that a microneedle-patterned surface can increase the anchorage of the device to a tissue by more than twofold. In summary, our soft lithographic patterning method can rapidly and inexpensively generate thin-film microneedle surfaces that can be used to produce balloons or enhance the properties of other 3D objects and devices.

Laser Cutting as a Rapid Method for Fabricating Thin Soft Pneumatic Actuators and Robots.

Pneumatically actuated soft robots address many challenges with interfacing with delicate objects, but these actuators/robots are still bulky and require many hours to fabricate, limiting their widespread use. This article reports a novel design and manufacturing method for ultrathin soft robots and actuators (∼70 µm) using a laser-cutting machine that cuts/welds sheets of thermoplastic polyurethane (TPU) from a 2D CAD drawing. Using this method, five different soft actuators (e.g., bending, rotating, contracting) are designed, fabricated, and characterized with both planar and nonplanar motions. Furthermore, we show how stacking multiple sheets of TPU enables rapid fabrication of multifunctional actuators. Finally, a portable four-arm swimming robot is designed and fabricated without any assembly steps. This rapid fabrication method enables soft robots to go from concept to operational within minutes, and creates a new subclass of soft robots suitable for applications requiring a robot to be ultrathin, lightweight, and/or fit within small volumes.

Patient-specific design of a soft occluder for the left atrial appendage.

3D printing has been used to create a wide variety of anatomical models and tools for procedural planning and training. Yet, the printing of permanent, soft endocardial implants remains challenging because of the need for haemocompatibility and durability of the printed materials. Here, we describe an approach for the rapid prototyping of patient-specific cardiovascular occluders via 3D printing and static moulding of inflatable silicone/polyurethane balloons derived from volume-rendered computed tomography scans. We demonstrate the use of the approach, which provides custom-made implants made of high-quality, durable and haemocompatible elastomeric materials, in the fabrication of devices for occlusion of the left atrial appendage-a structure known to be highly variable in geometry and the primary source of stroke for patients with atrial fibrillation. We describe the design workflow, fabrication and deployment of patient-specific left atrial appendage occluders and, as a proof-of-concept, show their efficacy using 3D-printed anatomical models, in vitro flow loops and an in vivo large animal model.

Measurement of the thermoelectric power factor of films over the 10-400 K range.

The design and development of a novel apparatus for the simultaneous measurement of electrical resistivity and Seebeck coefficient of films is reported here. Mounting stage is integrated inside a cryostat chamber enabling measurements over the 10-400 K temperature range, intended for organic thermoelectrics. Finite element method was used to analyze the thermo-mechanical response of the sample holder. The apparatus was validated against high purity nickel film, and a very good agreement was found.

Large-Area Semiconducting Graphene Nanomesh Tailored by Interferometric Lithography.

Graphene nanostructures are attracting a great deal of interest because of newly emerging properties originating from quantum confinement effects. We report on using interferometric lithography to fabricate uniform, chip-scale, semiconducting graphene nanomesh (GNM) with sub-10 nm neck widths (smallest edge-to-edge distance between two nanoholes). This approach is based on fast, low-cost, and high-yield lithographic technologies and demonstrates the feasibility of cost-effective development of large-scale semiconducting graphene sheets and devices. The GNM is estimated to have a room temperature energy bandgap of ~30 meV. Raman studies showed that the G band of the GNM experiences a blue shift and broadening compared to pristine graphene, a change which was attributed to quantum confinement and localization effects. A single-layer GNM field effect transistor exhibited promising drive current of ~3.9 µA/µm and ON/OFF current ratios of ~35 at room temperature. The ON/OFF current ratio of the GNM-device displayed distinct temperature dependence with about 24-fold enhancement at 77 K.

Thermal transport in phononic crystals and the observation of coherent phonon scattering at room temperature.

Large reductions in the thermal conductivity of thin silicon membranes have been demonstrated in various porous structures. However, the role of coherent boundary scattering in such structures has become a matter of some debate. Here we report on the first experimental observation of coherent phonon boundary scattering at room temperature in 2D phononic crystals formed by the introduction of air holes in a silicon matrix with minimum feature sizes >100 nm. To delaminate incoherent from coherent boundary scattering, phononic crystals with a fixed minimum feature size, differing only in unit cell geometry, were fabricated. A suspended island technique was used to measure the thermal conductivity. We introduce a hybrid thermal conductivity model that accounts for partially coherent and partially incoherent phonon boundary scattering. We observe excellent agreement between this model and experimental data, and the results suggest that significant room temperature coherent phonon boundary scattering occurs.

Thermal conductivity and nanocrystalline structure of platinum deposited by focused ion beam.

Pt deposited by focused ion beam (FIB) is a common material used for attachment of nanosamples, repair of integrated circuits, and synthesis of nanostructures. Despite its common use little information is available on its thermal properties. In this work, Pt deposited by FIB is characterized thermally, structurally, and chemically. Its thermal conductivity is found to be substantially lower than the bulk value of Pt, 7.2 W m(-1) K(-1) versus 71.6 W m(-1) K(-1) at room temperature. The low thermal conductivity is attributed to the nanostructure of the material and its chemical composition. Pt deposited by FIB is shown, via aberration corrected TEM, to be a segregated mix of nanocrystalline Pt and amorphous C with Ga and O impurities. Ga impurities mainly reside in the Pt while O is homogeneously distributed throughout. The Ga impurity, small grain size of the Pt, and the amorphous carbon between grains are the cause for the low thermal conductivity of this material. Since Pt deposited by FIB is a common material for affixing samples, this information can be used to assess systematic errors in thermal characterization of different nanosamples. This application is also demonstrated by thermal characterization of two carbon nanofibers and a correction using the reported thermal properties of the Pt deposited by FIB.

Microfabricated suspended island platform for the measurement of in-plane thermal conductivity of thin films and nanostructured materials with consideration of contact resistance.

A technique based on suspended islands is described to measure the in-plane thermal conductivity of thin films and nano-structured materials, and is also employed for measurements of several samples with a single measurement platform. Using systematic steps for measurements, the characterization of the thermal resistances of a sample and its contacts are studied. The calibration of the contacts in this method is independent of the geometry, size, materials, and uniformity of contacts. To verify the technique, two different Si samples with different thicknesses and two samples of the same SiN(x) wafer are characterized on a single device. One of the Si samples is also characterized by another technique, which verifies the current results. Characterization of the two SiN(x) samples taken from the same wafer showed less than 1% difference in the measured thermal conductivities, indicating the precision of the method. Additionally, one of the SiN(x) samples is characterized and then demounted, remounted, and characterized for a second time. The comparison showed the change in the thermal resistance of the contact in multiple measurements could be as small as 0.2 K/µW, if a similar sample is used.

Sub-pg mass sensing and measurement with an optomechanical oscillator.

Mass sensing based on mechanical oscillation frequency shift in micro/nano scale mechanical oscillators is a well-known and widely used technique. Piezo-electric, electronic excitation/detection and free-space optical detection are the most common techniques used for monitoring the minute frequency shifts induced by added mass. The advent of optomechanical oscillator (OMO), enabled by strong interaction between circulating optical power and mechanical deformation in high quality factor optical microresonators, has created new possibilities for excitation and interrogation of micro/nanomechanical resonators. In particular, radiation pressure driven optomechanical oscillators (OMOs) are excellent candidates for mass detection/measurement due to their simplicity, sensitivity and all-optical operation. In an OMO, a high quality factor optical mode simultaneously serves as an efficient actuator and a sensitive probe for precise monitoring of the mechanical eigen-frequencies of the cavity structure. Here, we show the narrow linewidth of optomechanical oscillation combined with harmonic optical modulation generated by nonlinear optical transfer function, can result in sub-pg mass sensitivity in large silica microtoroid OMOs. Moreover by carefully studying the impact of mechanical mode selection, device dimensions, mass position and noise mechanisms we explore the performance limits of OMO both as a mass detector and a high resolution mass measurement system. Our analysis shows that femtogram level resolution is within reach even with relatively large OMOs.