Nathan B. Crane

Self‐assembly in additive manufacturing: opportunities and obstacles

Purpose – Additive manufacturing (AM) offers substantial flexibility in shape, but much less flexibility in materials and functionality – particularly at small size scales. A system for automatically incorporating microscale components would enable the fabrication of objects with more functionality. The purpose of this paper is to consider the potential of self‐assembly to serve as an automated programmable integration method. In particular, it addresses the ability of random self‐assembly processes to successfully assemble objects with high performance despite the possibility of assembly errors.Design/methodology/approach – A self‐assembled thermoelectric system is taken as a sample system. The performance expectations for these systems are then predicted using modified one‐dimensional models that incorporate the effects of random errors. Monte‐Carlo simulation is used to predict the likely performance of self‐assembled thermoelectric systems and evaluate the impact of key process and system design param...

3D printed tooling for thermoforming of medical devices

Purpose – The purpose of this paper is to evaluate the performance of 3D printed materials for use as rapid tooling (RT) molds in low volume thermoforming processes such as in manufacturing custom prosthetics and orthotics.Design/methodology/approach – 3D printed specimens of different materials were produced using the Z‐Corp process. The parts were post processed using both standard and alternative methods. Material properties relevant to the 3D printed parts such as pneumatic permeability, flexural strength and wear rate were measured and compared to standard plaster compositions commonly used.Findings – Three‐dimensional printing (3DP) can replicate the performance of the plaster materials traditionally used in prosthetic/orthotic applications by using modified post process techniques. The resulting 3D printed molds can still be modified and adjusted using traditional methods. The results show that 3D printed molds are feasible for thermoforming prosthetic and orthotic devices such as prosthetic socket...

Continuous electrowetting via electrochemical diodes

We describe a novel method for droplet transport combining electrowetting on dielectric (EWOD) and the diode-like behavior of valve metals to achieve unique actuation performance. While traditional EWOD droplet transport requires switching of voltage between multiple electrodes, our method, which we term continuous rectified electrowetting, utilizes a simple single electrode and a DC voltage to move a 50 μl droplet 28 mm with velocities up to 32 mm s(-1).

Characterization of electrowetting processes through force measurements

A new method of characterizing electrowetting is presented. In this method, the electrowetting actuation forces are measured rather than the contact angle. The forces on the liquid are measured by trapping a droplet between a flat nanoindenter tip and the test substrate. When voltage is applied to electrodes in the substrate, lateral and normal forces are exerted on the tip and measured by the nanoindenter transducer. Proper selection of the tip geometry permits direct prediction of the resulting in-plane lateral forces using analytical formulas derived from the Young-Lippmann equation. Experimental results show good agreement with both analytical and numerical predictions. Numerical modeling using SURFACE EVOLVER shows that the lateral forces are relatively insensitive to most alignment errors and that the analytical model is most accurate when the flat tip is close to the substrate. Evaporation of the test liquid can introduce modest errors in long measurements, but compensation methods are presented. As the droplet undergoes almost no movement, the fluid dynamics have minimal impact on the measured forces and transient electrowetting events are readily detected. Experimental results show significant response at frequencies up to 40 Hz. This setup is useful in measuring electrowetting responses at high speeds and in measuring system degradation processes.

Meshed rectangular waveguide for high power, low loss and reduced weight applications

Additive manufacturing technologies are increasingly being demonstrated to be useful for microwave circuits, showing improved performance in multiple cases. In this work, a meshed rectangular waveguide structure is presented as an option for high power, low loss, but also reduced weight applications. A set of meshed Ku band waveguides was fabricated using binder jetting 3D printing technology showing that the weight can be reduced by 22% with an increase in loss of only 5%, from 0.019 dB/cm for the solid part to 0.020 dB/cm average across the band with the meshed design. Further weight reduction is possible if higher loss is allowed. To demonstrate the concept, a comparison is made between non-meshed and meshed waveguide 4 pole Chebyshev filters.

Floating electrode electrowetting on hydrophobic dielectric with an SiO2 layer

Floating electrode electrowetting is caused by dc voltage applied to a liquid droplet on the Cytop surface, without electrical connection to the substrate. The effect is caused by the charge separation in the floating electrode. A highly resistive thermally grown SiO2 layer underneath the Cytop enables the droplet to hold charges without leakage, which is the key contribution. Electrowetting with a SiO2 layer shows a memory effect, where the wetting angle stays the same after the auxiliary electrode is removed from the droplet in both conventional and floating electrode electrowetting. Floating electrode electrowetting provides an alternative configuration for developing advanced electrowetting-based devices.

Multimaterial and Multilayer Direct Digital Manufacturing of 3-D Structural Microwave Electronics

Direct digital manufacturing (DDM) is an emerging technology that is finding its place across a wide array of industries and applications as a cost-effective solution for low volume and mass customizable production. This technology encompasses a class of digital manufacturing techniques which can be combined to enable multimaterial fabrication and postprocessing. One of the promising applications for DDM is structural electronics, where lightweight printed plastics provide mechanical support as a fixture, package, or structural member and also host the electrical interconnects and devices, all in a contiguous fashion. Microwave structural electronics is a specific class of such systems for which the printing resolution as well as electrical and surface properties of the materials are especially important. This paper presents the current state of DDM technology, fundamental research into the electrical and mechanical properties of as-printed structures, and novel 3-D printed structures operating from C-band through Ku-band.

Analysis and Measurement of Forces in an Electrowetting-Driven Oscillator

Electrowetting is a promising method for manipulating small volumes of liquid on a solid surface. This complex phenomenon couples electrical and fluid properties and offers many potential surprises. The complex electrical and capillary interactions in electrowetting are illustrated by an analysis of an electrowetting configuration that produces an oscillating droplet motion from a steady DC voltage input. The paper presents an analysis of the electrowetting forces to explain the oscillation and presents a new method for measuring electrowetting forces using a Hysitron Triboindenter. Initial results are compared with predictions from numerical models and simplified analytical solutions.

Design of fluidic self-assembly bonds for precise component positioning

Self Assembly is a promising alternative to conventional pick and place robotic assembly of micro components. Its benefits include parallel integration of parts with low equipment costs. Various approaches to self assembly have been demonstrated, yet demanding applications like assembly of micro-optical devices require increased positioning accuracy. This paper proposes a new method for design of self assembly bonds that addresses this need. Current methods have zero force at the desired assembly position and low stiffness. This allows small disturbance forces to create significant positioning errors. The proposed method uses a substrate assembly feature to provide a high accuracy alignment guide to the part. The capillary bond region of the part and substrate are then modified to create a non-zero positioning force to maintain the part in the desired assembly position. Capillary force models show that this force aligns the part to the substrate assembly feature and reduces sensitivity of part position to process variation. Thus, the new configuration can substantially improve positioning accuracy of capillary self-assembly. This will result in a dramatic decrease in positioning errors in the micro parts. Various binding site designs are analyzed and guidelines are proposed for the design of an effective assembly bond using this new approach.

Perceived Cooling Using Asymmetrically-Applied Hot and Cold Stimuli

Temperature perception is a highly nonlinear phenomenon with faster rates of change being perceived at much lower thresholds than slower rates. This paper presents a method that takes advantage of this nonlinear characteristic to generate a perception of continuous cooling even though the average temperature is not changing. The method uses multiple thermal actuators so that a few are cooling quickly while the rest of the actuators are heating slowly. The slowly-heating actuators are below the perceptual threshold temperature change and hence are not perceived, while the quickly-cooling actuators are above the perceptual temperature change, hence are perceived. As a result, a feeling of decreasing temperature was elicited, when in fact, there was no net change in the temperature of the skin. Three sets of judiciously designed experiments were conducted in this study, investigating the effects of actuator sizes, forearm measurement locations, patterns of actuator layout, and various heating/cooling time cycles. Our results showed that 19 out 21 participants perceived the continuous cooling effect as hypothesized. Our research indicates that the measurement location, heating/cooling cycle times, and arrangement of the actuators affect the perception of continuous cooling.