Henry K. Chu

Parallel microassembly with a robotic manipulation system

This paper proposes a methodology to assemble multiple micro-components simultaneously with a robotic manipulator using a parallel assembly method. Through manipulating and assembling the micro-components, intricate, out-of-plane, three-dimensional micro-devices can now be fabricated. Use of a parallel microassembly process rather that a serial approach can significantly increase the productivity and reduce the cost of assembling micro-devices. The parallel microassembly operation proposed in this work was developed and implemented on a 6-DOF robot manipulator to attain considerable manufacturing flexibility. In this study, three passive microgrippers were bonded in parallel to the end-effector of the manipulator. Three microparts were then grasped by the grippers from the worktable of the manipulator, rotated 90°, and assembled onto the base substrate simultaneously. During the parallel microassembly operation, the visual image may not be able to monitor all three gripper-part pairs simultaneously due to the limited field of view of the microscope. Through the use of an alignment-calibration algorithm with only one gripper-part set, the remaining two sets were successfully manipulated and inserted onto the desired assembly location.

Rapid characterization of the biomechanical properties of drug-treated cells in a microfluidic device

Cell mechanics is closely related to many cell functions. Recent studies have suggested that the deformability of cells can be an effective biomarker to indicate the onset and progression of diseases. In this paper, a microfluidic chip is designed for rapid characterization of the mechanics of drug-treated cells through stretching with dielectrophoresis (DEP) force. This chip was fabricated using PDMS and micro-electrodes were integrated and patterned on the ITO layer of the chip. Leukemia NB4 cells were considered and the effect of all-trans retinoic acid (ATRA) drug on NB4 cells were examined via the microfluidic chip. To induce a DEP force onto the cell, a relatively weak ac voltage was utilized to immobilize a cell at one side of the electrodes. The applied voltage was then increased to 3.5 V pp and the cell started to be stretched along the applied electric field lines. The elongation of the cell was observed using an optical microscope and the results showed that both types of cells were deformed by the induced DEP force. The strain of the NB4 cell without the drug treatment was recorded to be about 0.08 (time t = 180 s) and the drug-treated NB4 cell was about 0.21 (time t = 180 s), indicating a decrease in the stiffness after drug treatment. The elastic modulus of the cell was also evaluated and the modulus changed from 140 Pa to 41 Pa after drug treatment. This microfluidic chip can provide a simple and rapid platform for measuring the change in the biomechanical properties of cells and can potentially be used as the tool to determine the biomechanical effects of different drug treatments for drug discovery and development applications.

Design of a high sensitivity capacitive force sensor

This paper presents the design and development of a MEMS based, capacitive sensor for micro-force measurement. The sensor has an overall dimension of 3600 mum times 1000 mum times 10 mum and was fabricated using the Micragem fabrication process. A displacement reduction mechanism is incorporated in this sensor design to increase the sensitivity of the sensor. Analysis from Finite Element software, COMSOL, confirms that a 10:1 displacement reduction ratio is achievable with this mechanism. Simulation results show that the sensor is capable of measuring a maximum force input of 11 milli-Newton, resulting from a 20-mum displacement on the sensing structure. A 6-DOF manipulator and an evaluation board were used to experimentally verify the performance the sensor. Experimental results show that a capacitance change of approximately 175 to 200 fF can be observed from a 20-mum displacement.

Dual-arm micromanipulation and handling of objects through visual images

Pick-and-place of micro-scale objects is essential for many microscopic tasks. For more sophisticated tasks, grasping and manipulating objects with two independent tools can enhance the capability and the dexterity of the tools. In this work, a dual-arm micromanipulation system equipped with two tungsten probes was employed for the manipulation of a sphere. In order to provide sufficient contact area for grasping, the two probes were positioned side-by-side to grasp a sphere lying on a glass substrate. Visual images were used to provide feedback for manipulating the sphere from one location to the desired location, finally releasing the sphere. Since the adhesion force is dominant in the micro-scale environment, the sphere adheres to the probe and could not be released. To resolve this issue, the two probes were reconfigured after the manipulation. The contact points of the two probes were reconfigured from a side-by-side contact, to a tip-to-tip contact with the sphere. Experimental results confirmed that through this probe reconfiguration, the success rate of sphere release from the probes is higher, allowing improved throughput with such an approach.

MEMS capacitive force sensor for use in microassembly

This paper presents the design and modeling of a MEMS capacitive sensor for use in microassembly processes. Force monitoring is an important aspect in microassembly processes. It can be used to control the movement of the microgripper to properly engage with and release the micropart without physical damages due to excessive force. In this paper, the proposed sensor structure has an overall dimension of 3600 mum times 840 mum times 10 mum and was fabricated using the Micragem fabrication process. A displacement reduction mechanism is incorporated in this sensor design to increase the sensitivity of the sensor. Experimental results showed that a capacitance change of 112.4 fF would result for a 20-mum input displacement.

Design and characterization of a conductive nanostructured polypyrrole‐polycaprolactone coated magnesium/PLGA composite for tissue engineering scaffolds

A novel biodegradable and conductive composite consisting of magnesium (Mg), polypyrrole-block-ploycaprolactone (PPy-PCL), and poly(lactic-co-glycolic acid) (PLGA) is synthesized in a core-shell-skeleton manner for tissue engineering applications. Mg particles in the composite are first coated with a conductive nanostructured PPy-PCL layer for corrosion resistance via the UV-induced photopolymerization method. PLGA matrix is then added to tailor the biodegradability of the resultant composite. Composites with different composition ratios are examined through experiments, and their material properties are characterized. The in vitro experiments on culture of 293FT-GFP cells show that the composites are suitable for cell growth and culture. Biodegradability of the composite is also evaluated. By adding PLGA matrix to the composite, the degrading time of the composite can last for more than eight weeks, hence providing a longer period for tissue formation as compared to Mg composites or alloys. The findings of this research will offer a new opportunity to utilize a conductive, nanostructured-coated Mg/PLGA composite as the scaffold material for implants and tissue regeneration.

Dynamic tracking of moving objects in microassembly through visual servoing

Precise micropart alignment is a crucial factor in most gripper-based microassembly processes. For the micropart to be grasped or manipulated, these processes require the micropart to be positioned and oriented properly for the microgripper. At present, many of the these processes still rely on operators to monitor and align the micropart manually through visual images provided by the camera on top of the assembly line. However, due to the limited field of view of the microassembly system microscope, the micropart may move outside of the visual monitoring area at some point during the manipulation process. The present work proposes an integrated microassembly algorithm that performs the assembly process regardless of the micropart initial orientation. The algorithm automatically aligns and tracks the micropart during the manipulation process. As the micropart rotates to the required grasping orientation, the algorithm projects the future motion of the micropart, repositions it, and simultaneously, using a PID control algorithm, maintains the micropart within the field of view of the microscope. The proposed algorithm eliminates the need for a manual alignment process, which is time consuming and is subject to error. The algorithm was implemented and evaluated on an in-house 6 DOF microassembly manipulator. Experimental results confirmed that the proposed algorithm successfully tracked and corrected a 45-degree misaligned micropart at a specified location within the camera field of view with a steady-state error of +/−15 pixels.

An electromagnetic system for magnetic microbead's manipulation

This paper presents the design of a magnetic micromanipulation system for wireless control of a single magnetic bead. Six orthogonally aligned electromagnetic coils were used to generate the magnetic fields for the manipulation and modeling equations were developed to evaluate the required inputs for each coil for precise closed-loop control of the magnetic bead. FEM simulation was performed to analyze the magnetic flux density and magnetic field gradient of the system and the results were compared with the actual measurement. To examine the performance of the proposed system, a super-paramagnetic microbead was placed in a salt medium and experiments were conducted to demonstrate the movements of the bead. The result suggests the ability of manipulating microbeads via the proposed magnetic manipulation system for various micro- and submicrometer scale applications.

Modeling and development of a magnetically actuated system for micro-particle manipulation

This paper presents the modeling and development of a micromanipulation system that can provide continuous force to manipulate a micro-particle through an externally applied magnetic field. This system consists of six electromagnetic coils and the size of the micro-particle to be manipulated is less than 30μm. The magnetic field maps generated from the electromagnets are first simulated using a finite element method. Based on the result, we then calculate the force that can be provided by the system. A prototype of the system is designed and constructed, which is used to precisely control the navigation of a super-paramagnetic micro-bead in a microfluidic environment. This system can be used for drug delivery as well as medical applications such as minimally invasive surgery, diagnosis and sensing.

Fabrication of a microcoil through parallel microassembly

This paper presents the fabrication of a three-dimensional microcoil through the technique of microassembly. The microcoil design is comprised of nine out-of-plane micro-sized windings. Each winding was assembled onto the base substrate orthogonally by a robotic manipulator through microassembly. In contrast to the conventional serial pick-and-place microassembly, this work incorporated the approach of parallel microassembly to grasp and assemble three windings onto the base substrate simultaneously for increased productivity. In addition, a vision-based algorithm was developed to automate the parallel grasping process of three windings. This algorithm utilized well-defined templates to provide high-precision position and orientation evaluations for the micro-sized components. The performance of the microcoil fabrication process was evaluated and discussed. To establish better electrical contact between the windings and the base substrate, conductive adhesive was introduced in the assembly process and the electrical properties of the assembled microcoil structure were examined.