Rajesh Surapaneni

Rapid prototyping of microfluidic systems using a PDMS/polymer tape composite

Rapid prototyping of microfluidic systems using a combination of double-sided tape and PDMS (polydimethylsiloxane) is introduced. PDMS is typically difficult to bond using adhesive tapes due to its hydrophobic nature and low surface energy. For this reason, PDMS is not compatible with the xurography method, which uses a knife plotter and various adhesive coated polymer tapes. To solve these problems, a PDMS/tape composite was developed and demonstrated in microfluidic applications. The PDMS/tape composite was created by spinning it to make a thin layer of PDMS over double-sided tape. Then the PDMS/tape composite was patterned to create channels using xurography, and bonded to a PDMS slab. After removing the backing paper from the tape, a complete microfluidic system could be created by placing the construct onto nearly any substrate; including glass, plastic or metal-coated glass/silicon substrates. The bond strength was shown to be sufficient for the pressures that occur in typical microfluidic channels used for chemical or biological analysis. This method was demonstrated in three applications: standard microfluidic channels and reactors, a microfluidic system with an integrated membrane, and an electrochemical biosensor. The PDMS/tape composite rapid prototyping technique provides a fast and cost effective fabrication method and can provide easy integration of microfluidic channels with sensors and other components without the need for a cleanroom facility.

A three-axis high-resolution capacitive tactile imager system based on floating comb electrodes

We present the design, fabrication and testing of a high-resolution 169-sensing cell capacitive flexible tactile imager (FTI) for normal and shear stress measurement as an auxiliary sensor for robotic grippers and gait analysis. The FTI consists of a flexible high-density array of normal stress and two-dimensional shear stress sensors fabricated using microelectromechanical systems (MEMS) and flexible printed circuit board (FPCB) techniques. The drive/sense lines of the FTI are realized using FPCB whereas the floating electrodes (Au) are patterned on a compressible PDMS layer spin coated on the FPCB layer. The use of unconnected floating electrodes significantly improves the reliability of traditional quad-electrode contact sensing devices by eliminating the need for patterning electrical wiring on PDMS. When placed at the heel of a boot, this FTI senses the position and motion of the line of contact with the ground. Normal stress readouts are obtained from the net capacitance of the cell and the shear-sense direction is determined by the amount of asymmetric overlap of the floating combs with respect to the bottom electrodes. The FTI is characterized using a high-speed switched-capacitor circuit with a 12-bit resolution at full frame rates of 100 Hz (~0.8 Mb s−1) capable of resolving a displacement as low as 60 µm. The FTI and the readout circuitry contribute to a noise/interference level of 5 mV and the sensitivity of normal and shear stress for the FTI is 0.38 MPa−1 and 79.5 GPa−1 respectively.

A highly sensitive flexible pressure and shear sensor array for measurement of ground reactions in pedestrian navigation

We present the design, fabrication and testing of a high-resolution ground reaction sensor cluster (GRSC) as an auxiliary sensor for pedestrian navigation. The GRSC consists of a flexible high-density array of compressible, elastomeric capacitive pressure and two-dimensional shear sensors. When placed at the heel of a boot, the 169-cell GRSC measures detailed information about the position and motion of the line of contact with the ground. The GRSC uses fingered capacitive sensors that provide a large sensitivity to shear stress. The GRSC sensors have pressure and shear sensitivities of 0.44 MPa−1 and 0.76 MPa−1, respectively.

Characterization of electrical interferences for ground reaction sensor cluster

This paper presents the characterization of electrical interferences for a high-resolution error-correcting biomechanical ground reaction sensor cluster (GRSC), developed for improving inertial measurement unit (IMU) position sensing accuracy. The GRSC is composed of 13 × 13 sensing nodes, which can measure dynamic ground forces, shear strains, and sole deformation associated with a ground locomotion gait. The integrated sensing electronics consist of a front-end multiplexer that can sequentially connect individual sensing nodes in a GRSC to a capacitance-to-voltage converter followed by an ADC, digital control unit, and driving circuitry to interrogate the GRSC. The characterization data shows that the single-ended (z-axis pressure) mode exhibits a large output interference due to the un-matched interconnect traces design, thus limiting sensing resolution to 8 bits. The differential mode (x/y-axes shear strain) shows a reduced interference effect, achieving a 10-bit resolution.

A high-resolution flexible tactile imager system based on floating comb electrodes

Flexible high-resolution contact force imagers are needed in many applications for robotic grippers and gait analysis, but its intrinsic intimate contact requirement often causes breaking of top metallization layers and failure in a short time. The use of floating electrodes has significantly improved the reliability of traditional quad-cell capacitive tactile sensing devices. In this paper we present a new type of high-resolution (676-sensors) flexible pressure/shear imager array based on floating combs. Each sensing cell consists of two sets of orthogonal comb electrodes connected in a differential capacitance configuration. The shear sense direction (+x, -x, +y, -y) is determined by the amount of asymmetric comb overlap. Pressure readouts are obtained from the net capacitance of the cell. The new comb configuration multiplies the shear capacitive signal by the number of combs per cell. The imager is read using a high-speed switched-capacitor circuit with a 12-bit resolution at full frame rates of 100 Hz (~ 0.8Mb/s).

Electrical characterization of 26 × 26 ground reaction sensor array interfaced with two parallel electronic detection channels

This paper presents the electrical characterization results of a 26 × 26 high-density ground reaction sensor array (HD-GRSA) interfaced with two parallel electronic detection channels. The system was developed for improving inertial measurement unit (IMU) positioning accuracy. The HD-GRSA is composed of 26 × 26 sensing nodes, which can measure dynamic ground force and shear strain associated with a ground locomotion gait. Each electronic detection channel consists of a front-end multiplexer that can sequentially connect individual sensing nodes from a 13 × 13 sub-array to a capacitance-to-voltage (C/V) converter followed by a 12-bit algorithmic ADC. The electronics were fabricated in a 0.35 μm CMOS process occupying an area of 7.7 mm2 for each channel while dissipating a DC power of 3 mW from a 3V supply. The HD-GRSA demonstrates the designed functionality achieving a gait ground velocity resolution of approximately 95 μmRMS/sec, limited by the electronic interference signals due to the long metal traces on the sensor array. Further performance improvement is expected by employing interference suppression techniques and better matching for critical wiring traces.

High-Performance Interface Electronic System for a 13

This paper describes a high-performance interface electronics system design for a high-density flexible biomechanical ground reaction sensor array (GRSA). The prototype system can be incorporated into a personal boot heel to measure real-time ground force, shear strain, and sole deformation associated with a human bipedal locomotion, thus providing zero-velocity correction to an inertial measurement unit placed in a close proximity. This approach can greatly reduce inertial error accumulation and improve positioning accuracy. The sensing electronics consist of a front-end multiplexer that can sequentially connect individual sensing nodes from a 13 × 13 GRSA to a capacitance-to-voltage converter followed by a 12-bit algorithmic ADC with a sampling rate of 66.7 k-samples/s. The entire sensor array can be scanned within 10 ms. The GRSA employs capacitive sensing scheme and is fabricated using PDMS deformable dielectric layer. The integrated sensing electronics are fabricated in XFAB 0.35 μm CMOS process and dissipate 3 mW power. The overall system sensing resolution is limited by electrical interferences coupled through long interconnect traces between a GRSA and an electronic sensing module. Dynamic and static pressure testing shows the prototype system functionality achieving a gait ground velocity sensing resolution of 100 μm/s.

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We present the microfabrication and characterization of an AFM-tip like device with integrated gas delivery microchannel for the generation of localized microplasmas. The device plasma is generated within a submicron region around its tip for direct-write micro and nanofabrication. The device is fabricated by forming a tall, sharp micromolded gold tip in a KOH etched inverted pyramid followed by thermo-compression bonding and consecutive tip transfer, microfluidic channel patterning and formation of supporting cantilever beam. The tall tip overcomes the height problems of previous designs. Preliminary experiments have been carried out demonstrating the generation of localized microplasma at atmospheric conditions with 1,000V AC stimulation. By mounting the device to a commercialized AFM station and operated in tapping mode, imaging with the same device has also been demonstrated.

13 Flexible Biomechanical Ground Reaction Sensor Array Achieving a Gait Ground Velocity Resolution of 100

This paper presents the design and characterization of an electronic sensing system interfaced with a high-density flexible biomechanical ground reaction sensor array (GRSA). The prototype system can be incorporated into a personal boot heel to measure real-time ground force shear strain and sole deformation associated with a human bipedal locomotion thus providing zero-velocity correction to an inertial measurement unit placed in a close proximity. This approach can greatly reduce inertial error accumulation over time and improve positioning accuracy. The electronic sensing system consists of a front-end multiplexer that can sequentially connect individual capacitive sensing nodes from a 13 × 13 GRSA to a capacitance-to-voltage converter followed by a 12-bit ADC sampled at 66.7 k-samples/sec a digital timing & control unit and a driving circuitry. The electronics were fabricated in XFAB 0.35 μm CMOS process and can achieve a gait ground velocity sensing resolution of 40 μm/sec while dissipating 3mW power.

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We present the development, fabrication and testing results of a new high-density flexible sensor array (HDFA) suitable of recording three-axis stresses with high spatial resolution. The new HDFA consists of 676 (26×26) sensing cells fabricated on top of a high-density flex circuit substrate. Each sensing cell is implemented using four floating comb electrodes separated from the flex substrate by a thin layer of a compressible PDMS film. Each sensing cell measures 2.77×2.55 mm2 thus packing 2704 capacitors in an area of ~ 50 cm2. The HDFA is read using a high-speed switched-capacitor circuit with a 13-bit resolution at full frame rates of 100 Hz (~0.8Mb/s). The new array is capable of detecting contact line displacements as low as 35 μm and contact line velocities as low as 38 μm/s.