5 research outputs found
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Development of a laser foaming process for high throughput three-dimensional tissue model devices
A three-dimensional (3D) porous structure on biodegradable or biocompatible polymers have attracted tremendous attention in numerous bio-related areas including 3D cell culturing and tissue engineering because of their microenvironment similar to ones in vivo. In this study, a novel fabrication process, named selective laser foaming, was developed to create localized 3D porous structure on a polymer chip. The effects of laser power and lasing time on the porous structure were studied both experimentally and through finite element modeling. A high throughput two-chamber tissue model platform was developed using the proposed selective laser foaming process.
For comparison, cell culture studies were conducted with both selective laser foamed and unfoamed polylactic acid (PLA) samples using T98G cells. The results show that by laser foaming gas-impregnated polylactic acid it is possible to generate an array of inverse cone-shaped wells with porous walls. The size of the foamed region can be controlled with laser power and exposure time, while the pore size of the scaffold can be manipulated with the saturation pressure. The finite element modeling results showed good agreement with the experimental data; therefore, the model could be used to optimize and control the selective foaming process. T98G cells grew well in the foamed scaffolds, forming clusters that have not been observed in 2D cell cultures. Cells were more viable in the 3D scaffolds than in the 2D cell culture cases, suggesting that the 3D porous microarray could be used for parallel studies of drug toxicity, guided stem cell differentiation, and DNA binding profiles.
As an example, a high-throughput two-chamber 3D tissue model platform driven by the centrifugal force was developed for drug screening. The selective laser foaming process was calibrated to fabricate 3D scaffold on a commercially available compact disc (CD) made of polycarbonate (PC). Laser foaming of gas saturated polycarbonate created inverse cone-shaped wells with micro-sized porous structure underneath the surface. The open pores were hundreds of micrometers in diameter and depth. The pore size of the underneath porous structure was several tens of micrometers. The size of the well was dependent on the laser power and laser exposure time. Two laser-foamed scaffolds were fabricated in tandem and two mechanically-machined chambers were placed adjacent to the scaffolds, respectively. All scaffolds and chambers were in line and all of them were connected with micro-channels. The surface was coated with polydopamine (PDA) in order to increase the hydrophilicity and biocompatibility. After sterilization, human glioblastoma multiforme (M059K) and hepatoblastoma (C3A sub28) were seeded in the two 3D scaffolds separately and cultured for up to four weeks. These cells grew well in the scaffolds and cell aggregates were observed, suggesting that the developed two-chamber tissue model array could be useful for high-throughput biochemical assays.Mechanical Engineerin
The development and application of superhydrophilic-superhydrophobic patterned polymer surfaces for high-density cell microarrays
In this work, the interesting and unique properties of superhydrophilic-superhydrophobic patterned surfaces were used to form high-density arrays of cells or liquids for cell screening applications
A fully integrated CMOS microelectrode system for electrochemistry
Electroanalysis has proven to be one of the most widely used technologies for point-of-care devices. Owing to the direct recording of the intrinsic properties of biochemical functions, the field has been involved in the study of biology since electrochemistry’s conception in the 1800’s. With the advent of microelectronics, humanity has welcomed self-monitoring portable devices such as the glucose sensor in its everyday routine. The sensitivity of amperometry/ voltammetry has been enhanced by the use of microelectrodes. Their arrangement into microelectrode arrays (MEAs) took a step forward into sensing biomarkers, DNA and pathogens on a multitude of sites. Integrating these devices and their operating circuits on CMOS monolithically miniaturised these systems even more, improved the noise response and achieved parallel data collection. Including microfluidics on this type of devices has led to the birth of the Lab-on-a-Chip technology. Despite the technology’s inclusion in many bioanalytical instruments there is still room for enhancing its capabilities and application possibilities. Even though research has been conducted on the selective preparation of microelectrodes with different materials in a CMOS MEA to sense several biomarkers, limited effort has been demonstrated on improving the parallel electroanalytical capabilities of these devices. Living and chemical materials have a tendency to alter their composition over time. Therefore analysing a biochemical sample using as many electroanalytical methods as possible simultaneously could offer a more complete diagnostic snapshot.
This thesis describes the development of a CMOS Lab-on-a-Chip device comprised of many electrochemical cells, capable of performing simultaneous amperometric/voltammetric measurements in the same fluidic chamber. The chip is named an electrochemical cell microarray (ECM) and it contains a MEA controlled by independent integrated potentiostats. The key stages in this work were: to investigate techniques for the electrochemical cell isolation through simulations; to design and implement a CMOS ECM ASIC; to prepare the CMOS chip for use in an electrochemical environment and encapsulate it to work with liquids; to test and characterise the CMOS chip housed in an experimental system; and to make parallel measurements by applying different simultaneous electroanalytical methods. It is envisaged that results from the system could be combined with multivariate analysis to describe a molecular profile rather than only concentration levels.
Simulations to determine the microelectrode structure and the potentiostat design, capable of constructing isolated electrochemical cells, were made using the Cadence CAD software package. The electrochemical environment and the microelectrode structure were modelled using a netlist of resistors and capacitors. The netlist was introduced in Cadence and it was simulated with potentiostat designs to produce 3-D potential distribution and electric field intensity maps of the chemical volume. The combination of a coaxial microelectrode structure and a fully differential potentiostat was found to result in independent electrochemical cells isolated from each other.
A 4 x 4 integrated ECM controlled by on-chip fully differential potentiostats and made up by a 16 × 16 working electrode MEA (laid out with the coaxial structure) was designed in an unmodified 0.35 μm CMOS process. The working electrodes were connected to a circuit capable of multiplexing them along a voltammetric measurement, maintaining their diffusion layers during stand-by time. Two readout methods were integrated, a simple resistor for an analogue readout and a discrete time digital current-to-frequency charge-sensitive amplifier. Working electrodes were designed with a 20 μm side length while the counter and reference electrodes had an 11 μm width. The microelectrodes were designed using the aluminium top metal layer of the CMOS process.
The chips were received from the foundry unmodified and passivated, thus they were post-process fabricated with photolithographic processes. The passivation layer had to be thinned over the MEA and completely removed on top of the microelectrodes. The openings were made 25 % smaller than the top metal layer electrode size to ensure a full coverage of the easily corroded Al metal. Two batches of chips were prepared, one with biocompatible Au on all the microelectrodes and one altered with Pd on the counter and Ag on the reference electrode. The chips were packaged on ceramic pin grid array packages and encapsulated using chemically resistant materials. Electroplating was verified to deposit Au with increased roughness on the microelectrodes and a cleaning step was performed prior to electrochemical experiments.
An experimental setup containing a PCB, a PXIe system by National Instruments, and software programs coded for use with the ECM was prepared. The programs were prepared to conduct various voltammetric and amperometric methods as well as to analyse the results. The first batch of post-processed encapsulated chips was used for characterisation and experimental measurements. The on-chip potentiostat was verified to perform alike a commercial potentiostat, tested with microelectrode samples prepared to mimic the coaxial structure of the ECM. The on-chip potentiostat’s fully differential design achieved a high 5.2 V potential window range for a CMOS device. An experiment was also devised and a 12.3 % cell-to-cell electrochemical cross-talk was found. The system was characterised with a 150 kHz bandwidth enabling fast-scan cyclic voltammetry(CV) experiments to be performed. A relatively high 1.39 nA limit-of-detection was recorded compared to other CMOS MEAs, which is however adequate for possible applications of the ECM. Due to lack of a current polarity output the digital current readout was only eligible for amperometric measurements, thus the analogue readout was used for the rest of the measurements.
The capability of the ECM system to perform independent parallel electroanalytical measurements was demonstrated with 3 different experimental techniques. The first one was a new voltammetric technique made possible by the ECM’s unique characteristics. The technique was named multiplexed cyclic voltammetry and it increased the acquisition speed of a voltammogram by a parallel potential scan on all the electrochemical cells. The second technique measured a chemical solution with 5 mM of ferrocene with constant potential amperometry, staircase cyclic voltammetry, normal pulse voltammetry, and differential pulse voltammetry simultaneously on different electrochemical cells. Lastly, a chemical solution with 2 analytes (ferrocene and decamethylferrocene) was prepared and they were sensed separately with constant potential amperometry and staircase cyclic voltammetry on different cells. The potential settings of each electrochemical cell were adjusted to detect its respective analyte