Subrata Kumar

Fabrication of microchannels: A review

Microchannels are primarily used in biomedical devices and microfluidic applications. Fabrication of microchannels has always been a tough task using conventional manufacturing technologies. Various types of materials are in use for fabricating microchannels in different types of applications including metals, polymers and ceramics. A number of methods are in use for fabricating different types of microchannels. These processes include both conventional and nonconventional fabrication techniques such as micromilling, lithography, embossing processes and laser ablation processing. During the recent years, some hybrid techniques have also been developed for fabrication of microchannels. This survey of various literatures reveals a broad spectrum of different processes used for fabricating microchannels. Currently, laser micromachining has been evolved as a potential technology for fabricating microchannels. Laser processing has been proved to be the most time efficient and clean. In this article, fabrication processes for creating microchannels have been reviewed with special emphasis on laser micromachining. This article mostly addresses the fabrication techniques for creating surface microchannels.

Profile and depth prediction in single-pass and two-pass CO2 laser microchanneling processes

Polymer based microfluidic channels are used in many chemical and biological devices. Polymethylmethacrylate (PMMA) has emerged as a key material for such devices owing to its high optical transparency and mechanical strength. The use of CO2 laser processing for fabricating microchannels on PMMA has been proved as an efficient and cost effective method. In this work, theoretical models for predicting microchannel profile and depth have been proposed. A model for single-pass laser processing has been proposed based on energy balance. A two-pass laser process for microchannel fabrication produces smoother microchannels with better surface topography and reduced bulging around the microchannel edges. An energy balance based model has also been proposed for two-pass processing. The experimental verification of the proposed models was conducted. Spectroscopic tests were carried out to determine the absorptivity, and simultaneous thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) tests were performed to determine the thermo-physical properties of the PMMA used in the proposed model. The results predicted using the model were found to be in close agreement with the actual values.

Experimental and theoretical analysis of defocused CO2 laser microchanneling on PMMA for enhanced surface finish

The poor surface finish of CO2 laser-micromachined microchannel walls is a major limitation of its utilization despite several key advantages, like low fabrication cost and low time consumption. Defocused CO2 laser beam machining is an effective solution for fabricating smooth microchannel walls on polymer and glass substrates. In this research work, the CO2 laser microchanneling process on PMMA has been analyzed at different beam defocus positions. Defocused processing has been investigated both theoretically and experimentally, and the depth of focus and beam diameter have been determined experimentally. The effect of beam defocusing on the microchannel width, depth, surface roughness, heat affected zone and microchannel profile were examined. A previously developed analytical model for microchannel depth prediction has been improved by incorporating the threshold energy density factor. A semi-analytical model for predicting the microchannel width at different defocus positions has been developed. A semi-empirical model has also been developed for predicting microchannel widths at different defocusing conditions for lower depth values. The developed models were compared and verified by performing actual experiments. Multi-objective optimization was performed to select the best optimum set of input parameters for achieving the desired surface roughness.

Energy Based Analysis of Laser Microchanneling Process on Polymethyl Methacrylate (PMMA)

CO2 laser micromachining provides low cost machining solution for fabrication of three dimensional microfluidic channels on poly-methyl-methacrylate(PMMA ). In this research work CO2 laser microchanneling process has been analyzed from the first principle. Considering the Gaussian distribution of laser beam, an energy based model has been proposed to predict the microchannel depth and channel profile. For fabricating microfluidic devices, PMMA has emerged as a cheap alternative to many other costly materials like silicon, quartz etc. Its material properties like absorptivity and thermal properties have been investigated. In order to physically verify the proposed model, experiments have been performed on a 3 mm thick PMMA sheet and actual and predicted results have been compared. Simultaneous TGA/DSC tests have been conducted to determine various thermal properties of PMMA. Since thermal conductivity of the PMMA is very low, the conduction loss has been neglected while developing the model. The proposed model successfully predicts the channel depth and profile without much loss of accuracy. energy based analysis has been found to be simple yet powerful method to predict the channel dimensions for low thermal conductivity materials.

Studies of laser textured Ti-6Al-4V wettability for implants

Wettability plays a notable role in success of any bio-implant. It influences tissue amalgamation, protein adsorption and cell attachment at the surface of an implant. Hence, wettability enhancement of the implant is a field of todays dynamic research. In this work, laser based direct melting approach was employed to generate four separate surface patterns on Ti-6Al-4V by means of nanosecond pulse fibre laser. The modification of surface morphology was assessed by means of SEM. Wettability was measured by the help of goniometer. The obtained results revealed that pulsed laser irradiation can substantially improve the biocompatibility of Ti-6AL-4V by making its surface super hydrophilic.

CO2 Laser Microchanneling Process: Effects of Compound Parameters and Pulse Overlapping

PMMA (Polymethyl methacrylate) is commonly used in many microfluidic devices like Lab-on-a-chip devices, bioanalytical devices etc. CO2 lasers provide easy and cost effective solution for micromachining needs on PMMA. Microchannels are an integral part of most of these microfluidic devices. CO2 laser beams have been successfully applied by many authors to fabricate microchannels on PMMA substrates. Laser beam power and scanning speed are the most important laser input parameters affecting the output parameters like microchannel depth, width and heat affected zone (HAZ). The effect of these individual parameters on output parameters are well known and already elaborated by many authors. However, these output parameters can more significantly be described by some compound parameters (combination of direct input laser parameters) like laser fluence, specific point energy, interaction time and P/U (power/scanning speed) ratio. The explanation of effect of these compound parameters was not found in earlier researches. In this work, several experiments were carried out to determine the effects of these compound parameters on output parameters i.e. microchannel width, depth and heat affected zone. The effect of pulse overlapping was also determined by performing experiments at different pulse overlaps and with two different energy deposition settings. The concept of actual pulse overlapping has been introduced by considering actual beam spot diameter instead of using theoretical beam diameter. Minimum pulse overlapping was determined experimentally in order to ensure smooth microchannel edges.

Monte-Carlo based Uncertainty Analysis For CO2 Laser Microchanneling Model

CO2 laser microchanneling has emerged as a potential technique for the fabrication of microfluidic devices on PMMA (Poly-methyl-meth-acrylate). PMMA directly vaporizes when subjected to high intensity focused CO2 laser beam. This process results in clean cut and acceptable surface finish on microchannel walls. Overall, CO2 laser microchanneling process is cost effective and easy to implement. While fabricating microchannels on PMMA using a CO2 laser, the maximum depth of the fabricated microchannel is the key feature. There are few analytical models available to predict the maximum depth of the microchannels and cut channel profile on PMMA substrate using a CO2 laser. These models depend upon the values of thermophysical properties of PMMA and laser beam parameters. There are a number of variants of transparent PMMA available in the market with different values of thermophysical properties. Therefore, for applying such analytical models, the values of these thermophysical properties are required to be known exactly. Although, the values of laser beam parameters are readily available, extensive experiments are required to be conducted to determine the value of thermophysical properties of PMMA. The unavailability of exact values of these property parameters restrict the proper control over the microchannel dimension for given power and scanning speed of the laser beam. In order to have dimensional control over the maximum depth of fabricated microchannels, it is necessary to have an idea of uncertainty associated with the predicted microchannel depth. In this research work, the uncertainty associated with the maximum depth dimension has been determined using Monte Carlo method (MCM). The propagation of uncertainty with different power and scanning speed has been predicted. The relative impact of each thermophysical property has been determined using sensitivity analysis.