John Garra

Dry etching of polydimethylsiloxane for microfluidic systems

A fluorine-based reactive ion etch (RIE) process has been developed to anisotropically dry etch the silicone elastomer polydimethylsiloxane (PDMS). This technique complements the standard molding procedure that makes use of forms made of thick SU-8 photoresist to produce features in the PDMS. Total gas pressure and the ratio of O2 to CF4 were varied to optimize etch rate. The RIE recipe developed in this study uses a 1:3 mixture of O2 to CF4 gas resulting in a highly directional and stable etch rate of approximately 20 μm per hour. Selective dry etching can be performed through a photolithographically patterned metal etch mask providing greater precision and alignment with preexisting molded features. The dry etch process is presented in this article along with a brief comparison to recently reported wet etch approaches.

A PDMS dermal patch for non-intrusive transdermal glucose sensing

Abstract This paper describes the fabrication of an adhesive bandage comprised of multiple, compliant polydimethylsiloxane (PDMS) micro-fluidic elements to perform controlled and non-intrusive transdermal sampling of glucose, or any other bio-molecule present in interstitial fluids. The patch-like device, to be worn on the skin, has PDMS component layers that form vertically oriented micro-fluidic channels and reservoirs. In addition, micro-heater elements are integrated onto the PDMS layer that will be in contact with the skin, and are used to thermally ablate tiny micro-pores through only the dead-skin layer, allowing for easier diffusion of normally trapped bio-molecules, such as glucose, to the skin surface. The dermal patch, containing micro-channels and fluid-encapsulated reservoirs, assist in the transport of glucose molecules from just beneath the dead-skin layer to a colorimetric detection membrane situated on top of the bi-layer PDMS patch. This paper will focus on the fabrication of the prototype PDMS patch and the challenges encountered during wafer-scale batch production. Preliminary in vitro studies using human graft skin samples are included to illustrate the non-inflammatory micro-ablation procedure.

Dry release of polymer structures with anti-sticking layer

A dry release method using a thin Teflon™ layer for SU-8 multilayered polymeric microstructures is presented. The low surface energy of Teflon makes the adhesion of SU-8 and substrate poor, enabling the SU-8 polymer photoresist to be removed after the devices have been fully processed. The surface energy was measured using the open-crack method, and the surface roughness and deformation of the released SU-8 were minimized in our processing. The dry release technique eliminates the diffusion limited problem in wet etching and is suitable to package complex three-dimensional polymer microfluidic devices. One such example, which provided the original impetus to formulate a dry release process, is a multilayered SU-8 structure that encapsulates small quantities of fluid. This device is being developed for a biomedical application, and will be used throughout this article as an example of a complex SU-8 structure that uses the dry release process.

Thermal ablation of PMMA for water release using a microheater

A new approach was developed in this work to ablate a micropore through a polymethylmethacrylate (PMMA) layer using a microheater for the on-demand release of water stored in a microreservoir. The size of the ablated micropore in the PMMA capping layer is mainly governed by the heater temperature profile and ablation time. Furthermore, the molten PMMA resulting from the heating energy tends to retract away from the micropore location, taking away the gold heater lines from the pore area. This prevents the possibility of any gold traces blocking the flow of water released from within the microreservoir. Simulation was conducted to find temperature profiles on the surface of the microheater, and the results were used to interpret the phenomena observed in the ablation process. Simulation was also conducted for the case when the microreservoir was filled with water. In addition, two alternative ablation materials, unexposed SU-8 and polydimethylsiloxane (PDMS), were examined as possible microreservoir capping layers. The approach developed has potential applications in microfluidic systems to release encapsulated fluids in a controllable manner.

Determining local residual strains of polydimethylsiloxane using ink dots, and stiffening polydimethylsiloxane using SU-8 particles

We have developed two methods to characterize and reduce the deformation of polydimethylsiloxane (PDMS) during its planar molding process. The first method was to determine the local residual strain of PDMS using ink dots. The information of the local residual strain was used to optimize the processing. The second method was to increase the stiffness of PDMS using SU-8 particles. The two methods were applied to a PDMS planar process. With the aid of the methods, the maximum local strain in the released PDMS has been reduced to meet the requirement of misalignment tolerance.

Fabrication of an epoxy based multi-layer bio-fluidic dermal patch

A multi-layer fabrication process using SU8 has been demonstrated for the realization of a robust dermal patch, called the Bio-Fluidic Integrable Transdermal (B-FIT) Microsystem. This device, which is worn in contact with the skin, samples and measures concentrations of bio-molecules such as glucose. The functional substructures of the device include fluidic reservoirs and capillaries, a micro heater element, and a calorimetric detection membrane. The heater elements are used to thermally ablate tiny pores through the surface layer of dead skin, allowing for easy diffusion of normally trapped bio-molecules to the skin surface. The capillaries and fluid containing reservoirs assist in the transport of bio-molecules from the skin surface to the detection patch situated on top of the device. The device offers the advantages of design flexibility, simplicity of fabrication and the integration of metals with polymers. Highly aligned structures and good adhesion between metals and cured SU8 layers have been achieved. A promising dry release process using Teflon has been developed for the removal of the device from a supporting glass substrate used during fabrication. This paper presents the wafer-scale fabrication and characterization of prototype B-FIT devices.

An innovative all-polymeric drug-supply device

In this paper, we report an innovative all-polymeric drug-supply device. The micro outlet of the device was ablated through a polymethyl methacrylate (PMMA) layer using a microheater. The size of the ablated micropore was mainly related to the heater temperature profile, and the molten PMMA took the gold heater lines away from the pore area, avoiding possible block of the gold lines to the flow out of the pore. Simulation was conducted to find the temperature profile on the surface of the microheater, and experimental results have a good match with simulation results.