Thomas W. Schneider

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.

A simple deflection-testing method to determine Poisson's ratio for MEMS applications

A method to determine Poissons ratio of thin films employing a simple experimental deflection test and a simulation program has been developed. According to the method, knowing Youngs modulus and measuring the deflection at an arbitrary point on the thin film, the Poissons ratio can be determined using any mechanical finite element analysis (FEA) program. Two examples are considered to test the method, with both yielding the same results. Due to the relative simplicity involved with this procedure, it is believed that the method can be used by MEMS researchers to determine Poissons ratios of their own thin films of interest. In addition, in the case that Poissons ratio is known and Youngs modulus is unknown, the method may be applied to find the Youngs modulus.

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.

Surface Patterning and Adhesion of Neuroblastoma X Glioma (NG108-15) Cells

A variable height flow cell was used to measure the adhesion properties of the neural cell line of neuroblastoma X glioma (NG108-15) cells cultured on substrates of organosilane self-assembled monolayers (SAMs). The SAMs tested in this study were 13F, 15F, PEG 550, OTS, DETA and APTS. Utilizing deep UV lithography, patterning of the SAMs create three regions for cell attachment; the original SAM, the backfilled SAM, and the interface between the two. Upon plating, the cell soma show no preference for any of the three regions. One exception was on PEG 550, which was found to resist cell adhesion upon normal plating conditions. The cell processes of the NG108-15 cells show a preference for growth at the interface between two patterned surfaces. A factor of three increase in adhesive properties was found for the patterned surfaces over an uncoated glass surface. Design rules of a single whole cell biosensor using the NG108-15 cells can be developed based on these findings.

Development of miniature electromechanical pressure sensor arrays (MEMPSA) for high-resolution thermospheric and mesospheric neutral wind measurement

We have developed a design for a neutral wind instrument for the upper atmosphere based on arrays of miniature pressure sensors. The elements of the array are tunnel displacement transducers (TDTs). We have characterized the TDT, a microelectomechanical system (MEMS), and evaluated TDT capabilities for the determination of pressure within a 2—D array. This is an approach for direct measurement of neutral winds. The sensitivity of the TDT is critical for this application. The output signal of a TDT is the bias voltage dependent tunneling current. This is a differential signal in a force balanced TDT due to the fact that a restoring force is applied to the diaphragm in order to restore the tunneling current to a fixed value. We have fabricated and assembled a custom test apparatus for the TDT based sensor arrays. We have demonstrated simultaneous collection of data from 4 TDTs that were hybrid assemblies of monolithic 2-TDT arrays. Results showed that correlation of the tunneling signal to the actual bias voltage difference required to balance the displacement force gives reasonable information for a pressure sensor and defines the required signal processing.

Polymethylmethacrylate membrane for fluid encapsulation and release in microfluidic systems

A multipolymeric fabrication process using polymethlymethacrylate (PMMA) as a membrane layer was demonstrated for a robust dermal patch, called the bio-fluidic integrable transdermal (B–FIT) microsystem. B–FIT is a noninvasive microdevice, in the form of an adhesive patch, which samples biomolecules from interstitial fluids using controlled thermal ablation and encapsulated fluid that is released during biomolecule collection. Thus, a membrane material was needed that could encapsulate the fluid and easily liberate it, on demand, with minimal energy and compatibility with the fabrication process. PMMA was selected as the most promising membrane material for the B–FIT as against SU-8 since it had the required properties of low melting temperature (120–150 °C), good adhesion to pre-existing B–FIT materials (gold and SU-8), and could be easily spin-cast into thin uniform films. However, PMMA is not inert towards various solvents like acetone, photoresist developers, hence, a process was developed to protect ...

Three Dimensional Thermal Effects in MEMS Devices

A three dimensional thermal imaging system is being developed for measuring temperature profiles in MEMS-biomedical devices. These devices rely on a thermal microablation of the dead-skin layer in order to sample transdermal fluids. This is accomplished using microheaters embedded into a PDMS microchannel device. In order to determine the proper functioning as well as long-term safety of the devices, a temperature profile of the device and the skin in contact with the heaters is needed. The results of simple analytical models are used to optimize a proto- type device. Using a three-dimensional chemical imaging microscope and temperature-depend- ent fluorophores, the temperature profile in a sample can be determined quantitatively as well. We demonstrate the technique on a model sample, and discuss extension to other applications such as thermal imaging in biological systems.

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.