Elisabeth Wilhelm

Connecting microfluidic chips using a chemically inert, reversible, multichannel chip-to-world-interface

In this paper we present a reusable, chemically inert, multichannel Chip-to-World-Interface (CWI). The concept of this interface is based on a force fit connection similar to the hollow screw connectors known from high-performance liquid chromatography (HPLC) instruments. It allows contamination free connection of up to 100 thermoplastic tubes to microfluidic chips made from various materials e.g., epoxy polymers, glass and polydimethylsiloxane (PDMS). The spacing of the tubes is fixed whereas the outer dimensions of the CWI can be adapted to the microfluidic chip it should be used with. We demonstrate that such a CWI with 100 tubes is pressure-tight up to (at least) 630 kPa (6.3 bar) pressure and the connection easily sustains flow rates above 4 ml min(-1). The presented CWI is designed such that the fluid probed in the microfluidic chip is in direct contact only with the tube material and the material from which the microfluidic chip is made. This not only enables fluid transport without dead volume, it also ensures that CWI itself will not be contaminated or contaminate the samples being probed. Using polytetrafluoroethylene (PTFE, Teflon®) tubing we demonstrate that the CWI can even be used with harsh organic solvents such as dichloromethane or dimethylformamide during continuous solvent probing over several hours without damage to the CWI or leakage. This CWI therefore effectively allows using almost all types of organic solvents in microfluidic applications.

Rapid bonding of polydimethylsiloxane to stereolithographically manufactured epoxy components using a photogenerated intermediary layer.

We describe a low cost, photo-induced, room-temperature bonding technique for bonding epoxy components to flexible PDMS membranes in less than half an hour. Bond strengths (~350 kPa) were characterized by ISO-conform tensile testing for a popular stereolithography resin and found comparable bond strengths as reported for PDMS/PDMS bonds.

Towards Displaying Graphics on a Cheap, Large-Scale Braille Display

Large-scale Braille displays will make participation in modern media society easier for visually impaired people. At the moment extensive research is done on developing new technologies for affordable refreshable Braille displays. However, the developed displays often do not match the user requirements. This is because most of the engineers entrusted with the development know little to nothing about the potential users of their systems. To bridge that gap we carried out an online survey. Within this survey 69 people who either are visually impaired themselves or take care of someone who has lost his/her sight stated their opinion on how a large-scale refreshable Braille display should be designed. The results of this survey were used to build a first prototype of a large-scale refreshable braille display for displaying text and tactile graphics. This prototype relies on cheap, energy efficient microfluidic phase change actuators.

Rapid bonding of polydimethylsiloxane (PDMS) to various stereolithographically (STL) structurable epoxy resins using photochemically cross-linked intermediary siloxane layers

In this paper we present a fast, low cost bonding technology for combining rigid epoxy components with soft membranes made out of polydimethylsiloxane (PDMS). Both materials are commonly used for microfluidic prototyping. Epoxy resins are often applied when rigid channels are required, that will not deform if exposed to high pressure. PDMS, on the other hand, is a flexible material, which allows integration of membrane valves on the chip. However, the integration of pressure driven components, such as membrane valves and pumps, into a completely flexible device leads to pressure losses. In order to build up pressure driven components with maximum energy efficiency a combination of rigid guiding channels and flexible membranes would be advisable. Stereolithographic (STL) structuring would be an ideal fabrication technique for this purpose, because complex 3D-channels structures can easily be fabricated using this technology. Unfortunately, the STL epoxies cannot be bonded using common bonding techniques. For this reason we propose two UV-light based silanization techniques that enable plasma induced bonding of epoxy components. The entire process including silanization and corona discharge bonding can be carried out within half an hour. Average bond strengths up to 350 kPa (depending on the silane) were determined in ISO-conform tensile testing. The applicability of both techniques for microfluidic applications was proven by hydrolytic stability testing lasting more than 40 hours.

Rapid prototyping of glass microfluidic chips

In academia the rapid and flexible creation of microfluidic chips is of great importance for microfluidic research. Besides polymers glass is a very important material especially when high chemical and temperature resistance are required. However, glass structuring is a very hazardous process which is not accessible to most members of the microfluidic community. We therefore sought a new method for the rapid and simple creation of transparent microfluidic glass chips by structuring and sintering amorphous silica suspensions. The whole process from a digital mask layout to a microstructured glass sheet can be done within two days. In this paper we show the applicability of this process to fabricate capillary driven microfluidic systems.

A latchable thermally activated phase change actuator and optimization of its response behaviour

In this paper we present a latchable, thermally activated phase change (PC) actuator. The actuator can be controlled using a heating resistor and two pressure sources. Thus several hundred actuators can be driven using a highly integrated platform which allows the individual control of several hundred heating resistors. To optimize the response time of the actuators we analyzed the influence of the heating power of the used resistors, the volume of the used phase change material (PCM) and the PCM itself.

A chemically inert multichannel chip-to-world interface to connect microfluidic chips

Within the last decades more and more microfluidic systems for applications in chemistry, biology or medicine were developed. Most of them need a connection between the chip and its macroscopic environment e.g., pumps. Numerous concepts for such interconnections are known from literature but most of them allow only a small number of connections and are neither chemically inert nor contamination-free. We developed a chemically inert, reusable, multichannel Chipto- World-Interface (CWI) based on a force fit connection. This principle is comparable to hollow screws as used in highperformance liquid chromatography. The CWI can be used to connect chips, made of different materials, e.g., glass, polydimethylsiloxane (PDMS), or epoxy polymers, with up to 100 thermoplastic tubes. The dimensions of the CWI and the number of connections can be individually adapted depending on the chip dimensions but the pitch between the tubes is fixed. Due to the design of the CWI the fluid is only in contact with the chip and the tubing material, thus leading to a contamination free and zero dead volume interconnection. Using tubes of polytetrafluorethylene (PTFE, Teflon®) even enables probing with organic solvents like dimethylformamide, dichloromethane or tetrahydrofuran over several hours without leakage or corrosion of the CWI. During experiments the CWI with 100 connections resisted pressure up to 630 kPa (6.3 bar) and sustained flow rates higher than 4 ml/min.