Claas Müller

Rapid prototyping of microfluidic chips in COC

We present a novel, cost-efficient process chain for fast tooling and small-lot replication of high-quality, multi-scale microfluidic polymer chips within less than 5 days. The fabrication chain starts with a primary master which is made by well-established cleanroom processes such as DRIE or negative SU-8 resist based surface micromachining. The formation of undercuts in the master which would complicate demolding is carefully avoided. Secondary PDMS masters or epoxy-based masters which are more suitable for common polymer replication schemes such as soft-embossing, hot-embossing or injection molding are subsequently cast from the primary masters. The polymer replica are mainly made of COC and show excellent fidelity with the conventionally micromachined master while displaying no degeneration, even after more than 200 cycles. The use of other polymers such as PMMA is also possible. The process chain further includes surface modification techniques for overall, long-term stable hydrophilic coatings and for local hydrophobic patches as well as a durable sealing based on thermal bonding.

High-resolution permanent photoresist laminate TMMF for sealed microfluidic structures in biological applications

We demonstrate the use of photosensitive epoxy laminate TMMF S2045 for the fabrication and sealing of tapered microfluidic channels. The 45 μm thick resist enables the fabrication of shallow sealed cavities featuring extreme aspect ratios of less than 1:40 (h = 45 μm, w = 2000 μm). It also provides high resolution and enables minimum feature sizes of 10 μm. For the fabrication of free-standing structures, an aspect ratio of up to 7:1 was achieved. The dry-film photoresist can be applied easily by lamination onto structured substrates. The total thickness variation of the resist across a 100 mm wafer was determined to be less than ±0.6 μm. Process parameters for the fabrication and sealing of various micro-channels are discussed and optimized in this paper. The main focus was to minimize thermal impact during lamination, soft-bake, exposure and post–exposure bake, which could lead to lid sagging or channel clogging due to liquefaction of uncured resist. We tested TMMF according to ISO 10995-5 and found it to be non-cytotoxic, enabling its use for biological applications. Swelling of less than 5% for incubation of the dry-film resist in several biologically relevant solvents, buffers and cleaning solutions was observed. (Some figures in this article are in colour only in the electronic version)

A versatile and flexible low-temperature full-wafer bonding process of monolithic 3D microfluidic structures in SU-8

We present a versatile fabrication process for the precise fabrication of embedded three-dimensional microfluidic structures in SU-8 photoresist. The full-wafer bond process based on a polyester (PET) handling layer enhances the previous low-temperature bonding technology. We achieved an extremely high bond strength of 45 MPa while requiring only small anchoring structures. Small channel structures with an aspect ratio >2 as well as wide membranes with an aspect ratio 80%) and enables the integration of microelectronics. The flexibility of the fabrication process is presented in two contrary applications. A completely freestanding and transparent SU-8 foil with a thickness of 225 µm featuring embedded 3D microchannels was fabricated. Also, high quality ink-jet dispensers were successfully fabricated whereas the dispenser quality mainly depends on the channel quality.

Smart wireless autonomous microsystems (SWAMs) for sensor actuator networks

Today a lot of different sensors and actuators are available which have an analog or digital, a standardized or not standardized interface. Many applications needs to measure sensor values, to control actuators and to interact between several sensors/actuators and other systems. Thus an intrinsic goal exists that the sensors and actuators can autonomously communicate with each other. A lot of applications are not suitable for wired communication. Hence there is an additional goal to realize the communication wireless. These goals lead to the challenge to implement smart, wireless, autonomous microsystems (SWAMs). Such a microsystem may capture the data of several sensors and control actuators. It will be intelligent and can communicate with a great number of other systems. Because the communication is wireless it is necessary that the SWAM is power aware to achieve a long lifetime. With SWAMs it is possible to build sensor actuator networks in the area of medical technology, environmental technology, process technology etc. This paper gives a survey about the requirements for the architecture of SWAMs. Then it will be pointed out, which feasibilities exist to realize such architectures. Finally the results from the implementation of such a microsystem will be presented and at the end of the paper novel applications will be showed.

Microthermoforming of microfluidic substrates by soft lithography (µTSL): optimization using design of experiments

We present a detailed analysis of microthermoforming by soft lithography (μTSL) for replication of foil-based microfluidic substrates. The process was systematically optimized by design of experiments (DOE) enabling fabrication of defect-free lab-on-a-chip devices. After the assessment of typical error patterns we optimized the process toward the minimum deviation between mold and thermoformed foil substrates. The following process parameters have most significant impact on the dimensional responses (p 40% relative impact. The DOE results in an empirical process model with a maximum deviation between the prediction and experimental proof of 2% for the optimum parameter set. Finally, process optimization is validated by the fabrication and testing of a microfluidic structure for blood plasma separation from human whole blood. The optimized process enabled metering of a nominal volume of 4.0 μl of blood plasma with an accuracy deviation of 3% and a metering precision of ±7.0%. The μTSL process takes about 30 min and easily enables the replication of 300 μm wide microchannels having vertical sidewalls without any draft angles in a well-controllable way. It proves to be suitable for multiple applications in the field of microfluidic devices. S Online supplementary data available from stacks.iop.org/JMM/21/115002/mmedia (Some figures in this article are in colour only in the electronic version)

An Integrated Power Supply System for Low Power 3.3 V Electronics Using On-Chip Polymer Electrolyte Membrane (PEM) Fuel Cells

A stabilized power supply realized by chip-integrated micro fuel cells within an extended CMOS process is presented in this paper. The fuel cell system delivers a maximum power output of 450 ¿ W/cm2. The electronic control circuitry consists of an LDO, an on-chip oscillator and a programmable timing network. The core system consumes an average power of 620 nW. The system reaches a current efficiency of up to 92% and provides a constant output voltage of 3.3 V.

Nanoimprint lithography for solar cell texturisation

The highest efficiency silicon solar cells are fabricated using defined texturing schemes by applying etching masks. However, for an industrial production of solar cells the usage of photolithographic processes to pattern these etching masks is too consumptive. Especially for multicrystalline silicon, there is a huge difference in the quality of the texture realized in high efficiency laboratory scale and maskless industrial scale fabrication. In this work we are describing the topography of a desired texture for solar cell front surfaces. We are investigating UV-nanoimprint lithography (UV-NIL) as a potential technology to substitute photolithography and so to enable the benefits resulting of a defined texture in industrially feasible processes. Besides the reduced process complexity, UV-NIL offers new possibilities in terms of structure shape and resolution of the generated etching mask. As mastering technology for the stamps we need in the UV-NIL, interference lithography is used. The UV-NIL process is conducted using flexible UV-transparent stamps to allow a full wafer process. The following texturisation process is realized via crystal orientation independent plasma etching to tap the full potential of the presented process chain especially for multicrystalline silicon. The textured surfaces are characerised optically using fourier spectroscopy.

Adaptive fluidic PDMS-lens with integrated piezoelectric actuator

We present the first adaptive microfluidic PDMS-lens system with an integrated piezoelectric pumping actuator. The system is fabricated in a highly streamlined fabrication process through casting via a hot embossing machine and subsequent oxygen bonding of the layers. With an optimized piezo actuator the achieved focal length ranges from 30 to 500 mm at a voltage from -9 to 44 V. We show that the lens resolution is crucially dependent on the thickness of the lens membrane. For an optimum thickness of 230 um at 5 mm diameter the lenses achieve a resolution of 120 linepairs/mm at a contrast of 50 %.

Implementation of Reconfigurable Micro-Sensor Interfaces Utilizing FPAAs

In the present paper, an easy way to implement reconfigurable microsensor interfaces for analog sensors with nonstandardized output signals is introduced. This is enabled by the usage of field programmable analog arrays (FPAAs). FPAAs contain all common components that are necessary for the analog signal processing. Commercially available or in the last years published programmable sensor interfaces have the disadvantage, that they are only programmable in its parameters or functionality. Hence they are normally limited to a special kind of sensor. Sensor interfaces based on FPAAs can be also programmed in its connections, thus it is possible to support various types of analog sensors. As a result of this work we present a developed universal programmable sensor interface (UPSI) for different sensors and its behaviour

Passive water management for µfuel-cells using capillary microstructures

In this work we present a novel system for the passive water management in polymer electrolyte fuel cells (PEMFC) based on capillary effects in microstructures. The system removes abundant water that occurs at low temperatures at a fuel cell cathode and secures the humidity of the electrolyte membrane on higher temperatures. Liquid water is removed by hydrophilic gas supply channels with a tapered cross section as presented previously, and further transported by a system of capillary channels and a layer of nonwoven material. To prevent the membrane from running dry, a storage area in the nonwoven layer is introduced, controlled by a novel passive capillary overflow valve. The valve controls whether water is stored or finally disposed by gravity and evaporation. Experiments in a model system show that the nonwoven material is capable of removing all liquid water that can be produced by the fuel cell. A miniaturized fuel cell utilizing the novel water removal system was fabricated and experiments show that the system can stabilize the performance during changes of electrical load. Clearing the drowned miniaturized fuel cell flow field was proven and required 2 min. To make the capillary effects available for the originally hydrophobic graphite composite materials that were used to fabricate the flow fields, hydrophilic grafting based on photochemistry was applied to the material and contact angles of about 40° could be achieved and preserved for at least three months.