Till Huesgen

A full-wafer fabrication process for glass microfluidic chips with integrated electroplated electrodes by direct bonding of dry film resist

A full-wafer process is presented for fast and simple fabrication of glass microfluidic chips with integrated electroplated electrodes. The process employs the permanent dry film resist (DFR) Ordyl SY300 to create microfluidic channels, followed by electroplating of silver and subsequent chlorination. The dry film resist is bonded directly to a second substrate, without intermediate gluing layers, only by applying pressure and moderate heating. The process of microfluidic channel fabrication, electroplating and wafer bonding can be completed within 1 day, thus making it one of the fastest and simplest full-wafer fabrication processes.

A self-sustaining micro thermomechanic-pyroelectric generator

Pyroelectric generators (PEGs) can be used for thermal energy harvesting and present a potential alternative to thermoelectric generators. However, in contrary to thermoelectric generators, the PEG principle requires thermal transients to stimulate the conversion process. Such suitable thermal transients are rare in nature, hindering the deployment. In this paper, we present a micro thermomechanic-pyroelectric energy generator (μTMPG) that converts a stationary spatial thermal gradient into the required transient temperature profile across the PEG. The measured power output of the μTMPG is 3 μW from a temperature difference of 79.5 K. However, with an optimized design, a power output of 39.4 mW is estimated for the same temperature difference.

Detailed study of a micro heat engine for thermal energy harvesting

This paper presents a micro heat engine fabricated in silicon micro technology. Its operation principle is based on a cavity filled with a liquid?gas phase-change working fluid that performs a self-controlled reciprocating motion between a heat source and a heat sink. A bistable buckling membrane generates the respective upward and downward driving forces upon expansion and contraction of the working fluid. For prediction of the engine performance a hybrid model is developed. This model predicts an operation frequency of 0.72?Hz and a mechanical output power of 1.29??W at a temperature difference of 37 K. Loss mechanisms are theoretically explored and ways to enhance the overall engine efficiency are discussed. To verify this model, a functional demonstrator is fabricated. In the experiments, an operation frequency of 0.71?Hz is found at a temperature difference of 37 K.

A self-sustaining pyroelectric energy harvester utilizing spatial thermal gradients

Pyroelectric generators (PEGs) present a potential alternative to thermoelectric generators (TEGs), to generate electric power from thermal fields. Pyroelectric generators have a predicted upper limit of 50 % of the Carnots efficiency, which exceeds that of TEGs. However, PEGs require an oscillating thermal field or thermal vibrations. Such vibrations are rare in technical systems, hindering their deployment for energy harvesting. In this paper we present a novel thermal energy harvester utilizing PEGs. The measured power output of such a harvester is 3 µW for a temperature difference of 79.5 K. By improving the harvester design, a power output of approximately 9 mW is predicted.

Fabrication of a normally-closed microvalve utilizing lithographically defined silicone micro O-rings

The focus of this work is on the development of a simple and variable process chain for the integration of flexible silicone material into silicon-based microfluidic devices. A normally-closed microvalve is chosen as a demonstrator device, as it combines features that are not easily obtained from silicon devices alone, especially, a high leak tightness of up to 1 bar pressure difference in the closed state and a high forward flow of several mL in the open state. For this purpose, a photopatternable silicone is used as a deformable circular valve lip between a piezoelectrically actuated membrane and a valve seat, similar to a micro O-ring with a width of 50 µm. The microvalve is piezo actuated by monolayer piezo actuators with a peak-to-peak driving voltage of = 200 V. The micro O-ring is pre-deformed by 2.8 µm during the valve fabrication process to yield the normally-closed behavior. A dry film resist lamination technology is developed for this critical process step to mate the two silicon wafers with the actuation membrane, the valve seat and the silicone O-ring in between at a well-defined distance. The dry film resist is used in a multifunctional way, not only to pre-deform the valve lip, but also to define the geometry of the valve chamber and to ensure a leak-tight connection of both wafers. Altogether, a peak value for the on- to off-ratio of the normally-closed microvalve higher than 30 000 is measured. This opens a wide range of potential applications, e.g. in micro-dosing, drug delivery, μ-TAS and microfluidics for biological or chemical applications in general.

Bistable switching with a thermoelectrically driven thermopneumatic actuator

We present for the first time a bistable thermopneumatic actuator that is based on thermoelectric temperature control. This concept shows several advantages over common thermopneumatic actuators: Due to an active cooling, the reaction time is decreased. Further, positive and negative displacements of the membrane are enabled and allow for a bistable switching. A novel fabrication technology is established and a prototype device is fabricated. Preliminary test results indicate a membrane displacement of 41 µm at an operation frequency of 3.8 Hz, which proves the feasibility of the concept.

A novel self-starting MEMS-heat engine for thermal energy harvesting

This paper describes a novel micro heat engine for thermal energy harvesting. The engine comprises a working fluid in a sealed cavity, that is bound by a bistable membrane. The engine chamber performs a self-controlled reciprocating motion between a heat source and heat sink. Two types of working fluid are investigated: pure gas and a liquid-gas phase-change medium. A theoretical mechanical output power of is 0.89 µW is calculated at a temperature difference of 37°C for the phase change type engine. Measurements at 37°C temperature difference indicate an operation frequency of 0.8 Hz for the phase change engine, compared to 0.16 Hz for the pure gas engine. A maximum of 1.6 Hz is found for the air-filled engine at 100°C temperature difference.

Bistable silicon microvalve with thermoelectrically driven thermopneumatic actuator for liquid flow control

This paper presents a bistable microfluidic valve with a thermopneumatic actuator that is thermoelectrically driven. A functional prototype is fabricated in silicon technology. The majority of the fabrication process, including the integration of the micro-Peltier devices, is performed at wafer level. The tested valves are switched with ±10 V pulses with an energy input of approximately 1 J for closing and 2 J for opening the valve. Leak rates below 1 µl/min are measured for inlet pressures up to 100 kPa.

Performance improvement of a micro thermomechanical generator by incorporating Galinstan??micro droplet arrays

In previous research we have demonstrated a micro thermomechanical pyroelectric generator (mu TMPG) as an alternative to thermoelectric generators to harvest ambient heat energy. In such a device, a thermal mass oscillates between a hot and a cold side by virtue of the bistability of its mechanical mount, thus generating a temporal thermal gradient over a pyroelectric material in between. The operational frequency as a major factor deciding the power output of the mu TMPG is in turn dependent on the thermal contact resistance (TCR) present at the mating regions of thermal mass, hot and cold sides. Hence, we have investigated the incorporation of an array of Galinstan droplets at the mating interfaces to reduce the TCR. These arrays are fabricated by selective deposition of Galinstan on a laser-micromachined silicon substrate. After incorporating such an array the operational frequency of the mu TMPG increases by at least 50%.