Leopold Georgi

Combination of channel- and droplet-based microfluidics for complex PoC devices

This paper presents a microfluidic device addressing the field of ambulant diagnostics in rural areas. Often, the diagnostic approach of micro-channel based point of care devices (PoCD) will target a certain marker - if e.g. on site another marker is to be checked against, the visiting doctor needs to use another test device. With the number of markers growing on a steady basis this will incur the need to transport a large number of individual test strips/cartridges, making the PoCD concept useless for this specific setting. The basis of the proposed novel device is a hybrid system combining the advantages of conventional channel-based fluidics with those of digitally controlled droplet-based fluidics. This hybrid concept uses a micro-channel based delivery partition for stored reagents with a disposable reaction partition based on electrowetting-on-dielectric to run the actual test protocol. It promises to realize a low cost approach for a Point-of-Care system with easy deployability. Conceptual implementation was done by roll embossing for the microchannels and direct structuring of the electrode elements for the EWOD substrate. The latter was laminated with a PTFE-film or coated with a nanoparticle loaded lacquer for hydrophobization. A variety of reagents were handled using a two-phase containment on the EWOD substrate, overcoming the issues associated with low surface energy fluids.

Enhancement of barrier properties of encapsulants for harsh environment applications

Medical devices with embedded highly miniaturized microsystems are used as implants in the human body or as non-invasive devices for sensor applications outside the body. Those devices bear quite a lot of economic opportunities but they also do offer unique challenges compared to consumer or automotive applications. Medical applications need to provide biocompatibility, highest miniaturization, rough treatment, autoclave sterilization and harsh environment e.g. humidity, wax, dust, blood or urine to be applicable. And microelectronics packaging needs to protect the functional elements of the microsystem against these rigid conditions. And, with a different set of media, packaging needs to fulfill the same task for automotive applications, where a growing number of control units and sensor systems under the hood in the transmission oil or petrol can be found. For both markets low cost packaging concepts with high media resistivity is needed. Polymer materials - mainly epoxy resins - are widely used in microelectronics packaging. They are established in microsystem manufacturing, for adhesives as die attach glues or for encapsulants as molding compounds, glob tops or underfill materials. Low cost and mass production capabilities are the main advantages of these materials. But like all polymers they cannot provide a hermetical sealing due to their permeability properties. The susceptibility to diffusion of liquids and gases through the polymer and along the interfaces is a drawback for polymer materials in general, as water or other media inside a microelectronic package might lead to softening of the material and to a decreasing adhesive strength and resulting delaminations close to solder bumps or wire bonds reducing package reliability by decreasing the package structural integrity. Therefore, plastic packaging materials with enhanced humidity resistance allowing the manufacturing of miniaturized microsystems for demanding applications as e.g. medical devices would increase package reliability during assembly and lifetime ideally without cost increase and with no changes in processing. As filler particles have an important influence on the final material properties of microelectronic encapsulants, they are well suited for material modifications. Typically micro-sized silica particles are incorporated into the polymer matrix as the thermo-mechanical properties could be well adapted to reliable packaging demands. However, there are a lot of nano-and micro-sized filler particles with potential to enhance the humidity barrier properties of encapsulants. Working principles of these particles may range from large surface impact of nano-particles, barrier functionality due to stacked layer formation (nano-clays), highly hydrophobic particle surface and molecular water catcher function. Micro- and nano-sized SiO2, bentonite, zeolites, Al2O3, carbon black and carbon nano tubes have been selected for a systematic study. To evaluate the potential of such additives concerning moisture resistance particles are mixed with a microelectronic grade epoxy resin. Neat particles as well as formulations are characterized regarding their water absorption, diffusion and barrier properties. Additionally multi-layer encapsulants with highest humidity barrier properties are introduced. Here, the mechanical or thermo-mechanical functionality is separated from humidity barrier characteristic. Polymer layers are processed wet in wet resulting in a homogeneous encapsulation with gradient material properties. Different methods for characterization of the diffusion properties close to microelectronics application have been developed and applied for material analysis. The pros and cons of simple weight measuring for absorption testing, sorption analysis, TGA desorption measuring, dielectric spectroscopy and encapsulated humidity sensors are presented and discussed along testing results with formulations with the different filler particles. The results of the measurement allow a modeling of the diffusion behavior of the characterized encapsulants and therewith a forecast on the later reliability of the overall system.

Variable Capacitor Energy Harvesting Based on Polymer Dielectric and Composite Electrode

Abstract This work focuses on a polymer based capacitive harvester which can be fabricated with help of roll-to-roll and low cost printing methods. In contrast to electrostatic MEMS based parallel plate transducers or dielectric elastomer systems here, the capacitance is varied as function of the mechanical load by changing of the top electrode area with help of an electrically conducting composite elastomer. In case of a composite elastomer electrode the maximum capacitance in compressed state does not only depend on the thickness and permitivity of the dielectric but first of all on the quality of the interface and the micro structure of the conducting phase in the composite electrode at the interface which was investigated by FEM Maxwell simulation. An equivalent circuit model is used to study the influence of the leakage current inside the dielectric and the bulk resistivity of the elastomer electrode. First experiments with state of the art polymer, thin film and novel printed dielectrics in contact with elastomer electrodes have been performed to prove the harvesting principle at low frequencies. Charges between 25 and 70 nAs per cm2 have been transferred per cycle at 100 V/200 V while the maximum capacity was between 0.4 and 0.8 nF/cm2.

A Hybrid Microfluidic-EWOD System for Complex PoC-Devices

A hybrid microfluidic-EWOD (electro-wetting on dielectrics) system has been developed to meet the demand of fast detection and analysis tools in the field of ambulant diagnostics. The microfluidic device generates discrete droplets at the exit of a micropipette in which the EWOD system coordinates the motions of discrete droplets using a planar array of electrodes. In this work, a dispenser microfluidic system based on a two level microfluidic system was designed and fabricated in silicon using clean room technology then replicated in poly-dimethylsiloxane (PDMS) and cyclic olefin copolymer (COC). This system improves the mechanism of breakup of droplet due to modification of hydraulic diameter.