Development of a Stretchable Platform for the Fabrication of Biocompatible Microsystems

Abstract

The focus of this PhD thesis is on the development of technologies for the fabrication of microsystems. More specifically, microsystems that are going to be used in a biocompatible and stretchable platform. The aim for the device is to be used in medical applications due to which biocompatibility is a necessity. Compared to rigid medical microsystems, stretchability has added values such as: ease of use for patients or application on curvilinear surfaces (note that biological surfaces like skin are almost never flat). Stretchability of conventional rigid electronics is a hot topic in recent literature as can be seen by the number of publications.
For a stretchable platform conventional rigid interconnects can not be used. Fabrication of stretchable interconnects has been one of the main works in this thesis. By a loose definition, stretchable interconnects are basically con- ductor lines embedded in elastomer substrates. Currently, metal interconnects embedded in polydimethylsiloxane are one of the major candidates. In this thesis, a novel straightforward method is presented that allows the fabrication of narrow 20 μm wide stretchable gold interconnects starting from double sided flexes (Cu–PI–Cu). Wider 100 μm gold interconnects have also been fabricated. The stretchable interconnects can be a single track of conductor or multiple parallel tracks. Different versions have been fabricated and presented in this thesis. Advantages and disadvantages of each have also been discussed. The starting double sided flexes are available commercially for a low cost which is another bonus of this technology. Cycling fatigue tests have been performed to verify the reliability of the stretchable interconnects. The results have shown that the stretchable interconnects are fully functional even after a minimum of 100 000 cycles at 40 % elongation. In addition to the outstanding reliability in this method of fabrication, fine pitch and biocompatibility are the added values paving the way for medical grade stretchable electronics.
It has been shown that integration of electrical printed circuit board com- ponents using the developed stretchable interconnect technology is possible. To demonstrate this, a stretchable array of LEDs has been fabricated. Surface mount LEDs are connected using the stretchable interconnect technology and are elongated up to 60 %. Other suface mount electronic components can be integrated using the same principle.
Apart form the stretchable interconnects, several other technologies and components have been developed. Polydimethylsiloxane (PDMS) is a widely used elastomer mainly known for encapsulation and moulding purposes. We have chosen biocompatible versions of this material for the fabrication of the components that are going to be used in the stretchable platform. Although the soft-lithography process for casting PDMS is a known process in literature, to fabricate for instance microvalves or micropumps in this material, other fabrication technologies are needed as becomes more clear in this thesis. The work which is done in this thesis is twofold:
• DevelopingnovelfabricationtechnologiestoformPDMSintovarious shapes.
• Fabricatingmicrocomponentsusingthecustommadefabricationtech- nologies.
Fabrication technologies have been developed to make PDMS membrane thick- nesses from less than 10 μm to millimeter range. Namely spinning, doctorblad- ing and hot embossing have been used to fabricate various thickness PDMS membranes. The thin membrane can be picked, transported, aligned and bonded correspondingly. The fabricated membranes can be patterned or non patterned. Fabrication processes have been developed to make through layer features in PDMS membranes as well. Through layer holes are mainly done using the hot embossing technique. Full details of the fabrication and charac- terization are provided. In this thesis, PDMS fabrication processes have been our enabling factor to realize micro components.
Transverse electroosmosis micropump and PDMS check valves have been fabricated using the developed fabrication technologies. In this thesis, a flex- ible micropump is fabricated which is aimed to be suitable in drug delivery applications. It provides a relatively high degree of biocompatibility, since the only employed materials are implantation grade polydimethylsiloxane elas- tomer and gold for the electrical interconnects. The working principle of the micropump is based on transverse DC electroosmosis which is a new variant of conventionally applied high voltage DC electroosmosis. This new technique is based on topography irregularities introduced in the channel resulting in a non-uniform charge distribution. The advantage is to drive the micropump using a relatively low DC voltage of 10 V while getting an effective flow speed of 60 μm s−1. In order to characterize the flow speed, dyed 3 μm beads are dispersed in the working fluid and their speed is measured by the line scanning technique using a confocal microscope. The full characterization process is discussed in the thesis. It is also observed that the flow has a helical profile which is an attractive feature for an efficient micromixer in active microfluidics and μ-TAS applications.
Another work which was mentioned is the fabrication of normally closed (NC) microvalves. NC check valves are of high importance in any medical device for leakage safety reasons. To do this, PDMS oxidation is used to re- duce the notorious PDMS to PDMS stiction. The fabricated microvalve is composed of only PDMS. The design, fabrication and characterization of the NC all-polymer membrane-type microvalve is described. The microvalve with a diameter and thickness of 5 mm and 3 mm, respectively, is fabricated com- pletely in biocompatible polydimethylsiloxane (PDMS). The opening pressure, which depends on the adhesion between the membrane and the slab, is reduced to the noticeably low value of 3.4 kPa by the surface oxidation of the PDMS layers. This technique in combination with a selective bonding by Polyethylene terephthalate (PET) shadow masking allows for the fabrication of a high reverse pressure microvalve. Up to 0.6 MPa reverse pressure, which was the maximum pressure that we could apply manually, no flowrate was observed. The presented microvalve features a minimum flow rate of 1 ml min−1.
Using the developed technologies many other biomedical applications can be realized. All in all, the work which is done in this thesis can be viewed in the framework of biocompatible stretchable platforms. The platform mainly consists of PDMS and metal electrodes.

Date of Award	Dec 2013
Original language	English