Jasbir N. Patel

PDMS as a sacrificial substrate for SU-8-based biomedical and microfluidic applications

We describe a new fabrication process utilizing polydimethylesiloxane (PDMS) as a sacrificial substrate layer for fabricating free-standing SU-8-based biomedical and microfluidic devices. The PDMS-on-glass substrate permits SU-8 photo patterning and layer-to-layer bonding. We have developed a novel PDMS-based process which allows the SU-8 structures to be easily peeled off from the substrate after complete fabrication. As an example, a fully enclosed microfluidic chip has been successfully fabricated utilizing the presented new process. The enclosed microfluidic chip uses adhesive bonding technology and the SU-8 layers from 10 µm to 450 µm thick for fully enclosed microchannels. SU-8 layers as large as the glass substrate are successfully fabricated and peeled off from the PDMS layer as single continuous sheets. The fabrication results are supported by optical microscopy and profilometry. The peel-off force for the 120 µm thick SU-8-based chips is measured using a voice coil actuator (VCA). As an additional benefit the release step leaves the input and the output of the microchannels accessible to the outside world facilitating interconnecting to the external devices. (Some figures in this article are in colour only in the electronic version)

A sacrificial SU-8 mask for direct metallization on PDMS

A new fabrication technology utilizing SU-8 as a sacrificial mask for metallization of the PDMS surface is presented. The sacrificial SU-8 layer process offers superior performance for reliable and repeatable metallization on the PDMS layer. Sacrificial SU-8 masks from 45 µm to 250 µm thickness are successfully fabricated on the PDMS layer to pattern gold on the PDMS surface. These layers are successfully peeled off from the PDMS surface after a metal deposition step. Metal lines from 10 µm to 500 µm wide and 1 mm to 50 mm long are successfully patterned and tested. Furthermore, the sacrificial SU-8 mask can be removed within minutes to realize metal patterns on the PDMS surface and does not leave any residue after removal of the SU-8 layer. As this new process is intended for use in fabrication of microfluidic and biomedical microdevices, electrodes of an electro-enzymatic glucose sensor are presented to demonstrate the technology.

Flexible Three-Dimensional Electrochemical Glucose Sensor with Improved Sensitivity Realized in Hybrid Polymer Microelectromechanical Systems Technique

Background: Continuous glucose monitoring for patients with diabetes is of paramount importance to avoid severe health conditions resulting from hypoglycemia or hyperglycemia. Most available methods require an invasive setup and a health care professional. Handheld devices available on the market also require finger pricking for every measurement and do not provide continuous monitoring. Hence, continuous glucose monitoring from human tears using a glucose sensor embedded in a contact lens has been considered as a suitable option. However, the glucose concentration in human tears is very low in comparison with the blood glucose level (1/10–1/40 concentration). We propose a sensor that solves the sensitivity problem in a new way, is flexible, and is constructed onto the oxygen permeable contact lens material. Methods: To achieve such sensitivity while maintaining a small sensor footprint suitable for placement in a contact lens, we increased the active electrode area by using three-dimensional (3-D) electrode micropatterning. Fully flexible 3-D electrodes were realized utilizing ordered arrays of pillars with different shapes and heights. Results: We successfully fabricated square and cylindrical pillars with different height (50, 100, and 200 μm) and uniform metal coverage to realize sensor electrodes. The increased surface area produces high amperometric current that increases sensor sensitivity up to 300% using 200 μ tall square pillars. The sensitivity improvement closely follows the improvement in the surface area of the electrode. Conclusions: The proposed flexible glucose sensors with 3-D microstructure electrodes are more sensitive to lower glucose concentrations and generate higher current signal than conventional glucose sensors.

SU-8- and PDMS-based hybrid fabrication technology for combination of permanently bonded flexible and rigid features on a single device

In this article, a novel hybrid fabrication technology is presented that uses both a flexible polymer (polydimethylsiloxane-?PDMS) and a rigid polymer (SU-8). A covalent bond between the flexible and rigid polymer layers is achieved using an oxygen plasma treatment during a layer-by-layer direct spin-on process. Precise alignment of the features in each layer and a highly repeatable method are achieved by this new process. As a proof-of-concept, we successfully fabricated PDMS-based flexible microfluidic devices with SU-8-based rigid world-to-chip/chip-to-world interconnects. The bond strength between the PDMS and SU-8 layers is measured by three methods: (1) Instron??microtester to pull apart the layers; (2) voice coil actuator to test the bond between interconnects and the substrate; and (3) microfluidic pressurization test to evaluate the bond strength along the channels. The bond strength between the flexible PDMS layer and the rigid SU-8 features is very strong; the bond between these two polymers does not fail during these evaluations although the integrity of the PDMS layer itself fails during the microtester evaluation. Additionally, the layer-by-layer direct spin-on process resulted in a repeatable process and precise alignment of the features in each layer, which are necessary in order to achieve consistent performance from the fabricated devices. The rigid SU-8 interconnects fabricated onto a flexible PDMS device serve as a world-to-chip/chip-to-world interconnects for the direct connection with Tygon??tubing. Three different designs of hybrid (PDMS and SU-8 based) microfluidic devices are designed, fabricated and tested. Each variation differed in the microchannel design in order to demonstrate the versatility of the process to make devices on multiple scales and patterns. These hybrid microfluidic devices are capable of functioning without leakage up to pressures of 85.85 ?3.56 kPa. Although microfluidic channels with interconnects are shown as a proof-of-concept, the fabrication process demonstrated herein could be utilized to develop a number of more sophisticated microfluidic and biomedical devices.

Flexible glucose sensor utilizing multilayer PDMS process

In this paper, a flexible glucose sensor with gold electrodes sandwiched between two polydimethyle siloxane (PDMS) layers is presented. PDMS is used as a flexible and bio-compatible sensor substrate material. Furthermore, PDMS is optically transparent and haemocompatible which is very important for implantable sensors.

Tungsten Lamps as an Affordable Light Source for Testing of Photovoltaic Cells

An improved Tungsten light source system for photovoltaic cell testing made from low-cost, commercially available materials is presented as an alternative to standard expensive testing equipment. In this work, spectral correction of the Tungsten light source is achieved by increasing the color temperature to ∼5200 K using inexpensive commercially available filters. Spectral measurements of the enhanced light source reveal that a better spectrum match towards the solar spectrum is achieved than what has been previously demonstrated. Specifically, the improved solar spectrum match is achieved by substantial filtering of the infrared range. The proposed setup is used to evaluate the performance of both silicon and organic based photovoltaic cells.

Hybrid polymer fabrication process for electro-enzymatic glucose sensor

We present a novel self-aligned and hybrid polymer fabrication process for an electro-enzymatic glucose sensor. The self-aligned fabrication process is performed using polydimethylsiloxane (PDMS) as a process substrate material, SU-8 as a sensor structural material, and gold as an electrode material. PDMS has many advantages as a process substrate over conventional substrates such as bare silicon or glass. During the fabrication process, SU-8 has good adhesion to the PDMS. However, after completion of all fabrication steps, the SU-8 based sensors can be easily peeled-off from the PDMS. The PDMS is prepared on a glass handle wafer, and is reusable for many process cycles. Such an SU-8 release technique from a PDMS substrate has never been proposed before. The novel process is employed to realize a glucose sensor with active and reference gold electrodes that are sandwiched between two SU-8 layers with contact pad openings and the active area opening to the top SU-8 layer. The enzyme glucose oxidase is immobilized within the confined active area opening to provide an active electrode sensing surface. After successful fabrication using the hybrid process, the overall thickness of the sensors is measured between 166.15 μm and 210.15 μm. The sensor area and the electrode area are 2mm x 3mm and 2mm x 2mm respectively. The resulting glucose sensors are mechanically flexible. A linear response is observed for the glucose sensors, typically between 50mg/dl and 600mg/dl glucose concentrations.

Optimized organic photovoltaics with surface plasmons

In this work, a new approach for optimizing organic photovoltaics using nanostructure arrays exhibiting surface plasmons is presented. Periodic nanohole arrays were fabricated on gold- and silver-coated flexible substrates, and were thereafter used as light transmitting anodes for solar cells. Transmission measurements on the plasmonic thin film made of gold and silver revealed enhanced transmission at specific wavelengths matching those of the photoactive polymer layer. Compared to the indium tin oxide-based photovoltaic cells, the plasmonic solar cells showed overall improvements in efficiency up to 4.8-fold for gold and 5.1-fold for the silver, respectively.

Electro-Enzymatic Sensor for Non-Invasive Glucose Measurement

In this article, a cost effective and simple electro-enzymatic sensor to measure glucose concentration non-invasively is proposed. Measurement of blood glucose concentration for early diabetes detection is one of the major health concerns. Usually, blood or other biofluid is collected by puncturing the finger or fore arm (invasive) which is a painful process. Hence, blood glucose measurement by non-invasive method is more desirable. Here, we propose a first step towards our final goal of non-invasive glucose measurement from human tears using contact lenses.

Authentication and process control system based on optical variable nanostructures

A new authentication and security system together with a nano-key concept using optical variable nanostructures (OVNs) are introduced. The three proposed designs are 1) the passive authentication, 2) the active authentication, and 3) the nano-key authentication. The passive authentication is obtained by insertion of an OVN image divided between multiple layers of the fabrication process. Authentic fabrication process is validated when the proper alignment (reconstructed image, for example) at the end of fabrication is achieved. The active and the nano-key authentication systems are implemented to directly interact with the active CMOS circuitry. A simple proof-of-concept for all introduced OVN-based authentication options are presented along with the optical images of the resulting authentication patterns.