Integration of Hydrogels and Plastics into Microfabrication Processes towards a MEMS RF-Interrogated Biosensor

Very inexpensive biosensors capable of monitoring biometrics data of a living organism have long been a goal of biomedical device technology. As such the integration of polymers and plastics are a good solution to achieve low cost and powerless devices. This dissertation presents experimental results on the integration of hydrogels (a class of polymer which swells in water) into microfabrication processes and the development of a new, all-plastic pH sensor. In particular, the work done on oxygen diffusion in hydrogels allows a major improvement in hydrogel spin-casting. It is shown that dynamic oxygen diffusion in hydrogel film before photo-polymerization is a strong inhibitor to polymerization. This effect can be used to pattern hydrogels. It is also demonstrated that hydrogels can be considered a microfabricated material while maintaining their swelling properties. These hydrogels can be spin-cast, patterned using UV light, plasma or oxygen. Finally, after deposition of the appropriate adhesion molecule, hydrogels can be deposited on silicon or plastics to fabricate sensors. The process outlined in this dissertation makes it possible to deposit and pattern thin hydrogel films onto silicon and then plastic wafers. In turn, these new methodsenable the fabrication of the first, all-polymer microfabricated pH sensor.
Low cost biosensors for pH or other analytes such as glucose have been extensively researched because they enable rapid prototyping and single-use devices. Moreover, such devices, if coupled with wireless capabilities, could be used to create a spatially-distributed network of sensors. This could be useful in healthcare, agriculture and environmental monitoring. In this instance, the integration of hydrogels and plastics into a device is the first step towards a MEMS RF-Interrogated Biosensor. The long range goal is to determine, in vivo, the pH and eventually the sucrose concentration (Brix) in grapes as they grow on the vine. The importance of this sensor when combined with environmental data for the improvement of viticulture and agriculture practices cannot be overstated. Current systems require a worker to walk along the vines, picking grapes and pressing them into a hand-held instrument. This is a destructive, ”snapshot” measurement at best and trend analysisis difficult to obtain. Bulky wireless environmental sensors are in development, yet none to date can make enological measurements in vivo. The proposed device, on the other hand, makes possible inexpensive real-time enological data from the field.
In future work, this technology can be integrated into a wireless device to create an inexpensive, wireless and powerless biosensor. The hydrogel volume swelling induces a change in capacitance. By integrating this capacitance into a LC tank circuit, the change in capacitance relates to a change in resonance frequency. This can be detected using a simple RF interrogation through a passive scheme (similar to Radio-Frequency Identification systems) in which the need for local power is made unnecessary. The final piece is the integration of this device onto a microneedle support. The microneedle allows to go through the skin of the grapes or maybe through a human skin. Such a device can be useful for the wine industry as a real-time, inexpensive and powerless sensor.
Publication date: 
December 31, 2007
Publication type: 
Ph.D. Dissertation
Payden, S. (2007). Integration of Hydrogels and Plastics Into Microfabrication Processes Towards a MEMS RF-interrogated Biosensor. (n.p.): University of California, Berkeley.

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