Further improvements have recently been made in the development of the devices described in “Improved Ion-Channel Biosensors” (NPO-30710), NASA Tech Briefs, Vol. 28, No. 10 (October 2004), page 30. As discussed in more detail in that article, these sensors offer advantages of greater stability, greater lifetime, and individual electrical addressability, relative to prior ion-channel biosensors.
In order to give meaning to a brief description of the recent improvements, it is necessary to recapitulate a substantial portion of the text of the cited previous article. The figure depicts one sensor that incorporates the recent improvements, and can be helpful in understanding the recapitulated text, which follows:
These sensors are microfabricated from silicon and other materials compatible with silicon. Typically, the sensors are fabricated in arrays in silicon wafers on glass plates. Each sensor in the array can be individually electrically addressed, without interference with its neighbors. Each sensor includes a well covered by a thin layer of silicon nitride, in which is made a pinhole for the formation of a lipid bilayer membrane. In one stage of fabrication, the lower half of the well is filled with agarose, which is allowed to harden. Then the upper half of the well is filled with a liquid electrolyte (which thereafter remains liquid) and a lipid bilayer is painted over the pinhole. The liquid contains a protein that forms an ion channel on top of the hardened agarose. The combination of enclosure in the well and support by the hardened agarose provides the stability needed to keep the membrane functional for times as long as days or even weeks.
An electrode above the well, another electrode below the well, and all the materials between the electrodes together constitute a capacitor. What is measured is the capacitive transient current in response to an applied voltage pulse. One notable feature of this sensor, in comparison with prior such sensors, is a relatively thick dielectric layer between the top of the well and the top electrode. This layer greatly reduces the capacitance of an aperture across which the ion channels are formed, thereby increasing the signal-to-noise ratio. The use of a relatively large aperture with agarose support makes it possible to form many ion channels instead of only one, thereby further increasing the signal-to-noise ratio and effectively increasing the size of the available ionic reservoir. The relatively large reservoir makes it possible to measure AC rather than DC. This concludes the recapitulation from the cited previous article.
The improvements include the following:
- The microfluidic channels through which agarose is wicked into the lower halves of the wells are fabricated in a reusable layer of polydimethylsiloxane [PDMS (commonly known as silicone rubber)]. This layer of PDMS forms a hermetic seal with the underlying glass plate and the overlying silicon chip, but can be removed and washed, making the array reusable.
- Before forming the lipid bilayer over the pinholes in the silicon nitride layer, the silicon nitride is coated with a self-assembled monolayer, which serves to stabilize the lipid bilayer, thereby making the array into an even more stable device.
- The lipid bilayer is formed rapidly by means of a spin-coating procedure that can be performed by a worker without special skill.
This work was done by Jay L. Nadeau, Victor E. White, Joshua A. Maurer, and Dennis A. Dougherty of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-40560, volume and number of this NASA Tech Briefs issue, and the page number.