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Parylene Gets Promoted

Fri, 02/05/2010 - 6:10am
Bill Boyd, Equipment Manager, Specialty Coating Systems



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SCS Labcoter 2 provides a convenient system to facilitate experimentation with parylene in the realm of biomedical and life sciences. Photo courtesy of Specialty Coating Systems
Parylene basics

Most forms of parylene coatings are inert and biocompatible, and they have excellent moisture, chemical and dielectric barrier properties. These ultra-thin polymers are applied using a vapor deposition polymerization process, which allows precise control of coating rate and thickness.

The process begins with a powdered precursor or “dimer,” which is vaporized under vacuum with heat. This creates a dimeric gas that is pyrolized to cleave the dimer into its monomeric form and deposited as a transparent polymer film onto the receiving surface, which can be any dimension and format. As a vapor, parylene deposition can ingress into all areas—even nano-crevices, holes and niches—to provide 100% coverage without bridging of openings or pooling.

The spontaneous polymerization process takes place at ambient temperatures and does not involve solvents, catalysts, plasticizers or cure forces. Typical thickness for parylene coatings ranges from several hundred angstroms to 75 microns.

For laboratory research, development, and testing, portable systems such as the SCS Labcoter 2 from Specialty Coating Systems facilitate experimentation with parylene in biomedical and life science applications. Such systems feature closed-loop monomer pressure control and continuous process monitoring to ensure reliable and repeatable application of parylene coatings. Other options include interchangeable chamber modules, tumble coating capabilities and chart recorders.
Application examples

Yu-Chong Tai, Ph.D., professor of electrical engineering and bioengineering at the California Institute of Technology (Caltech), has been using parylene for a range of applications to create unique nano-devices.

In his cleanroom laboratory, Tai can deposit a variety of standard thin-film materials and add parylene to the mix. This allows him to combine mono-layers of various types of materials to assemble interesting composite structures.

Basically, Tai is working with “combined semiconductor” deposition technology on a laboratory scale and adding the unique properties of parylene. Laboratory systems provide the same process as basic semiconductor deposition, only on a convenient, research-based level.

For the past ten years, the National Institutes of Health (NIH) has been funding Tai to make micro implants made partly from parylene. These devices, which differ from traditional implants such as pacemakers, have been implanted into the brain for neurostimulation and recording.

Cornell University is also involved in research using parylene materials. The Cornell NanoScale Science and Technology Facility (CNF) focuses on a wide range of semiconductor processing equipment for building nano-devices.

Supported by the National Science Foundation (NSF), the National Nanotechnology Infrastructure Network (NNIN)—an integrated partnership of fourteen user facilities, including the CNF—provides superior opportunities for nanoscience and nanotechnology research. The network provides support in nanoscale fabrication, synthesis, characterization, modeling, design, computation, and training in an open, hands-on environment available to all qualified users.

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Figure 1. Parylene filter membranes aid in cancer cell research. Graphic courtesy of California Institute of Technology Click to enlarge.
According to the CNF’s Rob Ilic, Ph.D., research associate and user program manager, parylene is an excellent combination tool. Without the use of an adhesion promoter, it forms a loose bond that becomes a flexible membrane, which enhances patterning applications. Using an adhesion promoter in the deposition process, parylene forms a tight, pinhole-free coating that conforms to the exact dimensions of the device. This makes it is a versatile tool for a variety of projects and experiments because it can be used both as a coating and as a structural component.

Ilic was the first at Cornell to promote using parylene across a range of applications. Today it is widely used in numerous areas of research, and each has its own laboratory deposition systems.

Ocular implants

Recently, a sensor has been developed in Tai’s lab that can be put inside the eye to measure intraocular pressure. The device measures 2 x 3 mm in size and can be injected via a needle. Using this device, doctors can wirelessly read the pressure within the eye. Other biomedical materials are combined with parylene to make this device biocompatible.

Tai notes that in his lab, parylene is never just a coating but also a structural material. The retinal implant goal is to make blind people see again, and the device uses parylene because of its biocompatibility.

Cornell has also contributed to developing ocular devices using parylene but in a more standard use, combining leading edge technology with the coating benefits of parylene. The goal of this retinal implant project is to create an artificial eye. The device comprises a micro CCD image sensor that is attached to the back of the eye using tiny electrodes. Attached to the retina, the electrodes are coated with biocompatible parylene to minimize rejection.

Cell patterning

Early on, Ilic recognized the benefits of parylene for patterning biological modules. If no adhesion promoter is used in the parylene deposition process, the deposited film can be lithographically shaped and easily removed from the underlying substrate. This creates a biocompatible membrane for which researchers have many uses.

Patterning biological modules is a difficult process that involves the cell and solvents typically used to pattern surfaces or pattern metals using blister-off processes.

Recently, Cornell published a paper describing the use of parylene as a dry lift-off material to pattern biological modules and study the interactions between cells. Many different biological modules can be patterned using this technique, which borrows from the semiconductor deposition layering process.

The dry lift-off technique uses a conformal parylene layer deposited without an adhesion promoter. It is photolithographically patterned using standard UV-sensitive photoresists and dry etching in a reactive ion etch (RIE) chamber using oxygen plasma. The patterned parylene layer acts as a template to physically pattern biomolecular materials.

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Figure 2. Process flow schematic of dry-peel fabrication steps:
A = Photoresist patterning using optical lithography.
B = Reactive ion etching of parylene and removal of top photoresist layer.
C = E. coli antibody and cell immobilization.
D = Peeling of parylene, resulting in antibody-cell pattern.
Graphic courtesy of Cornell University

Click to enlarge.
When the parylene film is lifted from the substrate, the result is a geometrically defined region, the negative image of the parylene pattern of immobilized biological material. Dry lift-off provides an efficient and economical method for reproducing large-scale patterns of biological materials using established lithographic processes.

According to Ilic, parylene does not interact with acetone or other solvents used to remove resist layers. Because parylene is both chemically inert and thermally stable over a range of temperatures, selective immobilization of a variety of biomolecular materials and other chemically sensitive layers is possible.

Cancer cell filtering

Caltech is developing some interesting filter membranes out of parylene, and the biggest application is in studying cancer in circulating tumor cells, says Tai. During metastasis, tumor cells are released from the main tumors into the bloodstream. The parylene filters separate the tumor cells from the healthy cells.

Traditional semiconductor deposition technology is used to make these filters, drawing on the innate properties present in parylene. Caltech is currently in the stage of giving these filtering devices to physicians who are validating the results observed in the lab.

Lab-on-a-chip nanotechnology

Parylene’s vapor deposition process makes it applicable to semiconductor-type engineering, particularly for micro sensors and micro actuators. In device engineering, the three types of micro-electromechanical systems (MEMS) are electronics or circuits; sensors, including pressure, flow, magnetic field, thermometers and gyroscopes; and actuators, which provide power to move things. Putting these technologies together opens up interesting areas such as lab-on-a-chip devices and micro-sized biomedical sensors.

Tai first made micro valves and micro pumps out of parylene and then moved on to flow sensors and other flexible, three-dimensional devices needed for lab-on-a-chip devices. In this area, he is personally working on a project for blood analysis where a single drop of blood on one microchip can tell everything about the blood in a matter of minutes, whereas a full laboratory utilizing multiple vials of blood requires days.

For example, Tai’s chip can use one drop of blood to tell how many white blood cells are present in each type: neutrophil, eosinophil, basophil, lymphocyte and monocyte. It can also check other information, such as the ion and oxygen concentration.

These types of tests typically involve numerous liquids—blood, urine and reagents. But this analysis can be done on a tiny chip as long as the micro-valves, pumps and sensors are constructed of parylene and other materials and are on the chip.

Conclusion

For the last fifteen years, Cornell and Caltech have been working with coating systems and materials from SCS and are taking parylene to a new level using systems specially designed for labs. There are hundreds of polymer formulations—some are FDA-approved, some are not. Because of their inherent biocompatibility and unique deposition method, the parylenes present many opportunities to the biomedical and life science fields.

Coating systems developed specifically for laboratory use are ushering in a new generation of applications for parylene, many of which are now moving from the laboratory into commercialized testing.

For more information, contact Bill Boyd, bboyd@scscoatings.com; Yu-Chong Tai, Ph.D., yctai@mems.caltech.edu; or Rob Ilic, Ph.D., rob@cnf.cornell.edu.

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