This invention relates to microscale systems and methods for molding and grafting materials on a planar substrate. More specifically, this invention relates to the molding and grafting of highly uniform and very thin layers of polymeric materials onto the surface of electronically addressable microchips and other small substrate surfaces.
The following description provides a summary of information relevant to the present invention. It is not an admission that any of the information provided herein is prior art to the presently claimed invention, nor that any of the publications specifically or implicitly referenced are prior art to the invention.
The application of thin films to substrate surfaces on a microscale level has predominantly been an issue in the electronics industry. Such application has also predominantly involved the use of spin coating and masking lamination technologies. However, with respect to applying thin films comprising permeable polymer layers to electronically addressable microchips used in the isolation and detection of biomaterials, neither masking lamination or spin coating provide particularly optimal results.
For example, the use of spin coating of permeation layers on substrates comprising electronically addressable microchip cartridge designs that are not perfectly planar often results in problems obtaining uniform layers in selected regions of the microchip. Consequently, spin coating onto nonplanar surfaces results in wide thickness variations of the applied permeation layer.
The working requirements of the microchips include very tight tolerances in applied and realized electronic potentials and currents at the surface of the layer above the electrodes of the microchip. Therefore, permeation layer thicknesses are required to be uniform. Variations in the thickness result in uncontrollable variables when attempting to transport biomaterials among the electronically addressable capture sites of the microchip.
In another aspect, when agarose is applied via spin coating, the process must be kept within a specific temperature range for proper spreading of the agarose. If the temperature is too low, the agarose will congeal prematurely and not spread properly. Where synthetic hydrogels based on monomeric solutions are to be applied by spin coating and subsequently crosslinked, it is necessary to add soluble polymers to the hydrogel solution in order to increase the viscosity so that films of appropriate thickness will be realized. However, addition of viscosity enhancing polymers changes the final composition of the permeation layer as well as the performance characteristics of the layer in allowing polyelectrolyte and ion electrophoresis between the electrode and the top surface of the layer. Spin coating is further problematic in that it requires high velocities for radial spreading of the monomers or monomer/polymer mixtures. Such high velocities can cause damage to the substrate.
Other methods have also been employed to provide a more uniform thickness. For example, in an attempt to cast thin films onto microchips, a coverslip method has been used wherein a coverslip is applied directly to a solution of a polymerizeable material prior to actual polymerization with the idea that the coverslip would provide for a uniform polymerized layer on top of the microchip. Although such a method improves surface uniformity, there is a large variability of thickness which contravenes the application of such a method where highly sensitive electronic addressing and high volume manufacturing is of concern.
In still other microscale molding applications, some processes use pressurized molds where the mold is pressurized between 1 and 50 atm to prevent the formation of voids or volume shrinkage upon polymerization. (Micro and Nano Patterning Polymers, Oxford University Press, 1993, ISBN 0841235813) Still other systems use solution injection or component mixing in the microreaction mold.
In contrast, we have developed a simple microreaction molding system that generates highly uniform films directly on the surface of a substrate, such as a microchip, which avoids thickness variation problems experienced in spin coating and coverslip oriented polymerization techniques. Moreover, we have developed a means for directly forming permeation layers having reproducible thickness onto electronically addressable microchips and other substrates. This invention may be applied to attachment of multiple thin films or layers of a hydrogel and grafts of polymeric materials on a substrate in a manner that is applicable to high volume and inexpensive manufacture. This molding system further provides for creating a substrate having multiple permeation layers having a multiplicity of characteristics.
In one embodiment of the invention, a molding system is provided comprising a two-part mold having a transparent window element and a metal or polymer mold casing or frame. Preferably, metal used for the casing can be 304 stainless steel, 316 stainless steel or titanium. For embodiments using a polymer casing, examples of a suitable polymer include polytetrafluoroethylene, polyfluoroalkoxane (PFA), and polyetheretherketone (PEEK).
In this two-part mold embodiment, the transparent window can be made of any material that will allow the transmission of at least one wavelength of electromagnetic radiation, in particular ultra violet (UV), visible (Vis), and infrared (IR). In preferred embodiments, acceptable window materials may comprise crystalline substances such as fused silica, quartz, sapphire, geranium, silicon, or glass, or organic polymeric materials such a plexiglass, polyacrylates, and polycarbonates. The window element further comprises an upper surface that serves as a xe2x80x9cbase platexe2x80x9d or mold bottom that is fitted into the mold frame. This surface generally can comprise any contour for making a patterned surface to the material being molded as a thin film. In a preferred embodiment, such surface is planar. By planar is meant a surface that has vertical height variations less than 1 xcexcm. By patterned is meant a surface which has a vertical contour variations greater than 1 xcexcm. Whether planar or patterned, the window element and its upper surface affording the radiation access to the mold cavity.
In order for the window element to form the mold bottom, the upper surface of the window element is offset from the top of the frame element thereby forming a mold cavity. In a preferred embodiment, the offset is between 100 nanometers and 100 xcexcm below the frame surface. Additionally, this offset is variable by the fact that the window element is xe2x80x98slidablyxe2x80x99 connected to and encased by the frame element. By slidably is meant that the position of the window element is adjustable by sliding the window inside the frame element.
In another embodiment, the system couples photopolymerization and molding into a single process. In this embodiment, the window is transparent to UV light energy which is used to initiate photopolymerization of a reactive monomer solution that upon polymerization becomes the thin film which in turn becomes attached to the surface of the microchip or other substrate during polymerization. In this system an UV light initiator such as 50% 2,4,6-trimethylbenzoyl diphenylphosphine oxide and 50% 2-hydroxy 2-methyl-1-phenyl-propan-1-one (D 4265) may be used with a polymerizable monomer to initiate polymerization.
In another embodiment, polymerization initiation may be carried out using a window that is transparent to thermal energy in the form of IR irradiation. In this embodiment, the monomer solution and mold are heated to a specific temperature range thereby causing activation of heat sensitive polymerization initiators such as azobisisobutyronitrile (AIBN).
In a further embodiment, polymerization may be carried out using chemical energy wherein monomer polymerization takes place over a short period of time in the mold cavity using chemical initiators such as ammonium persulfate/tetramethyl ethylenene diamine (APS/TEMED).
In still further embodiments, window elements that are transparent to visible light may be used where chemical initiators that are sensitive to visible light wavelengths are used in the polymerization reaction.
In another embodiment, the microchip or other substrate forms a top or xe2x80x98capxe2x80x99 of the mold system. Having the substrate comprise one of the sides of the mold acts to simplify the mold system while at the same time providing a mechanism for defining the thickness of the film, as determined by the space between the substrate surface and mold cavity bottom.
In another embodiment, the two-part mold system comprises a means for filling the mold well using ports that allow the monomer solution to flow into the mold from one side to the opposite side of the mold where there is an exit port for excess fluid.
In still another embodiment, the mold system may also comprise instead of a two-part mold, a single-part mold or xe2x80x98noncompositexe2x80x99 mold comprising a plate that is transparent to at least one wavelength of electromagnetic radiation. In this embodiment, a mold cavity is formed in an upper surface of the plate to a depth of between 100 nanometers and 100 xcexcm. In a preferred embodiment, the plate may be made of materials such as fused silica, quartz, sapphire, geranium, silicon, glass, polyacrylates, polycarbonates, and plexiglass. In this embodiment, the substrate also serves as a top to the mold. In a further embodiment, such noncomposite molds may comprise either individual xe2x80x98mold blocksxe2x80x99 which can further be attached to a surface so as to make an array of molds, or an array of individual molds can be formed directly into the surface of a single block of such material.
In yet another embodiment where UV radiation initiation of polymerization is used, thin films may be molded to a substrate within seconds after UV irradiation. Moreover, such films having high uniformity with one another may be reproduced from the molds onto individual substrates.
In still other embodiments, the system of molding thin films to substrate surfaces may be carried out in a repetitive fashion so as to allow for the formation of a multiplicity of thin film layers onto the surface of a single substrate. In this embodiment, it is contemplated that each layer will provide a specific property to the permeation layer as desired.