Patent Application
20120219727
The present invention relates to methods of surface treating microfluidic devices and more particularly to surface treating of channels or cavities within a fabricated microfluidic device by provision of a layer thereon. Depending on the nature of the materials forming the layer such a layer may provide a barrier layer preventing subsequent material ingress to the substrate material of the microfluidic device, or may just selectively modify the properties of that portion of the channel or cavity on which the layer is provided.
Microfluidic devices are well known in the art and typically comprise a plurality of individual cavities or fluid channels defined within a substrate and through which a fluid may be stored or flow. The dimensions of the individual cavities or channels are typically of the order of a human hair. Usually the length of such channels is much greater than their width. Such kinds of channels, their geometrical variations and their networks are used in the microfluidic devices for various purposes such as DNA sequencing, separation by electrophoresis, cell sorting and culturing, biomolecular analysis, biological and chemical synthesis. Within the art, the development of microfluidic devices not only introduced possible miniaturization of the existing analytical technologies but also new opportunities to conduct novel experiments in non conventional formats for mining information otherwise difficult to obtain.
Among the materials for the fabrication of microfluidic devices, polydimethylsiloxane (PDMS) has been a most popular material, offering a range of advantages such as, rapid prototyping, inertness, biocompatibility, optical transparency, and permeability, etc. Despite these advantages, the surface chemistry of the microfluidic channels in PDMS remains a major issue since organic solvents or small molecules can penetrate through PDMS substrate. Furthermore, it provides a hydrophobic surface which promotes nonspecific adsorption or even absorption of small molecules into the bulk mass of the PDMS substrate. This non-desired adsorption can affect the ultimate analysis for which the microfluidic device is being used. It will be appreciated that PDMS is an example of a silicon based organic polymer
A further disadvantage arising from the use of PDMS substrates is related to the absence of functional groups on the PDMS surface which reduces the possibility of covalently immobilizing bio-molecules or other custom designed functionalities. Such functionalities are available using other substrates such as for example glass.
Despite these apparent limitations, PDMS remains a preferred choice for use in fabricating microfluidic devices and attempts have been made to address the issues associated with the nature of the substrate material. Examples of known attempts which are used to resolve the hydrophobic issue is to expose the channels defined within the PDMS substrate to ozone or oxygen plasma for a short time which renders the surface hydrophilic usually for less than an hour. While this addresses the issues of hydrophobicity, it is very much time delimited and requires processing concurrently with the testing that is to be conducted. Other approaches which do not require such concurrent processing include various wet chemical strategies such as graft polymerization, silanization and, adsorption of detergents, proteins, polyelectrolytes, and sol-gel based coating. While these approaches allow for a surface treatment of the channel or cavity surfaces in advance of use of the microfluidic devices, these techniques are tedious, require skilful handling, Moreover, multiplexing of such methods on large scale may perhaps induce complications in process parameters which involve flushing/storing reagents or chemicals that are not environmentally friendly.
There is therefore a continued need for devices and methodologies for fabricating such devices which overcomes these and other problems.
These and other problems are addressed by surface treating surfaces of fluidic channels or cavities of a previously fabricated microfluidic device having a plurality of fluidic channels defined therein by providing the microfluidic device within a gas plasma reactor and using the gas plasma reactor to generate a surface layer on individual surfaces of the plurality of fluidic channels, the surface layer resultant from a gas diffusion process. Judicious selection of a precursor compound used within the gas plasma reactor can provide for generation of a layer on the individual surfaces. If the properties of the layer are such as to provide a barrier layer that barrier layer will operably minimise the ingress of materials through the layer from the fluidic channels and into the bulk substrate of the microfluidic device. The invention also provides a method of fabricating a microfluidic device comprising providing one or more channels or cavities within a substrate and effecting through a plasma generated gas deposition process the generation of a surface or barrier layer on individual ones of the channels or cavities. Desirably the surface or barrier layer is generated through the chemical adsorption of a metalloid oxide such as a silicon oxide layer on the surface of the individual channels or cavities.
The invention therefore provides a method according to claim 1 with advantageous embodiments being detailed in the dependent claims. The invention also provides a method of surface treating surfaces of channels or cavities defined within a microfluidic device according to claim 34 or 37 with advantageous embodiments provided in the dependent claims thereto. These and other features of the invention will now be described with reference to exemplary embodiments of the invention.
The teaching of the present invention will now be described with reference to an exemplary arrangement whereby a silicon oxide barrier layer is provided on individual surfaces of channels or cavities defined within a PDMS based substrate microfluidic device. Using this exemplary arrangement a method of formation of a barrier layer composed of silicon oxides in microcavities of assembled polymeric microfluidic devices for use in biological assays or chemical analyses known as biochips or lab-on-a-chip devices will be described. Furthermore, methodologies providing for the surface modification of covered microfluidic channels in polydimethylsiloxane (PDMS) substrate using a dry chemical process of plasma enhanced chemical vapour deposition (PECVD) will be described. The surface modified PDMS showed presence of silicon oxide (SiOx) moieties on surface responsible for inducing surface functionalization to Si—O− (silanol groups) and hydrophilic character to the channels. The presence of Si and O in the barrier was characterised by using energy dispersion x-ray spectroscopy. It will be understood and appreciated by the person skilled in the art that these exemplary arrangements are provided to assist in an understanding of the teaching of the present invention and it is not intended to limit the teaching to such exemplary arrangements as modifications can be made without departing from the scope of the present teaching.
In accordance with this exemplary arrangement, it is possible to fabricate and form a glass like highly efficient barrier on PDMS microcavities. It will be appreciated that the microfluidic device may be provided in a monolithic structure or may be fabricated from two or more layers that are bonded to one another. These layers may be fabricated from the same or different materials. In contrast to wet chemical treatments employed by prior art arrangements, the methodology employed in accordance with the present teaching is based on gas phase reaction.
It will be appreciated by those of skill in the art that plasmas with complex chemistry are composed of a multitude of atomic and molecular species. Within the context of the present teaching the make-up of the plasma is determined by a capacity of specific atomic and molecular species to react with the surfaces of channels or cavities within a microfluidic device to form a thin film or etch the surface. These ‘radicals’ are usually formed by dissociation of the feedstock gases introduced into the plasma chamber 131. The relative densities of these radicals depend on the power coupled into the plasma, the precise gas mixture, and the chamber geometry.
Within the context of the present teaching, it is understood that the species which form the film in non-line-of-sight or high aspect ratio regions of the device, reach their reaction site by diffusion. This diffusion process if governed by several parameters, including the energy and cross sections of the species, surface sticking coefficients, and pressure.
The densities of the species that result in the formation of the film determine the deposition rate and the relative densities of the depositing species determine the film stoichiometry. The presence of other species can often result in a net removal rather than a deposition rate due to the presence of energetic species which cause sputtering or chemical etch.
Using this understanding the present inventors have provided for an optimisation of the plasma process to ensure the desired chemical composition of the plasma is achieved. There are several sensor techniques for monitoring and controlling the relative density of species in a plasma such as optical emission spectroscopy (OES), mass spectroscopy, laser absorption spectroscopy, etc. In the case of optical emission spectroscopy, species emit light at characteristic wavelengths which indicate the presence of these species. The intensity of the emitted peak is often an indicator of the density of the species, albeit it is not a representation of the actual absolute density of the species. This can be achieved by more advanced analysis such as actinometry.
By controlling parameters such as the density of the depositing species (and the chemical etch species), through a combination of process set-points and process sensors, it is possible within the context of the present teaching to optimize the densities of the radical species available for diffusion into the channels.
It will be understood that the potential applied within the gas plasma reactor may be selected dependent on the source available and the intended process parameter requiring control; for example it may be an RF source but microwave or other potential sources may also be used. When precursors in the form of O2 gas and HMDSO vapours are introduced (Step 155) into the chamber, the fragments (radicals, ions or elemental species) 110 of these molecules are produced, and as will be appreciated by those skilled in the art, as a result of the bombardment of energetic electrons and ions in the plasma. It is believed that since the charged species oscillate with the RF power in the plasma, that the radicals of Si and O are more likely to migrate into the microcavities by diffusion These radicals but nonetheless reactive fragments diffuse into the cavities defined within the previously assembled or fabricated microfluidic device through their open inlets, survive, and react on the surfaces within the channels 105 to form a thin glassy barrier 120 (Step 160). It will be appreciated that the formation of the “glassy” barrier is representative of the properties of the silicon oxide layer that is formed. The layer formed is through a gas phase diffusion process, and the radicals being adsorbed onto the surface to form chemical bonds at favorable sites to raise an amorphous network. It has been found that providing oxygen to HMDSO ratios greater than 10, more homogeneous SiOx radicals are produced and the film property approaches to that of silica-like. In one exemplary arrangement, the flow rates of O2 and HMDSO, were selected to be 500 sccm and 16 sccm respectively.
Characterisation of the barrier layer was done using energy dispersion x-ray spectroscopy (EDX). Since the cavities are in strong dielectric medium, the probability of charged particles migrating into the cavities is low. While it is not intended to limit the application of the present teaching to any one specific methodology or understanding it is believed that the radicals or elemental species are more likely to migrate into the cavities and subsequently react on the walls to form the barrier. In the plasma, besides other fragments the presence of Si radicals and elemental O could be verified, using optical emission spectroscopy, from their specific emissions near 519 nm, 777 nm and 884 nm respectively.
Since PDMS contains Si as one of its constituents, to confirm that the observed Si characteristic of the EDX spectrum was resultant from a generated barrier layer as opposed to a silicon constituent of the PDMS a further experiment was conducted whereby a barrier layer was formed on channels within a microfluidic device fabricated from a plastic (Zeonor) substrate.
It will be understood that the analysis of the EDX spectrum both supports the generation of the SiOx barrier layer on the PDMS substrate of
The quality of the barrier was examined by introducing a dye which exhibits affinity for PDMS into a microfluidic device fabricated from a PDMS substrate. Rhodamine B (RhB) is such a dye and
where, ‘D’ is the diffusion coefficient, ‘dn/dx’ the density of particles along the cavity length. The flux of particles (φ) impinging an area could be estimated by using the relation (φ=3.513×1022PA/√{square root over (MT)}, where P is the pressure in torr, M the molecular weight, and T the temperature). Estimated flux of O and Si impinging the respective cross sectional area is plotted in
It will be appreciated that the present invention relies upon a gas diffusion of constituent materials of the ultimate surface layer into the individual channels of the microfluidic devices and then their ultimate chemical binding with one another and adsorption onto the surfaces of the channels to define a layer on those surfaces.
Individual ones of the constituents may deposit initially on the surfaces and then provide a reaction site for combination with the other constituents. In this way it will be appreciated that it is not intended to limit the present invention to a methodology that requires reaction and then adsorption. It will be appreciated however that by introducing elemental materials in a gas phase into defined channels, that the ultimate formation of the barrier layer will be affected by surface interactions by those elemental materials with the surfaces of the channels or cavities. In this way it would be expected that channels having a plurality of bends would provide more reaction surfaces than substantially straight channels. To evaluate the effect of such bends and the depth at which a barrier layer may be generated within a microfluidic device, generation of a SiOx barrier was studied in a device comprising bends within the individual channels. For this experiment we used long cavities of 5,000 μm2 inlets cross section. Here we have to consider that pressure in the plasma chamber was maintained near 300 mTorr. At such a pressure mean free paths (λmfp) for Si and O were estimated as 1.42×10−4 m and 2.72×10−4 m, respectively, (λmfp=RT/√{square root over (2)}πd2NAP, where Tis the temperature, d the atomic diameter, NA the Avogadro's number, and P the pressure). In steady equilibrium state we expect no pressure difference between the reaction chamber and in the cavity. Gas flow in the cavity may not be applicable here because the cavity has either one dead end or both ends open in the plasma. These experimental conditions suggest that mass transfer in the cavity is possible through a diffusion phenomenon. If this is true a bend in the cavity should not affect the barrier formation deeper in the cavity past an initial bend. To evaluate this long channels were fabricated with a duplex of intentional bends of 45°, 90°, 135°, 180° as shown in
Reproducibility of the barrier formation was studied in a multiplex of very long meandering cavities of 40 individual units (
Thickness of the barrier was determined with respect to time of the reaction and pressure in the chamber maintaining the ratio of O2 and HMDSO. A slab of PDMS bearing grooves was bonded to Si wafer so that the microcavities have a bottom of Si wafer. The barrier was formed and the device was disassembled. The Si wafer was used for determining the thickness of the barrier using a surface profilometer. As shown in
The thickness curves in
The barrier in PDMS channels was tested for various applications such as absorption of small biomolecules, resistance to organic solvents, and electrophoresis. It is known that PDMS exhibits an affinity for certain small molecules which absorb in PDMS surface. Once absorbed, the surface may be considered contaminated with such molecules and it is difficult to wash them away. Tetramethyl rhodamine isothiocyanate (TRITC) is a derivative of rhodamine and is frequently used for biomolecular labeling. TRITC labelled biotin solution was stored in bare and SiOx barrier PDMS channels and after 1 hour the channels were washed with 0.1 M NaOH and then with 1.0 M NaOH solutions. The state of the channels could be compared by fluorescent images after each washing as shown in
For reproducible electrophoresis experiments in narrow channels a stable charge on the surface (zeta potential) is required for stable electro-osmotic flow (EOF), however, bare PDMS shows an unstable EOF due to its uncontrollable characteristic of nonspecific absorption property. Electrophoresis experiments in a SiOx barrier channel using a pH 9.0 buffer solution showed a stable EOF (
It will be appreciated that the generation of microfluidic devices with a barrier layer formed on the surfaces of individual channels or cavities has been described with reference to an exemplary arrangement whereby the device is fabricated in a PDMS substrate and the barrier layer is generated through gas diffusion of elemental Si resultant from HMDSO precursors and oxygen introduced into a plasma environment. Such barrier layers have been described as the result of diffusion of gas phase highly reactive chemical species into long microcavities leading to a surface reaction within the polymeric cavities. The reactive species created from fragmentation of O2 and hexamethyldisiloxane (HMDSO) in an RF plasma environment diffused into microcavities of polydimethylsiloxane (PDMS) to form an efficient glass like thin film barrier. The reactive species like silicon radicals and elemental oxygen maintained their reactivity for sufficiently longer time and survived random walks in the cavities. The barrier was observed at significantly deep distances along the length of the cavities. The barrier thickness and the growth length could be controlled by the reaction time, and the operating pressure in the chamber. It has been described how increasing the cross section area of the cavity inlet and/or decreasing the mean free path, such as by increasing the pressure, increases the thickness of the barrier. The barrier showed a strong resistance to organic solvent like toluene and prevented the PDMS microfluidics from swelling and deformation. Moreover, the formation of glass like thin film on PDMS microfluidic channels solved the stability problem of electro-osmotic flow (EOF) in naked PDMS microfluidic devices. Reproducible separations by electrophoresis, which was comparable to that in conventional fused silica capillaries were also demonstrated. In this way the present teaching provides a dry physicochemical method of creating SiOx barrier in polymeric microfluidic channels which is reproducible, robust, and up scaling on industrial scale may be more straightforward than for the conventional wet chemical methods. A wide range of applications of such techniques are possible in various fields for example for coating the covered surfaces of microfluidic channels, tubes, capillaries, medical devices, catheters, and advanced electronic and opt-fluidic packaging.
Therefore while such exemplary arrangements have been described to assist the person skilled in the art in an understanding of the benefit and teaching of the present invention, such exemplary arrangements are not provided to limit the teaching to such exemplary arrangements, Modifications can be made to that described herein without departing from the spirit and or scope of the present teaching. For example the channels or cavities could be fabricated in other materials such as plastics, metals, polymers, and elastomeric materials. Furthermore in the context of a generation of a Si based barrier layer, HMDSO represents a suitable precursor material other organo silicon precursors such as any of hexamethyldisiloxane, tetramethylsilane, tetraethoxysilane, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, and tetramethylcyclotetrasiloxane or similar compounds could be utilised.
While the generation of a glassy barrier layer is predicated on the use of silicon (silicon being a constituent of glass), other barrier layer could also be fabricated by varying the choice of elemental materials introduced into the individual channels or cavities. Indeed silicon is exemplary of a metalloid material that can be used to generate a metalloid oxide layer within the microfluidic device and the use of other metalloid materials will result in the ultimate fabrication of other metalloid oxide layers. Furthermore using such gas plasma reactions it is possible to generate metallic oxide layers within the microfluidic devices. The generated silicon oxide layer is typically provided as an amorphous film on the surface of the individual channels and by replacing the organo-silica precursor with a metallic precursor such as titanium tetraisopropoxide Ti(OC3H7)4 it is possible to generate amorphous metallic layers of titanium oxide on the surfaces of the individual channels. Titanium oxide coatings have particular application in biomedical applications. By combining the HMDSO precursor with a Ti precursor in the presence of oxygen it is possible to further improve the wettability of the final layer provided on the surface of the individual microchannels beyond that achievable using pure silicon precursors.
While the geometries of individual channels have been described in little detail it will be appreciated that the surface layer could be formed onto individual features within the micro-channels such as micro pillars, trenches, and wells etc., and it is not intended that the application of the methodologies of the present invention be construed as being limited only to surface treatment of fluidic passages. By providing surface treatment of individual sites within the channels or cavities it is possible to provide target reaction sites within the microfluidic devices at specific locations. In the context of vaporizable organo precursors and O2 in highly energetic plasma environment the subsequent barrier is resultant from a deposit on the substrate surfaces forming the thin film barrier SiOx coating giving rise to ionisable silanol (Si—O−) groups so that the thin film barrier of SiOx can be a substitute of glass like surface.
It will be appreciated that heretofore the surface treatment of the channels or cavities within the microfluidic device has been described with regard to a single process effected within a gas plasma reactor that subsequent to the formation of the surface layer other surface treatments could be effected to further modify the response characteristics of the microfluidic device. For example, if a channel is coated with a SiOx layer through methodologies such as that described heretofore, that the coated layer of SiOx bearing silanol groups can be further functionalized. Examples of the type of functional groups that could be generated include amine, polyethylene glycol, proteins or DNA that could be generated using techniques such as wet chemical or gas phase reactions.
It will be appreciated that where the surface treatment of cavities or channels within a device has been described herein that it is not intended to limit that surface treatment to individual channels or cavities and it is intended to include networks of such channels or cavities within the general nomenclature of channels or cavities. Within this context it will be appreciated that by use of a diffusion process that it is possible to coat meander patterns defined by the channels or cavities within the microchannel. The use of a diffusion process does not require direct concurrent exposure of the target surfaces as the radicals will enter the channels at one end and migrate along the path defined by the channel, coating along the way. Using such a process allows the surface treating of channels or cavities having aspect ratios (the ratio of the width of the channel to its length) greater than 10. Indeed using a process in accordance with the teaching of the present invention it has been possible to surface treat microchannels having lengths of the order of cm, representing aspect ratios >>10. By using an optimised diffusion process to surface treat the channels, it is possible to coat the channels despite the channels and their side walls or surfaces being wholly defined within the microstructure. In this way it will be appreciated that a process in accordance with the present teaching may be used to treat channels or cavities having surfaces extending fully about their perimeter, those channels or cavities being fully defined within the interior volume of the microfluidic device, Channels or cavities within the context of the present teaching having individual surfaces that extend fully about the channel or cavity are in this way not open channels, where the length of the channel is accessible from an exterior of the microfluidic device.
Therefore although the invention has been described with reference to exemplary illustrative embodiments it will be appreciated that specific components or configurations described with reference to one figure may equally be used where appropriate with the configuration of another figure. Any description of these examples of the implementation of the invention are not intended to limit the invention in any way as modifications or alterations can and may be made without departing from the spirit or scope of the invention. It will be understood that the invention is not to be limited in any way except as may be deemed necessary in the light of the appended claims.
Similarly, the words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0910626.1 | Jun 2009 | GB | national |
This application is a U.S. national stage application filed under 35 U.S.C. §371 of International Patent Application PCT/EP2010/058631, accorded an international filing date of Jun. 18, 2010, which is incorporated herein by reference in its entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/058631 | 6/18/2010 | WO | 00 | 5/18/2012 |
