WO2004092048A1

WO2004092048A1 - Microfluidic sealing

Info

Publication number: WO2004092048A1
Application number: PCT/AU2004/000458
Authority: WO
Inventors: Abdirahman Ali Yussuf; Jason Hayes; Matthew Solomon; Igor Sbarski
Original assignee: Microtechnology Centre Management Limited
Priority date: 2003-04-15
Filing date: 2004-04-13
Publication date: 2004-10-28
Also published as: AU2003901779A0

Abstract

A method of laminating polymeric microfluidic devices in which a conductive polymer is deposited in channels (15) adjacent the microfluidic channels (11) and subjected to microwave radiation sufficient to locally heat the polymer adjacent the conductive polymer to thermally bond the polymeric layers (10, 14) together. Polyaniline is a preferred conductive polymer.

Description

MICROFLUIDIC SEALING

This invention relates to the fabrication of microfluidic devices and in particular to forming fluid tight seals between layers in laminated microfluidic devices.

Background to the invention

Extensive work has been done on producing microfluidic devices on various substrates with range of materials such as amorphous silicon, glass, quartz, and metals and polymers. To date, the majority of the devices reported in the literature, as well as those that are commercially available, have been fabricated- using silicon and glass based materials. However, polymeric materials are increasingly being used in microfluidic systems.

One major problem in the area of plastic microfluidics is bonding of components made of different materials. Most existing bonding techniques in microfluidics focus on the bonding of silicon, glass and ceramic material that can sustain high processing temperature but these schemes are not applicable to temperature- sensitive plastic materials.

Polymer substrates are seen as a better alternative for the fabrication of microfluidic devices because these materials are less expensive, possess good processibility for mass production, are recyclable and easier to manipulate than silica-based substrates. Various polymers such as polydimethylsiloxane (PDMS), polymethylmethacrylate (PMMA), polycarbonate (PC), polystrene (PS) and polyethylene (PE) have been used for microfluidic fabrications. The most commonly used material is PDMS which is tough, optically transparent, amenable to fabrication using a number of procedures, inexpensive and excellent optical properties of PDMS. Ail these relevant characteristics of PDMS render this material an excellent alternative to the commonly used glass and quartz substrate. Laminated microfluidic structures need the layers to be bonded together and this may be achieved by adhesive or by welding. Most thermoplastics can be heat sealed or welded by heating the plastic surface to be joined to high temperatures depending on the plastic. The surfaces should be brought into contact and sufficient pressure applied to allow intimate contact. The heat may be applied from both sides of the joint to be bonded or from only one side. In thermal bonding of solid polymeric substrates, one or more of the substrates to be bonded is heated to the glass transition temperature of the substrate surface. For polymeric substrates substantial deformation can occur during thermal bonding at substantially lower temperatures, and this should be taken into account. Adhesive bonding is used in microfluidic devices technology mainly to attach components. Adhesives have an ability to seal as well as bond the interior parts of the assembly from the environment. While a hermetic seal is possible, care is necessary in the surface preparation (surface modifications) of the joint and the application of the adhesive. The disadvantage of this technique is when using it, care needs to be taken in order to prevent the adhesive from flowing into the micro channels.

Lamination is the process of combining two or more polymeric materials into a new composite. The polymers may be alike or different. Several methods such as wet lamination and hot melts lamination are in common practice today. Lamination process is widespread in the microfluidic devices for fabricating polymers in a polymer film and works well for sealing channels. It is particularly well suited to fabrication of micro-scale components because the laminates can often be cut or machined using macroscale processes combined with a wide variety of microfabrication techniques. Frequently a thermoplastic welding technique will be selected for reasons which may include cost, short joining times, good joint mechanical properties and high level of consistency. Today there are numerous joining (welding) techniques available, which can be broadly divided into three groups : those where heat is generated by mechanical movement of components to be joined (friction), those where heat is generated by an external source and those using electromagnetism. All these provide the means to join most thermoplastic components. However the issue of sample contaminants needs to be considered in some methods; for example, if the two surface to be joined are contaminated with release agents or other chemicals, a weld will not take place as the friction heat to melt the polymers will not be generated by the ultrasonic vibrational energy. In addition, chemical contaminants may prevent the softened/molten plastics from joining. One form of adhesiveless bonding is to use plasma treatment of the surface. USA patent 6171714 is one example where a polymer film is treated to improve adhesion of a metal layer

USA 6284329 also discloses plasma treatment for bonding polyimide and copper. It is an object of this invention to provide an inexpensive means of bonding layers in microfluidic devices that do not interfere with the integrity of the microwave channels and is easily adaptable to manufacturing techniques employed in making polymer based microfluidic devices.

Brief description of the invention

To this end the present invention provides a method of bonding polymeric layers in a microfluidic device in which at least one polymeric layer includes microfluidic channels and a second polymeric layer is bonded to the first to seal the channels which method includes the steps of a) Applying a microwave heatable substance adjacent to said microfluidic channel, b) irradiating the microwave heatable substance with microwave radiation of a wavelength effective to heat said substance for a time sufficient to heat the polymer adjacent said microwave heatable substance to bond the two layers together.

Microwaves are electromagnetic waves in the frequency band from 300 MHz to 300 GHz. Microwave heating and conventional heating techniques differ fundamentally. In the former heat is absorbed over the entire cross-section of the weld, whereas in the latter heat is diffused from the surface through the material. The microwave welding technique employs concentrated microwave energy affecting only the area to be bonded, which means selective zone heating or localized heating. The major advantage of using microwave is that it raises the bulk material temperature volumetrically instead of conduction from the surface such as conventional oven heating. This method is applicable to the polymeric materials usually used in microfluidic devices such as polymethylmethacrylate (PMMA) and polycarbonate (PC).

Microwave welding of plastics can be achieved by placing a conductive polymer- heating element at joint interface. This heating element absorbs the microwave energy and converts it into heat, which determines the heat generation and peak temperature during welding.

This invention provides new approaches to improve the physical properties of materials; provides alternatives for processing materials that are hard to process; reduces the environmental impact of material processing; provides economic advantages through the saving of energy, space, and time; and provides an opportunity to produce new materials and structures that can not be achieved by other methods.

Microwave possess several characteristics that are not available in conventional processing of materials, including:

♦ Penetrating radiation;

♦ Controllable electric-field distributions;

♦ Rapid heating;

♦ Selective heating materials through differential absorption; and ♦ Self-limiting reactions

These characteristics, either singly or in combination, present opportunities and benefits that are not available from conventional heating or processing methods and provide alternatives for the processing of a wide variety of materials, including rubber, polymers, ceramics, composites etc. In some processes and products heating of a specific component while leaving the surrounding material relatively unaffected would be of great advantage; this selective heating process is not possible by conventional heating techniques.

In the last 20 years, a new class of electrically conductive polymers such as polythiophene, polyptrrole and polyaniline have been studied. These materials have a unique combination of mechanical and electrical properties making them very useful for welding. Among the conducting polymers, liquid polyaniline has been considered as one of the most promising materials because the electromagnetic parameters can be adjusted by changing both oxidation and protonation state. The microwave heatable material may be applied or printed onto the surface of the layer containing the microfluidic channels or onto the layer to be laminated to the microfluidic channel layer. The amount of the conducting polymer applied determines the heat which can be generated. An alternative method includes the steps of a) forming at least one channel adjacent said microfluidic channel, b) filling said channel with said conductive polymer. The channel may be in the microfluidic layer or the layer to be bonded to the microfluidic layer. This enables a greater quantity of material to be located at the optimal distance from the microfluidic channel and allows the time required for heating to be reduced.

Detailed Description of the Invention

Preferred embodiments of the invention are shown in the drawings in which

Figurel is a plan view of microfluidic channel in a first polymer film;

Figure 2 is a plan view of a second polymer film having sealing channel according to this invention; Figure 3 is a plan view of the two films of figures Hand 2 superimposed on each other;

Figure 4 is a side view of figure 3;

Figure 5 is an end view of figure 3; and

Figure 6 is a view along the line A-A of figure 3. As seen in figure 1 a microfluidic channel 11 is formed in a film 10 which is the upper substrate in the microfluidic device. In the lower substrate 14 a sealing channel 15 is formed and filled with a conductive polymer such as liquid polyaniline. The formation and filling of the channels may be achieved using conventional lithographic and printing techniques. The polymer layers are preferably each 0.1 to 2 mm thick, the channels are 0.5 to

0.6 mm in width and 0.1 to 0.3 mm in depth.

The preferred conductive polymer used in this invention is polyaniline. The polyaniline is deposited in channels 15 on either side of the microfluidic channel

11. The channel 11 may be 0.2 to 1mm wide and 0.2 0.6mm deep. The channel 15 is spaced sufficient distance away from the microfluidic channel 11 to prevent any distortion of the channel due to the localized heating of the conductive polymer.

This distance is preferably 1 to 4mm. Instead of using channels the polyaniline may be printed lithographically onto the surface of the substrates 10 or 15 in the same relative locations. The printed layer may be about lOOmicrons thick and 50 to 100 microns wide. Because less material is used the time required to achieve bonding may be longer. Since the microwave field will interact with any electrically conductive materials present, it may not be compatible with devices including electrical contacts. This process is most suited to thermoplastic polymers which do not absorb microwave radiation to a significant level. This includes the majority of polymers currently used in microfluidics except for polyimide and PDMS. A suitable clamping method is required to produce spatially uniform bonds to avoid deformation due to uneven loading.

The preferred microwave frequency is 2.45 GHz and a Single Mode Microwave System is preferred. Due to the dimensions of the waveguide, the number of wave modes inside the applicator is limited. A single mode microwave heating provides faster heating and welding than multi-mode systems. Also the field distribution can be calculated by using Maxwell's equations with the associated boundary conditions.

At microwave heating frequencies the energy transfer is accomplished in special channels called waveguide, the dimensions of which depend upon the operating frequency. The energy thus, once generated, is confined to travel in these enclosed structures and, with the exception of the horn type radiator; microwave applicators are designed to confine the microwave within them. Waveguide is normally made of conductive materials such as aluminium or any metal alloys and has the shape of either a hollow rectangular or cylindrical, which is designed as such to meet the propagation modes of interest. It has an advantage over other transmission lines such as coaxial cable as it can handle electromagnetic waves more efficiently, leading to low electromagnetic energy losses at higher frequency operation. In fact at higher frequency operation, the use of coaxial cable to carry out microwave transmission is not recommended because it could result in the leakage of microwave radiation. In general transmission line system there are three modes, known as Transverse Electric Magnetic Mode (TEM Mode) as is found in a coaxial cable, Transverse Electric mode (TE Mode), and Transverse Magnetic (TM Mode) which are found in a waveguide. In a waveguide transmission line system, only two modes, namely, TE and TM modes separately, can occur inside a waveguide due to the dimensional constraints. In order successfully apply microwave energy for materials processing and effectively control the level of the microwave energy to be absorbed by the materials under processing, the knowledge of material-microwave interaction must be understood well. The microwave heating process is an electromagnetic interaction between the incident microwave radiation and the target material. The microwave energy absorbed within materials depends, among other things, on the incident microwave frequency, distribution of electric fields within the material and dielectric properties of the material. Non-conducting materials are transparent to microwave energy whilst highly conductive materials are opaque, resulting total reflection of the microwaves. The material property that indicates the degree of absorption is called the dielectric loss. The microwaves are absorbed by the component that has high dielectric loss while passing through the low loss material with little drop in energy. The dielectric constant mostly determines how much of the incident energy reflected at the air-sample interface, and how much enters the sample. The important factor in microwave processing of materials is the loss tangent, tan δ, which is indicative of the ability of the materials to convert absorbed energy into heat depending on electric-field intensity, frequency, loss factor and permittivity. This is defined as follows;

tan δ = -

Where; ε^" is the dielectric loss or loss factor ε is the dielectric constant

Chen et al. 1993 have investigated the relationship between polymer structure and microwave absorptivity. Dielectric loss factor, ε , loss tangent, and tan δ were used to evaluate potential material processability under applied microwave radiation. In general, the heat-ability was found to be a direct function of the dielectric loss dispersion dependence on temperature and frequency. The dielectric loss factor obtained at low-frequency measurements was found to be directly proportional to the heat-ability of the polymers.

Microwave parameters such as power, time have been optimised for a polyaniline containing channel of 400μm and (300W for 15 sec). The time required has been found to increase with lower volumes of polyaniline.

PMMA and PC substrates have been sealed using this technology without channel blocking and distortion. Different channel widths such as 600, 400, 200μm have been sealed. A range of polyaniline channels widths have been tested such as 600, 400, 200μm, and all of them resulted in a good bond. Pressure and leak tests of the sealed channels were performed and results showed no leakage up to 200 psi. Patterning of the polyaniline may be carried out by screen printing The cost of developing equipment to implement this technique is likely to be relatively low. The key task in development is in producing a sufficiently uniform microwave field over a suitable area. This technique allows the hermetic sealing of a range of thermoplastic polymers without the presence of a third material in contact with the channels. This solves many of the channel filling and non-material compatibility issues present with many adhesives. Bonding occurs over a few seconds and requires only a low clamping force. This suggests that the technique could be scaled up to allow medium to large scale manufacture on a continuous web.

This technique may be applied not only to microfluidic sealing, but also many other fields of manufacture such as medical devices where it can replace techniques such as RF heating of metallic susceptors, and UV cured adhesive bonding. The ability to use printing techniques to dispense the polyaniline may improve mass- manufacturability.

From the above it can be seen that the present invention provides a unique method for achieving effective sealing of polymeric microfluidic channels whether the materials are in the two polymer films are the same or different.

Those skilled in the art will realize that the invention can be put into practice in a number of ways without departing from the essential teachings of the invention.

Claims

1. A method of bonding polymeric layers in a microfluidic device in which at least one polymeric layer includes microfluidic channels and a second polymeric layer is bonded to the first to seal the channels which method includes the steps of a) Applying a microwave heatable substance adjacent to said microfluidic channel, b) irradiating the microwave heatable substance with microwave radiation of a wavelength effective to heat said substance for a time sufficient to heat the polymer adjacent said microwave heatable substance to bond the two layers together.

2. A method as claimed in claim 1 in which microwave heatable substance is a conductive polymer,

3. A method as claimed in claim 2 in which the conductive polymer is polyaniline.

4. A method as claimed in any preceding claim which includes the steps of a. forming at least one channel adjacent said microfluidic channel, b. filling said channel with said conductive polymer,

5. A method as claimed in any preceding claim in which the polymeric material in each layer is different.

6. A method as claimed in claim 4 in which the microfluidic channel is formed in one polymer and the channels containing the conductive polymer are in the layer to be bonded to the layer containing the microfluidic channel.

7. A microfluidic device incorporating a microfluidic channel formed in a first polymeric substrate and a second polymeric substrate bonded to the first substrate in which a microwave heatable substance is located adjacent said microfluidic channel to facilitate the thermal bonding of the substrates.

8. A device as claimed in claim 7 in which the microwave heatable substance is a conductive polymer.

9. A device as claimed in claim 8in which the conductive polymer is polyaniline.

10. A device as claimed in claim 7 to 9 in which the polymeric material in each layer is different.

11. A device as claimed in claim any one of claims 2-10 in which the microfluidic channel is formed in one polymer and the conductive polymer is printed on the layer to be bonded to the layer containing the microfluidic channel.