Matthew Solomon

Microwave welding of polymeric-microfluidic devices

This paper describes a novel technique for bonding polymeric-microfluidic devices using microwave energy and a conductive polymer (polyaniline). The bonding is achieved by patterning the polyaniline features at the polymer joint interface by filling of milled microchannels. The absorbed electromagnetic energy is then converted into heat, facilitating the localized microwave bonding of two polymethylmethacrylate (PMMA) substrates. A coaxial open-ended probe was used to study the dielectric properties at 2.45 GHz of the PMMA and polyaniline at a range of temperatures up to 120 °C. The measurements confirm a difference in the dielectric loss factor of the PMMA substrate and the polyaniline, which means that differential heating using microwaves is possible. Microfluidic channels of 200 µm and 400 µm widths were sealed using a microwave power of 300 W for 15 s. The results of the interface evaluations and leak test show that strong bonding is formed at the polymer interface, and there is no fluid leak up to a pressure of 1.18 MPa. Temperature field of microwave heating was found by using direct measurement techniques. A numerical simulation was also conducted by using the finite-element method, which confirmed and validated the experimental results. These results also indicate that no global deformation of the PMMA substrate occurred during the bonding process.

Fabrication of multilayered microfluidic 3D polymer packages

The realization of microfluidic packages by stacking and bonding several layers of microstructured polymer films opens up the potential of creating complex three-dimensional microfluidic structures based on relatively simple two-dimensional manufacturing processes. Whereas a multitude of microstructuring techniques have been developed, packaging and bonding technologies for multilayer microfluidic devices are still underrepresented. Bulk bonding processes like thermal diffusion bonding fit well into a lab environment, but feature extensive bonding times. With increasing fluidic complexity, bonding technologies that enable selective bonding and sealing at pre-selected areas (e.g. around channel walls or process chambers) are required. Selective bonding technologies enable a localized heat generation exactly at the desired bond position and thus significantly reduce the risk of structure deformation and channel clogging. In this paper, experimental results for a variety of bulk and selective bonding methods are reported and compared. Surface modification of polymers and lasers welding of polymer sheets are identified as suitable technologies for integration with high-throughput production environments.

Laser LIGA for serpentine Ni microstructures

A pulsed excimer laser (248 nm) based LIGA-like process is presented for the fabrication of Ni serpentine microstructures, such as those that might be used for micro-heaters. The structures were produced on both Cu (60 micrometers ) clad PCB and on Cu/Ti (up to 4 micrometers /15 nm) sputtered Si (100) substrates. The substrates were coated with a Laminar dry film (35 micrometers ) photoresist, which was then patterned by laser ablation to produce the mould for Ni electroforming. The optimal ablation conditions were identified for laser patterning to prepare the micro polymer mould. Beam fluence (~ 1 J/cm2) and number of shots (~ 60 pulses) for 50 micrometers wide features on this photoresist were established, and it was observed that an increased number of shots and increased fluence were needed for features less than 20 micrometers wide. Additionally, the Cu layer surface was cleaned by the use of 5 -10 laser pulses at the same fluence. Ni electroforming has been carried out using standard Ni sulfamate bath at a current density of ~ 10 mA/cm2. After Ni electroforming, both the Laminar dry film and the Cu layers around the electroformed Ni patterns were removed using a combination of acetone, laser and Cu selective etching. Finally, a series of Ni microstructures were fabricated consisting of up to 50 micrometers wide and 35 micrometers thick serpentine tracks. The devices were measured using a scanning confocal microscope and it was found that using the excimer laser to remove the remaining dry film laminate also smoothed the electroplated Ni surfaces from a pre-laser treated Ra of 1.20 micrometers to 0.19 micrometers . Laser ablation also released the finer features from the substrate.

Packaging of disposable chips for bioanalytical applications [microfluidics]

In this paper, the realization of microfluidic packages by bonding several stacked layers of microstructured polymer films in a reel-to-reel manufacturing system is shown to be a promising approach. The smart division between disposable and reusable system parts as well as the careful selection of materials, based on microstructuring, bonding, biocompatibility and auto fluorescence criteria may lead to bioanalytical devices with a very competitive cost of ownership per test. For the realization of such microfluidic disposables, microstructuring technologies based on UV laser micromachining, bonding technologies based on thermal diffusion, adhesives and microwave sealing using conductive polymers, and surface modification approaches for the reduction of non-specific protein binding are discussed.

Laser-LIGA for Ni microcantilevers

This paper presents our design and experimental results of nickel microcantilevers, which were fabricated using a laser-LIGA process, based on KrF (248 nm) excimer laser micromachining. A chrome-on-quartz mask, containing the desired mask patterns was prepared for this work. The substrate of copper (30 μm thick) clad printed circuit board (PCB) was laminated with Laminar 5038 photopolymer to be laser patterned. Following laser patterning and laser cleaning, all the samples were electroformed with nickel on top of the copper layer. To release the Ni microcantilevers, the excimer laser was employed again to remove the polymer in the localised area to facilitate Cu selective etching. Here, copper acted as the sacrificial layer as well. The Cu selective etching was carried out with ~ 20 % (wt) aqueous solution of ammonium persulfate. Because the Cu selective etching is isotropic, some undercuts happened next to the anchor area. The samples were characterised using optical microscope, confocal laser scanning microscope and SEM, and some of Ni cantilevers were tested electro-thermally. Their performance was analyzed with respect to the simulation results.

Single mode microwave sealing of polymer-based microfluidic devices using conductive polymer

Polymer based microfluidic devices have an important potential use in BioMEMs applications due to the low cost and biocompatibility. However, sealing the devices hermetically without blocking the channels, altering their dimensions or changing the surface properties is a challenging issue in their fabrication. In this paper a microwave-based sealing technique using a polymethylmethacrylate (PMMA) substrate and conductive polymer (polyaniline) is presented. The developed novel bonding technique has achieved precise, well-controlled and selective heating, which causes localized melting of the polymer substrates. At the joint interface, patterned polyaniline features absorb electromagnetic radiation and convert it into heat, which facilitates the microwave bonding of two PMMA substrates. This new approach can easily seal microfluidic devices with micron-sized channels without blocking or destroying the integrity of the channel. Microfluidic channels of 400 μm and 200 μm wide were sealed using a microwave power of 300 Watts, in less than 20 seconds. The microfluidic channel fabrication techniques, polyaniline patterning method at the interface and bonding evaluation such as sample cross section and leak test are discussed. The dielectric properties of polyaniline and PMMA at 2.45 GHz frequency are also evaluated by using the open probe technique, which shows PMMA is essentially transparent to microwave energy.

Hysteresis and drift in a carbon-polymer composite strain sensor

A conductive polymer strain gauge was screen printed to produce an active area of 3mm × 4mm. The graphite and titanium dioxide loaded thermoplastic device was found to have a resistance of 4.3kΩ and a gauge factor of up to 20. The higher resistivity and gauge factor result in a lower power consumption and higher sensitivity when directly compared to metal foil strain gauges. However, a substantial hysteresis of approximately 80με was identified in a complete strain cycle from 0me to 730με. The source of this hysteresis was considered to be the thermoplastic matrix. Subsequently the viscoelastic nature of the polymer matrix was analysed using the gauges resistive signal as it changed under applied strains, and this output was then compared to the standard linear solid (or Zener) model from linear viscoelastic theory. This model was applied to the data and with some limitations was found to make an improvement to the reported hysteresis.

Real-time monitoring of excimer laser ablation of multilayer thin films by detecting acoustic emission

In micromachining, excimer laser ablation was a key process for producing featured by removing parts of a photoresist layer. One hurdle was that the seed layer (e.g., copper), on which the photoresist was spun, was easily attacked or damaged in laser ablation of the photoresist. To overcome this, an acoustic emission transducer (AET) was coupled to the X-Y stage of an excimer laser system to acquire surface acoustic waves (SAWs) arising from pulsed laser-material interaction. The characteristics of such a process could then be investigated by analysing this feedback AE signal. Analysis of the frequency spectrum showed that there was a dominant frequency correlating with the ablation process through one material to another. Specifically, the amplitude of the dominant frequency had an abrupt change when laser beam approached the interface of two layers. The RMS values and the variance values of raw acoustic waves were also indicative of such a process. The exact number of shots machining through one material was indicated by properly calibrating such a correlation at given laser parameters. Furthermore, the etch rate of machined material could be calculated by averaging the thickness of this material with the associated number of shots. Finally, a real-time monitoring scheme of complex laser micromachining process was addressed on the basis of taking SAWs as feedback signals.