Lasers contribute to next-gen microfluidic devices

Microfluidics is a multidisciplinary field encompassing physics, chemistry, engineering, flow dynamics and, frequently, biology. It deals with the manipulation and flow of liquids and gasses in miniature systems—typically submillimeter-sized. Microfluidic-device channels range in diameter from hundreds of nanometers to a couple hundred microns.

Common microfluidic materials include polymers (nylon, PMT, PMMA and silicone), sapphire, glasses (quartz, fused silica, Pyrex) and silicon. Among the desirable characteristics of microfluidic devices are sharply defined features, smooth walls and optically clear surfaces. Plus, the parts need to have high reproducibility and be producible at high speed in order to make the process economically feasible. Features of interest include holes, channels, sample chambers and cones—both 2-D and 3-D.

Microfluidic devices generally are divided into continuous-flow and digital, or droplet-based, styles. (See "Digital microfluidics: an alternative to continuous flow" below.) Continuous-flow devices are adequate for well-defined and simple biochemical applications, but they are less suitable for tasks requiring a high degree of flexibility or sensitivity. These closed-channel systems are inherently difficult to integrate and scale up.

Digital microfluidic devices offer more flexibility and scalability. They allow many procedures to be performed simultaneously with one device and permit more-sensitive measurements to be taken.

Flow properties are the same at the macro and micro scales. But surface tension, viscosity, electrical charges and other factors have greater impact at the microscale because of the much larger surface-to-volume ratio. And, unlike macroscale devices, the impact of flow inertia is negligible at the microscale. As a result, micro designs cannot be scaled-down versions of macro designs.

Assorted microfluidic chips, provided by Dolomite. Image courtesy The Dolomite Centre Ltd.

Alternative manufacturing methods are needed to exploit microscale physical properties. Lasers are one such process because they meet three criteria for successfully manufacturing next-generation microfluidic devices:

Small feature size. Lasers, especially ultraviolet ones, can produce smaller features than other methods, such as EDMing and ultrasonic and hard-tool machining.

Wide range of materials. Lasers can process metals and nonconductive materials, like glass, plastics and ceramics. EDMs, for instance, only work with conductive materials.

Low cost. Lasing is an extremely cost-effective manufacturing process.

Laser options

Infrared (IR) lasers, primarily CO2 units, have proven to be very flexible and fast when processing polymeric materials. On a dollars-per-photon basis, these lasers are much-less expensive than the options discussed below. However, they remove material by thermal vaporization, and the resulting melt may lead to issues with feature quality and attainable feature size.

The best results with IR lasers are obtained in PMMA, which has a high IR-absorption rate. Features can be produced at high speeds, with attainable channel depths from 100µm to 300µm and widths from about 100µm to 250µm. Because of the melt, surface roughness is small—in the micron range—but recondensed polymers on the structure edges can cause problems.

Most microfluidic manufacturing is performed with ultraviolet lasers, which, like IR lasers, produce invisible beams. (Visible-beam lasers are only used in special applications.) The UV choices are 355nm and 266nm DPSS (diode-pumped solid-state) lasers, and 248nm, 193nm and 157nm excimer lasers. The lasers in the preceding sentence are ordered from low to high in terms of cost and complexity, and high to low with respect to attainable spot size on target. Cut quality increases as wavelength decreases.

DPSS lasers have a Gaussian beam and usually are used with galvanometers, which make programming complex patterns a straightforward procedure. A galvo with Z-axis motion allows focusing the beam to different points in space. Such a setup is especially useful when the substrate partially absorbs the beam because it lets the user focus into and etch inside the bulk of the material. This occurs because the energy density at the surface (unfocused beam) is insufficient to exceed the ablation threshold of the material, while the energy density at the focal point (inside the bulk material) surpasses the threshold.

The absorption rate for many materials is too low when laser wavelength exceeds 200nm. One option in these cases is to use a VUV (vacuum-ultraviolet) laser with a 193nm or 157nm wavelength. The 193nm laser will require, at minimum, use of a purged-beam-delivery system because air—specifically, oxygen—absorbs about 50 percent of the available photons when the path length is approximately 1m. A 193nm wavelength is good for materials such as nylon and PET.

A 157nm laser can process even more difficult-to-machine materials, like PTFE, quartz and fused silica. It will, however, require the use of an evacuated or highly purged-beam-delivery system, as even a trace of oxygen in the beam path will kill the process.

Until recently, the above options were the only ones that merited consideration. That has changed in the past few years with the commercialization of picosecond and femtosecond lasers. Their ultrashort pulse lengths introduce nonlinear effects that make absorption possible in otherwise “nonabsorbent” materials. (These effects occur because ps and fs lasers deposit photon energy into the material so rapidly that more than one photon is absorbed almost simultaneously, permitting otherwise “forbidden” quantum transitions.)

NASA is studying the feasibility of producing microfluidic cards with Raydiance 800-fs lasers. If successful, the cards will be used for space-based experiments during a mission in 2011 or 2012. Image courtesy Raydiance Corp.

Next-gen devices

Microfluidics experiments have been ongoing since the 1980s. In the early 1990s, the first major application, inkjet print heads, was commercialized.

In the early 2000s, scientists at Sandia National Laboratories developed µChemLab, a portable, handheld chemical-analysis system for homeland security, defense, environmental and medical applications. The unit can detect chemical-warfare agents and proteins, as well as biotoxins such as ricin, staphylococcal enterotoxin B and botulinum. It also can identify viruses and bacteria via protein fingerprinting.

A lot of microfluidics research is being conducted in the medical sector. The technology has great appeal for the medical community, particularly those in the diagnostics world. The reason is that, compared to conventional plate assays, microfluidic chips require only a tiny fraction of the sample and reagent for each process performed—typically nanoliters instead of milliliters. Also, microscale reactions occur much more quickly and, because the functions performed are easy to automate via microchips, human intervention is minimized. This, in turn, reduces costs and the potential for contamination.

Others are working on integrating a host of analytical procedures onto a single substrate—lab-on-a-chip systems. A subset of microfluidics, LOCs combine multiple lab functions onto microchips. They allow experiments, such as DNA analysis, drug detection and patient screening for illness, to be performed in a single operation and with minimal fluid.

Aerospace is another microfluidics growth area. In January, NASA’s Ames Research Center and Raydiance Corp. announced a joint project wherein 800-fs lasers manufacture microfluidic devices. The goal is to deploy the devices in free-flying nanosatellites, the International Space Station, and future lunar and planetary research laboratories.

Looking ahead, microfluidics will revolutionize many industries. Processes will become more efficient and chemical reactions not currently possible, because the reagents are either too unstable or cost too much, will become routine.

As an enabling process, lasing will play a major role in the evolution of these technologies, which will permit the use of new workpiece materials and the design of smaller and more efficient devices. They will also facilitate the necessary scaling up of manufacturing volumes to allow full commercialization of microfluidic devices. µ

About the author: Ronald D. Schaeffer, Ph.D., is CEO of PhotoMachining Inc., a high-precision laser job shop and systems integrator in Pelham, N.H. E-mail: rschaeffer@photomachining.com.

Digital microfluidics: an alternative to continuous flow

Digital microfluidics is an alternative paradigm for lab-on-a-chip systems that is based upon micromanipulation of discrete droplets. Microfluidics processing is performed on unit-sized packets of fluid that are transported, stored, mixed, reacted or analyzed in a discrete manner using a standard set of basic instructions.

As an analogy to digital microelectronics, these basic instructions can be combined and reused within hierarchical design structures so that complex procedures (e.g., chemical synthesis or biological assays) can be built up step by step.

In contrast to continuous-flow microfluidics, digital microfluidics works much the same way as traditional bench-top protocols, only with much smaller volumes and much higher automation. Thus, a wide range of established chemistries and protocols can be seamlessly transferred to a nanoliter droplet format.

—Reproduced with permission from the Web site of Duke University’s Department of Electrical and Computer Engineering (http://microfluidics.ee.duke.edu/).