Biochemical IC chips What happens when microfluidics moves into three dimensions? If Koji Ikuta and colleagues at Nagoya University (Japan) are doing the work, the answer is a threedimensional honeycomb-like structure made of "biochemical integrated circuit (IC) chips". Ikuta likens the chips— whose concept dates back to 1994—to standard electronic IC chips, which the user mixes and matches to build the desired circuit. Ideally, he says, if a biochemist wants to perform a new reaction, he or she would simply add a new biochemical IC, which is similar to adding memory to a computer. However, that's in the future. At this point, Ikuta and co-workers have built a 4-layer stackable chip set, where each layer includes multiple microdevices. At the top, an interface chip with 9 microconnectors takes samples from the outside world and delivers them to a second chip with 18 oneway valves. The third chip contains 9 mii croreactors with 18 inlets and 9 outlets, which then feed into 9 concentrators in the bottom chip. Each layer is 14 mm across and 1-3 mm tall, and is made from UV-curable polymers and other materials. Ikuta says bonding is not needed to prevent leaks between layers. To test the system, the researchers conducted a simple synthesis reaction to produce luciferase, the firefly's photoprotein. Because they detected luminescence, which has a long time scale, background fluorescence from the chip was not an issue, Ikuta says. The early chips were fabricated using a single-photon microstereolithography pro-
Connector Chip
Valve Chip
- Reactor Chip
Concentrator Chip
Biochemical IC Chip Biochemical IC chip set, including connector, valve, reactor, and concentrator chips.
cess, which allows the researchers to solidify the liquid polymer at a pinpoint. With this technology, the group has fabricated freely movable components, such as a gear with a shaft and stopper, without sacrificial layers or supports, Ikuta says. More recently, the group has upgraded to a twophoton process, which will allow submicrometer resolution in three dimensions. Ikuta notes that the two-photon method is bigger and more expensive because it uses Ti:sapphire lasers, but he says it is neces-
sary to make smaller, more complicated chips. Ikuta says the group is now finishing a micropump chip—the next step toward his long-term goal of creating the biochemical equivalent of the electronics industry's large-scale integration. He acknowledges that quite a bit of work needs to be done before that will happen. However, if he has his way, lab researchers will one day construct their own microscale chemical labs right on the bench.
NEWS FROM THE AUSTRALIAN INTERNATIONAL SYMPOSIUM ON ANALYTICAL SCIENCE AlisonDownardreports from Melbourne, magnitude can be influenced in several Australia. ways. Varying the concentration of a competing anion in the mobile phase is a conChromatography venient method. For example, when the chloride concentration is >500 mM, CE with tunable selectivity is observed, and this gradually selectivity for shifts to ion-exchange selectivity as the inorganic anions chloride concentration decreases to 20 mM. Other handles for tuning selectiviWhen features of two standard chromatoties (and, hence, elution orders) include graphic techniques based on different the internal diameter of the capillary and selectivity mechanisms are combined, the nature of the competing anion. Deunique selectivities for inorganic ions creasing the capillary diameter to 25 um can be achieved. Paul Haddad and research student Michael Breadmore from increases the ion-exchange contribution, and using a stronger eluent ion has the the University of Tasmania (Australia) have investigated how ion-exchange cap- opposite effect. Perchlorate exhibits very strong ion-exchange displacement, and illary electrochromatography (IE-CEC) changing the retention characteristics can be tuned to give the selectivity of an ion-exchange separation, a capillary elec- from CE-like to ion-exchange-like trophoresis (CE) separation, or somea much smaller concentration range thing in between. than for chloride The goal of CEC is to introduce a staThe mobilities of analyte ions under tionary phase into the capillary of a CE particular elution conditions can be modsystem. C]8 is the most common choice of eled mathematically using well-known stationary phase; however, for IE-CEC, an ion-exchange and CE principles. Excelion-exchange material is used. Haddad's lent agreement between calculated and group has experimented with packed-colexperimental electrophoretic mobilities umn, pseudostationary-phase-, and openare obtained, allowing the model to be tubular (coated-wall)-CEC and favors the used to optimize selectivity. latter format due to the simplicity of colHaddad says he is somewhat surprised umn preparation. When anion-exchange that sharp elution peaks are obtained unmaterial is pumped through the 75-um-i.d. der most conditions, because sharp peaks capillary, the positively charged particles imply rapid mass transfer through the stick to the wall via electrostatic interacelectrolyte and at the capillary wall. Howtions. In contrast to the packed-column ever, a limitation of the method is that method, frits are not required, which peak tailing occurs for analytes with strong avoids a significant technical challenge. ion-exchange interactions when me eluent anion is present at a low conccntration. NevUsing a mixture of seven common inertheless, Haddad is enthusiastic about the organic ions, Haddad and Breadmore have demonstrated the principle of tuning technique and believes that it offers the best chance so far for selectivity control in the separation selectivity. The ion-exchange separation of inorganic anions. interaction is the variable factor, and its
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