AC Research
Accelerated Articles Anal. Chem. 1997, 69, 543-551
Characterization of an Inkjet Chemical Microdispenser for Combinatorial Library Synthesis Anthony V. Lemmo, Jeffrey T. Fisher, H. Mario Geysen, and Donald J. Rose*
Glaxo Wellcome Research Institute, Inc., Research Triangle Park, North Carolina 27709
Solenoid inkjet valves are characterized and implemented in an automated chemical microdispenser, the ChemJet, to be used for combinatorial library synthesis. A novel high-density format for chemical synthesis and assay is also presented. The “ChemSheet” consists of a 48 × 48 array of 8-µL wells, molded as an 8 1/2-in. × 11-in. sheet of polypropylene. The development of the ChemSheet created the need for high-speed, parallel microdispensing. The ChemJet addresses this need by providing the ability to dispense 8-µL volumes of chemical reagents to the 2304 wells of the ChemSheet in under 10 s, with an average reproducibility of ∼2.2%. Results from both quantitative and qualitative characterization of inkjet solenoid valves are presented. Characterization of the ChemJet instrument is also presented.
Recent advances in combinatorial chemistry and molecular biology have created the need for new types of dispensing devices. Combinatorial chemistry applications require a robust, chemically inert dispenser which can rapidly dispense microliter amounts of liquid to high-density formats. For example, to synthesize a library of 1 million discrete compounds, each composed of three monomers, a total of 3 × 106 dispenses would be required. If each dispense consumed 2 s (dispense time and time for movement between locations), then the total time to dispense all of the reagents would be 6 × 106 s, or 1667 h, or 21 8-h days. Two ways to reduce this dispense time would be to dispense in parallel or to decrease the individual dispense time. Doing 10 dispenses/s with 48 dispensers in parallel would reduce the total dispense time to under 4 h, which is on the same order as the other steps in the synthesis (e.g., reaction and washing). Our need for high-speed, parallel microdispensing arose during the development of a novel high-density format for chemical synthesis and assay. The “ChemSheet”, shown in Figure 1, S0003-2700(96)00808-6 CCC: $14.00
© 1997 American Chemical Society
Figure 1. ChemSheet format. The format consists of a molded polypropylene sheet containing 2304 wells, 8 µL in volume, arranged in a 48 × 48 array. The spacing between the wells is 4.5 mm, identical to that of a 384-well microtiter plate. The overall dimensions are 8 1/2 in × 11 in.
consists of a 48 × 48 array of 8-µL wells molded as an 8 1/2-in. × 11-in. sheet of polypropylene. Each well is 3.5 mm in diameter and 0.9 mm deep. The shallow well depth is required for easy washing of the well surface. The resulting 2304 wells are spaced 4.5 mm apart (the spacing of a standard 384-well microtiter plate). One compound is synthesized in each well and then screened either by measuring the binding to that tethered compound or by releasing the compound from the surface and measuring its activity in solution. Chemical libraries are created on the ChemSheet in the following manner (assuming each component in the library is composed of three monomers): (1) n number of ChemSheets are functionalized with n number of monomers Analytical Chemistry, Vol. 69, No. 4, February 15, 1997 543
(monomer set 1) such that each sheet has the same monomer attached to the surface of each well; (2) 48 unique monomers (monomer set 2) are dispensed across the 48 rows of each ChemSheet, where they are coupled to monomer 1, and then each sheet is washed; (3) 48 unique monomers (monomer set 3), the same as or different from those in step 2, are dispensed across the 48 columns of each ChemSheet, where they are coupled to the dimer (monomer 1 and monomer 2), and then each sheet is washed. The result is (n)(48)(48) unique compounds, ∼5 nmol of each. By processing 48 ChemSheets, over 110 000 compounds can be produced in a spatially encoded array of wells. This spatial encoding provides rapid decoding of compounds which yield a positive result in a biological screen. Inkjet-type dispensing, both solenoid and piezoelectric, have been used in many areas of chemistry and science including fabrication of biosensors,1,2 in vitro neurophysiology,3 sample introduction for mass spectrometry,4 sample introduction for capillary electrophoresis,5 dispensing droplets of solder for fabrication of printed circuit boards,6 and fraction collection in an ionexchange chromatography system.7 Liquid dispensing using a solenoid inkjet valve involves connecting the valve inlet to a pressurized (5-10 psig) fluid source and connecting the valve outlet, via a delivery tube, to a restrictive orifice. The dispense occurs by momentarily opening the valve for several milliseconds, which launches an acoustic or pressure wave down the delivery tube to the restrictive orifice. Upon reaching the restrictive orifice, the velocity of the liquid stream increases, causing the liquid to “jet” from the orifice, as shown in the series of video frames in Figure 2. Based on the time scale of the video frames (one frame every 1/60 s, or 17 ms), the dispense event shown is complete after ∼30 ms. The use of inkjet-type dispensers for delivering chemical reagents has several distinct advantages over syringeor pump-based pipet dispensing: (1) The dispense process is a noncontact dispense (see Figure 2), eliminating the need for washing between additions of reagents. (2) The dispense time is rapid, essentially limited only by the response time of the solenoid valve (typically
