by Catherine Shaffer
Microplate washers are often not appreciated for the important work they do, because they don't produce the end result of an assay. But their performance can significantly affect that end result. Microplate washers come in the form of strip washers, full plate washers and combination washer-dispensers. They can handle a variety of plate types, including deep well plates and 96-, 384- and 1536-well plates. Applications that use microplate washers include ELISAs, as well as biochemical, cell-based and microsphere-based assays.
Making a poor choice in a microplate washer can have many consequences. Some washers cannot reach a low enough residual volume or cannot adjust the flow rate of wash liquid and vacuum in the aspirating system, resulting in damaged cells or poor wash quality, high background, large standard deviations and wasted time. Clogged manifolds can cause frustration and failed runs, and uneven dispensing leads to unpredictable, unreliable results.
Must-have features
Washing performance. Even very small amounts of residue (from insufficient washing) can increase the background signal of an assay and can lead to false positive or false negative results. Residual volumes can be minimized by tweaking dispensing and aspiration height and aspiration speed. Most microplate washers offer this feature.
“Very low residual volumes eliminate high background signal to ensure reliable assay results without the possibility of decreased assay sensitivity or false positive or negative results,” says Reija-Riitta Harinen, Product Specialist for Thermo Scientific, which makes Wellwash and Wellwash Versa microplate washers. A cross-wise sweep mode can reduce those residual volumes further.
Cleaning. It is important to keep in mind how easy it is to clean the instrument; dried buffer salts can create jams and blockages. In fact, according to Cathy Olsen, PhD, Applications Scientist at Molecular Devices, improper cleaning is the main source of failure in microplate washers. Molecular Devices offers the AquaMax 2000 and AquaMax 4000 microplate washers, which feature automated cleaning programs that keep the washers clean and running smoothly. The one-touch clean program lets users keep the entire fluid path clean so that assays work consistently. There is also an automated shutdown program that leaves the system empty and dry after a user-defined interval, preventing the buildup of contaminants. The AquaMax instruments also have features to suit every need and application. “For ELISAs, [customers] need thorough washing to minimize background, and the ability to keep their system clean so that assays give consistent results,” said Olsen. Users doing both biochemical and cell-based assays need an instrument that can do both with minimal fuss. Interchangeable heads make the transition simple. The heads can be changed in 30 seconds with no tools or calibration.
Matrical Bioscience's SQUIRT microplate washer offers a pre-programmed wash routine and is lidded for easy access to the internal workings of the instrument. In addition, a computer interface provides a real-time graphical display depicting parameters such as wash cycles, wash pressure and wash speed. These features make the washer easy to learn and operate.
Safety. Safety is an often-overlooked factor when selecting a microplate washer. Hazards include wash- and waste-bottle spillage and liquid being drawn into the aspiration pump, damaging the instrument. Washers that come with liquid-level sensors and plate sensors can prevent mishaps in the course of use. An aerosol cover can shield a plate containing infectious material, preventing contamination of the instrument and laboratory.
“Our washers use diaphragm pump technology, which makes for quieter and safer operation,” says Tristana Von Will, an applications specialist for Biochrom.
The Biochrom Anthos Fluido 2 washer is optimized for absorbance and luminescence assays. Its primary application is for ELISA. It comes with 8- and 12-channel manifolds for 96-well plates and 16-channel manifolds for 384-well plates. Its four liquid lines are configurable to many microplate formats and wash programs. Its safety features include a stop function, a hinged transparent aerosol shield and liquid-level sensors. Biochrom also offers the Asys Atlantis washer, which is compatible with any type of reader or application.
Gentle dispensing for adherent cells. If the washer is going to be used to wash adherent cells, it needs to have the ability to gently dispense and remove excess liquid without disturbing the cell layer. A good feature to have in a microplate washer used with such cells is an angled dispensing tip, so that weakly adherent cells are not washed away with the solution. A number of manufacturers offer that feature.
Networking and automation
Most washers will need to integrate into an established laboratory workflow or an automated workflow, and many have features to make that integration easier.
BioTek Instruments, for example, supplies strip washers, full plate washers and combination washer/dispensers—each powered by onboard or PC-based software.
BioTek’s most popular model, the ELx405 Microplate Washer, includes a dual-action manifold to provide individual control of aspiration and dispensing manifolds, and ultrasonic cleaning technology
to eliminate assay failure based on poor maintenance. The ELx405 can be used for various
applications, from vigorous and complex wash routines to gentle cell-based washing, magnetic-
and polystyrene-bead washing and even deep well plate washing. Applications for the instrument include plasmid DNA purification, serum sample prep, cell counting and cell proliferation assays.
“Versatility is built into each instrument so that users don’t have to compromise their process to
suit an instrument’s scope,” says Gary Barush, director of marketing and sales at BioTek. “It can be daunting to sort through microplate washer options on a website or in literature, so we encourage potential customers to schedule an on-site demonstration.”
The Titertek-Berthold Zoom and Zoom HT washers are designed to handle all known applications. The high-throughput version, Zoom HT, can process 60-plate batches. Zoom FP is used for binding assays, bead-based assays and other applications that require filtration plates.
“In our Zoom HT, we have a linear shuttle that quickly shuttles plates from storage position into wash position and back. This makes our Zoom HT washer the fastest washer for high-throughput plate wash,” says Jon Call, Product Manager, Liquid Handling Equipment, Titertek-Berthold.
Titertek-Berthold encourages investment in a quality washer. “A washer is a washer. Why pay $15,000 when there is something for $5,000, right?” says Golovleva. However, the problems that result from a low-end washer include inability to reach low residual volumes; non-adjustable flow rates of wash liquid and vacuum, which clogs manifolds; and uneven dispensing of wash liquid. “In other words, poor wash quality ultimately leads to high background, huge standard deviations between replicates of the same sample and valuable time lost on maintenance and troubleshooting,” Golovleva says.
Details and extras
Although the broader issues of washing efficiency, workflow and application should be primary considerations when choosing a microplate washer, there are several smaller considerations that can make daily use of the instrument more pleasant and/or efficient. These include:
Speed. Microplate washers come in the form of standard plate washers and “strip washers.” A 96-channel wash head can process a plate at least 12 times faster than a washer with an 8-channel wash head.
Setting your own configuration. The ability to use your own wash bottles and the configurability of bottles and wash heads are features that may interest users.
Upgradability. Some instruments will need to be upgraded eventually, whereas others include components to accommodate future needs for different plates and assays. “Customers should consider the plates they are using now and those they might be using in the future,” says Bob Alexander, a marketing specialist for Matrical Bioscience. “Conventional washers may require the purchase of additional manifolds, when switching between plate types.”
The image at the top of this page is BioTek Instrument's ELx405 Microplate Washer.