Josh P. Roberts has an M.A. in the history and philosophy of science, and he also went through the Ph.D. program in molecular, cellular, developmental biology, and genetics at the University of Minnesota, with dissertation research in ocular immunology.
For those who came of age washing microplates with a handheld multichannel pipettor and a stack of paper towels, an automated washer can seem like a godsend. Of course, there’s the walk-away benefit of having a piece of equipment that will repeatedly fill a plate with buffer and then remove it for you. Add to that the reproducibly low inter- and intra-plate variability—with concomitant better overall assay performance—and you have a strong case for adding a washer to your already crowded benchtop.
Automated washers are typically divided into strip washers and full-plate washers. The former generally accomplish their task by sequentially washing a strip of 8 or 12 wells (sometimes 16, or 2 x 8 or 2 x 12) until the entire plate is complete. Full-plate washers, as the name implies, will wash the whole plate simultaneously. (There are also some instruments that will sequentially wash sections of a plate. For example, among BioTek’s offerings are a 96-tube array option that “comes down a single time for a 96-well plate, or it moves via cam mechanism to wash a 384-well plate in four sectors,” explains senior product marketing manager Jason Greene.)
For the most part, each can accomplish the same tasks, so choosing one may be a matter of price—strip washers tend to cost considerably less than full-plate washers—and speed. “If you have only a few samples, and you don’t run the assays every day, a strip washer is a good choice for you,” says Tuula Jernstrom, product manager with Thermo Fisher Scientific. “If you’re running a lot of plates or many assays, or you have a lot of samples, then a whole-plate washer is a faster choice for you.”
Assays
Although the lion’s share of automatic microplate washing is for ELISA and other colorimetric biochemical assays in 96-well plates, those are not the only things washers are used for. A variety of different plate types and configurations, hosting different types of experiments, can also be processed—assuming you have the appropriate washer.
It’s important to look at what kind of plates you plan to run, says Celeste Glazer, product manager for bioresearch at Molecular Devices. Low volume? V-bottom? U-bottom? Deep well? 384- or 1,536-well? “A lot of different types of plates don’t run in all different kinds of washers.”
Some washers can accommodate cell- and bead-based experiments, too. When washing cells that are adhered to a plate bottom, “you don’t want to touch them,” cautions Jernstrom. “You want to aspirate as much liquid as you can, but you have to do it very gently. And when you are adding the wash buffer, you want to do it very gently so that you don’t break the cell layer.”
Different manufactures have differnet ways to accomplish this. Some offer interchangeable cell wash heads or manifolds. Molecular Devices, for example, offers one head with angled dispensing pins that curve in rather than come straight down, and other vendors may offer different sized bores or other solutions, Glazer explains.
Often, the way the liquid is delivered and aspirated is controlled by the instrument’s programming. Tecan’s Cell Protection wash settings, for example, call for gradually raising the dispense-tip position as the tips release single drops and allow for reduced vacuum pressure. Other vendors may tilt the head, sweep it across the well or otherwise modify the x, y and/or z location or speed of dispensing or aspiration. Even with pre-set programs, users can generally “go in and look [at] what it’s doing,” Glazer says. “There is a lot of room for optimization.” For cells, that may be as simple as executing more cycles of gentler washes.
Separation technology
The popularity of bead-based assays has prompted some microplate-washer vendors to specifically address that application, providing tools that can aspirate liquid without also losing the beads.
There are two types of beads typically used in multiwell assays: magnetic and polystyrene. EMD Millipore offers assays that use both formats but currently is transitioning its portfolio to magnetic beads “just because of the ease of cleaning,” says Steven Suchyta, product manager for multiplexing and immunoassays. The company partners with automation firm BioTek, marketing a selection of the latter’s instruments with a choice of high-energy neodymium-iron-boron magnets, vacuum filtration, or both, and pre-loaded with parameters validated for EMD Millipore’s assays.
“Magnetic-bead assays would be run in conventional polystyrene-bottom plates, aspirating from the top, whereas the plastic-bead assays would be run in the filter bottom plates, aspirated from below,” explains Greene.
The interface
Instrument manufacturers increasingly are emphasizing ease of use, and microplate-washer makers are no exception. Many washers come equipped with (sometimes optional) Windows-like touch-screen graphical interfaces from which virtually all of the instrument’s adjustable parameters can be controlled. Some have a built-in help menu and even feature video tutorials that can be viewed on-screen. Programs can be imported or updated via USB flash drive. The resistive technology-based screen of the BioTek 405 TS even enables users to program the instrument while wearing gloves, Suchyta says.
In addition to washing parameters, the interface lets users control maintenance and other operational parameters and view warning messages that they are running low on buffer or that their waste bottle is getting full—safety features built into many washers currently on the market.
If the instruments are to be integrated with automation (and some instruments are better suited for that than others, Glazer says) control may be ceded to an external PC running vendor-provided or third-party software.
Bells and whistles
Researchers sometimes forget they need to take good care of their washer. “Best practice should be that every time you finish working, you wash the whole liquid path twice with distilled water,” says Jernstrom. Many instruments offer rinse protocols, some of which are automatically performed after a given interval or upon shutdown.
BioTek offers the optional Ultrasonic Advantage, a “full-fledged ultrasonic cleaner” that creates cavitation bubbles that break down salt crystal or protein buildup, explains Greene.
For labs that run multiple assays or require more than one type of wash buffer (with and without BSA, for example), it’s best to choose a washer with multiple liquid channels, each fed by its own bottle.
For more specialized applications, options such as plate shaking, temperature control and other add-ons can be had. Vendors may also tout parameters such as CV (coefficient of variation, or reproducibility), residual volume (how much liquid is left in the well after a wash cycle), dispense increments and the like. But the key may just be to figure out what your needs are and put the instruments to the test.