Abstract
The Society of Biomolecular Screening Proposes New Design Standards for one of the Industry's Most Used Tools - the 96-Well Plate.
V grooves allow high tolerance positioning by robotic grippers.
The Society of BioMolecular Screening is endeavoring to create a consensus to establish a level of standardization for 96-well microplates. More mechanization is being provided to meet the increasing use of microplates in biological assays. This involves various forms of stackers and robotic hands. These mechanical systems are far less tolerant of plate variations than manual systems. The variation of plates results in increased handling costs and less reliability.
It should be possible to achieve various levels of standardization over a period of time, which will benefit all concerned. When the plate supplier makes a new mold, he will know the plate will function in automated systems. The equipment designer can design around fewer variables, resulting in greater reliability. The end user will benefit by having fewer plates jam or drop on the floor.
Care must be exercised that standardization does not limit meaningful creativity in the design and function of microplates. The Society of BioMolecular Screening proposes approaching standardization in two stages. The first stage would deal with standardization from the point of view of the machine designer, i.e., would concentrate on factors related to automated handling of microplates. The second stage would approach standardization from the perspective of the end user.
The following proposals are intended as a starting point to solicit comments and suggestions for the first stage of standardization. A committee of the Society for BioMolecular Screening will consider all suggestions, and a final proposal will be submitted for approval in October 1996, at the annual meeting of the Society.
A common dimension among all plates is the prime requisite of the machine designer. It is felt there are fewer limitations maintaining the external footprint than any other dimension. The chosen dimensions are based on the original Cooke Microtiter and Linbro plate (Figure 2). The straight line tolerance should only be a problem on plates molded of softer resins, such as polypropylene. Hopefully, additional ribs can be used to maintain the edge to the stated tolerance.
Proposed dimensions of standardized 96-well plates.
These dimensions would allow the machine designer to use a nest of 5.050 inches long and 3.380 inches wide. The smallest plate in that nest would give a total movement of 0.030 inches length and width, while the largest would have 0.010 movement.
The outside flange around the bottom serves as a nest for the plate below when stacked on top of another. However, this flange is effectively used in escapement type devices on plate stackers. A defined dimension facilitates the escapement design. Limiting the height of the flange provides a larger known area of the plate side wall for a robotic hand to grip, particularly if the plate is to be used with a lid.
The plate sidewall is commonly used to grip the plate with robotic hands, thus the need for a plain surface without protrusions. A plate stacking mechanism is also facilitated by having a smooth, plain surface above the outside flange. As the volume of testing increases, more bar code labels are being used. These also require a plain smooth surface for adherence.
It is not felt that the height of the plate should be defined by a standard. This would put an unnecessary restriction on the creativity of the plate designer.
When gripping a plate with a robotic hand, it is desirable to have it in exactly the same position within the hand each time. By putting mating V projections on the fingers of the hand (Figure 1), they will locate the plate at the same point as the hand closes. The inclined planes of the Vees will slide the loose plate to the same exact position within the hand each time the fingers close.
Putting the Vees between columns 2 and 3, and 10 and 11, leaves room between these columns on the sidewall for a bar code label.
The same Vees could be located on the ends between A and B, and G and H. However, this would restrict the allowable bar code application area. The Vee could be moved out to the centerline of A and H, instead of in between rows.
A third function of the Vees is to aid in lid removal. The sidewall of the lid would be straight, spanning the opening of the Vee. The lidded plate could be dropped through an opening that had four projections coming into the plate bottom, but clear of contact with the Vee. This would allow the plate to pass through.
The lid, without the Vee, would be retained on the four projections, and thus separated from the bottom. It can be retained in such a manner to allow the bottom of the plate to pass back up through the opening to allow lid replacement. This provides another means of handling lidded plates on readers, and even stacker designs.
Leaving a 0.200 inch distance between the bottom edge of the lid and the plate base line, assures a gap of at least 0.100 inches between the lid and the plate outside flange. This allows a robotic hand to grip a lidded plate and move the pair in one move. It also should leave enough room on the lid to allow the hand to grip the lid only, while leaving enough closure of the lid over the plate to minimize evaporation.
The cutoff corner assures the lid can only be replaced one way. Any condensation on the lid would remain over the same well.
From the user's standpoint, it is desirable to have one lid fit all plates. It eliminates having to keep lids matched to the plate. To do this would require setting standard dimensions for the top of the plate, so as to obtain a fit from a common lid. This would probably also lock in many of the lower dimensions. This is an area in which more feedback is needed. For instance, what impact does that have on the plate molders?
There is some value in automated equipment knowing the depth of the well. More correctly, it is knowing where the bottom of the well is, in relation to a known dimension. In this case, that is the distance, from the nest upon which the plate base line sits, to the bottom of the well. Setting this dimension still leaves the depth of the well dependent on the total height of the plate, which is left as a variable dimension.
Questions and suggestions regarding standards for the 96-well microplate may be directed to Tom Astle at Tomtec, 1010 Sherman Avenue, Hamden, CT 06514, tel. (203) 281-6790, fax (203) 248-5724.