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Lateral-flow assays: Designing for automation

David L. Carlberg

Low-tech assembly may dominate the world of IVD test strips today—but the companies that will flourish in the future are those that plan ahead for automation.

Diagnostic test strips are everywhere these days. And much like the universe, the market for these products continues to expand at an astronomical pace. But when the manufacturer of such a lateral-flow assay suddenly finds that it has more customers than it could ever have imagined, it can also find that scaling up to meet the demands of success can be an undertaking of galactic proportions.

The biggest difficulty for a company deciding to scale up its test-strip manufacturing process is often a failure to see the big picture. Too often a product is designed, and its manufacturing processes developed and validated, with little or no thought to the growth of the product or how it will be manufactured in large volume. Once this process has begun, it can be very difficult and expensive for the company to change existing validated processes in order to accommodate the mandates of automation.

Often, the root of such problems lies with the laboratory scientists involved in developing the assay. Their preoccupation is to optimize the membrane flow rate, to establish the proper location of the capture and control lines, and to negotiate the right combination of reagents, antibodies, and conjugates (see Figure 1). But because they are creating test strips a handful at a time, the last thing on their minds is to design the test to optimize its manufacturing processes.

Figure 1. Typical lateral-flow test-strip construction.

Process Scale-Up

Using commercially available benchtop modules for dispensing, laminating, and strip-slitting, the development scientists optimize the design of the test strip, taking care to carefully document and validate the process (see Figures 2 and 3). Suddenly demand exceeds marketing projections and panic hits. The question becomes "Do we add more assemblers and duplicate the existing processes, or do we automate?"

Figure 2. Typical module for manual lamination of diagnostic components.

Adding more assemblers is a mixed blessing. On the plus side, it can be done quickly, and it allows the manufacturer considerable flexibility (that is, as demand for the product changes, staffing can vary proportionally). On the minus side, it means many more assemblers to train, ongoing labor costs, greater potential for personnel disputes, greater pressure on existing facilities (or even for larger facilities), and, in the end, quality and yield are generally reduced.

Figure 3. Typical module for cutting cards or webs into individual test strips.

Automation can mitigate many of these concerns. But manufacturers should consider such a decision carefully and with their eyes open. They should open a dialog with equipment vendors, and begin to develop project budgets and schedules. And based on those budgets and schedules, they should then be in a position to determine just how much automation is sufficient—and which processes are best automated.

But even better than waiting until scale-up is forced upon them, manufacturers should consider the automation of a product's processing on the day the product is conceived. Decisions made at the very beginning can have a marked impact on the viability of automated processes later on. Elements as basic as materials selection and component dimensioning should be evaluated carefully and should be discussed with a reputable automation company—one that understands the subtleties of test-strip manufacturing—early in the design process.

The benefits of automation are many, not the least of which are improved quality, reduced product cost, higher yields, and smaller facilities requirements. These improved efficiencies add up to streamlining a company's overall operations, giving the firm a competitive market edge.

Materials Selection

In the materials selection process, functionality is the primary concern. The membranes, filters, absorbents, adhesives, desiccants, and other components must enable the product to function as desired. Everything else being equal, however, the best materials are those that minimize potential processing problems.

In test-strip manufacturing, the biggest material-related problems are generally those involving the use of adhesives (see Figure 4). Adhesives tend to build up on laminator guide surfaces and on the cutting surfaces of strip-slitters. Loose fibers from the various filter components can be picked up by adhesives and, before long, the manufacturing equipment is covered with balls or strings of adhesive and filter fibers that can attach themselves to test strips or otherwise interfere with efficient processing. Frequent cleaning is often a necessity. Of course, it is impossible to avoid adhesives entirely, but the amount and tack of the adhesives should be kept to a minimum whenever possible.

Figure 4. The biggest material-related problems in test-strip manufacturing are generally those involving adhesives, shown by the adhesive buildup on a rotary slitting module.

If it is anticipated that market demand will ultimately require web-based (reel-to-reel) manufacturing processes, it is important to select materials that are available in web form and that can be efficiently processed in this way. Some materials are very difficult to handle in web form.

Conjugate pad materials include paper-based products, glass fibers, and polypropylene. Sample and absorbent pads are usually paper-based products. Each material has its place in test-strip construction. But when optimizing strip design, manufacturers should consider the limitations of each material, carefully evaluating their weaknesses where automatic processing is concerned. It is a good idea to test the materials against one another, paying careful attention not only to how they function, but also to how they handle.

In general, paper products have very low tensile properties, especially when wet. This translates to difficulty in web coating and laminating processes.

Glass fibers are generally difficult to cut or slit, have poor tensile properties (which makes them difficult to guide on web systems), and cause excessive wear on machinery, especially cutting blades.

Although a wide variety of alternative materials are available, nitrocellulose membranes remain the most commonly used substrate for lateral-flow tests. Both supported and unsupported versions of these materials are common, but it is important to note that unsupported nitrocellulose always presents a problem. It is very fragile and difficult to guide in web processes. Because unsupported membranes are much more prone to fracture than supported membranes, yields are significantly reduced. In short, unsupported nitrocellulose should be avoided whenever possible.

The backing materials used in test-strip construction are generally polyester, styrene, or PVC with a pressure-sensitive adhesive layer on one side. Sometimes—especially in the case of free, unencased test strips—there is a desire to have a very thick and stiff backing. It is important to keep in mind that thick backing materials are more difficult to cut. As the strips are slit, the excessive cutting pressures required to cut a thick backing can cause undesirable marks along the edges of some of the strips. Such problems can generally be minimized by using a thinner backing. Again, it is wise to test prospective materials prior to making a final selection.

Manufacturers can maintain their ability to efficiently scale up their manufacturing process by considering the following additional points related to materials selection.

• Make sure that all vendors intend to produce and support the proposed materials in sufficient volumes, preferably well beyond the expected life of your product (recognizing, however, that there are no guarantees).

• Second-source materials whenever possible.

• Use only well established, reputable vendors with a history and reputation for quality and customer support.

• Avoid experimental materials; they may be unavailable in sufficient manufacturing quantities or may be discontinued, leaving no alternatives.

Dimensional Considerations

The nominal dimensions of a test strip—including its length, width, and the position of its capture and control lines—are generally driven by the sample volume and speed requirements of the test. Varying the width or length of a test strip by a few percent may have little effect on its functionality. If such a strip is to be enclosed in a housing or cassette, however, these dimensions must be maintained much more accurately.

For example, consider an assembly that requires the test strip to fit between containment pins or between the walls of a plastic housing. If the strip is too wide, it will be a tight press-fit, making automated assembly difficult and unreliable. If the strip is too narrow, it may slide around too much and not be covered entirely by the sample well or read-window. If the length of the strip is not accurate, the test lines may not line up with corresponding features on the cassette top. Proper dimensional tolerances of both the strip and cassette are therefore very important.

When determining the location and dimensions of the various test-strip components (e.g., the nitrocellulose substrate, conjugate pad, sample pad, and absorbent), it is important to keep in mind that each of these elements comes with its own set of dimensional parameters (see Figure 5). It may be difficult, for example, to achieve a 1-mm overlap of conjugate or absorbent to the nitrocellulose when each component has a slit-width tolerance of ±0.25 mm and the tolerance of the backing web is also ±0.25 mm.

Figure 5. Typical test-strip construction showing recommended lamination dimensioning and tolerancing. (Notice that overlaps are not dimensioned, but rather one edge of each component.)

Most lamination processes guide one edge of each component relative to a specific datum—usually one edge of the backing material. During lamination, each of the components may move within its respective tolerance range. Add to this the tolerance variation in the widths of the components, and it is easy to see how a theoretical overlap of 1 mm can suddenly drop to as low as 0.25 mm (all tolerances 'under') or increase to as much as 1.75 mm (all tolerances 'over').

Optimizing Cassette Design

Among the lateral-flow assays on the market today, there is a large variation in the types and styles of cassette designs. The simplest are small rectangular assemblies comprising a top, a bottom, and a test strip. Others are designed in more elaborate, asymmetrical shapes, and may also include a midstream urine wick or a desiccant tablet. Some devices may even contain multiple test strips.

If automated assembly of the device is a goal and low-cost automation is a requirement, then it is best to stick with a basic rectangular design using as few components as possible. This doesn't mean that complex designs can't be accommodated. It simply means that automation of these designs will, in general, be more expensive (see Figure 6). This is because the simplest and most efficient machine design will generally employ simple, straight-sided part conveyors and containment mechanisms. If a device is tapered or has asymmetrical or curved sides, then locating and holding the part at critical placement positions becomes more difficult. This leads to higher machine complexity and hence, higher cost.

Figure 6. Five device configurations. Because of their straight parallel sides, automated assembly of C and D is simpler than for A, B, and E.

To improve the chances of accurate and reliable assembly, manufacturers should design their devices so that the top and bottom halves are generally the same size and shape, with the bottom being a few thousandths of an inch larger in both length and width. Additionally, the pin-socket arrangement that secures the top and bottom halves together should be easy for automated equipment to locate; this can be accomplished by chamfering both the pin and the socket by about 0.010 in. x 45°. To allow for automatic part orientation, neither the top nor the bottom half should be completely symmetrical; each should have at least one feature that can enable an optical sensor to distinguish one end from the other.

To simplify automated pick-and-place of the device's components, the strip-locating features in the housing bottom should be dimensioned and toleranced from one outside edge of the part (see Figure 7). The same is true for the locating pins of the two housing halves as well as any for any features used to position a midstream urine wick or desiccant tablet.

Figure 7. Recommended dimensioning and tolerancing for a typical test-strip device. Illustration shows a device containing a hypothetical 0.200 x 2.500–in. strip and a desiccant. Dimensions are in inches.

The test strip should nest within the cassette bottom closely, but loosely. We recommend a nominal clearance of the strip to the nesting features of about 0.005 to 0.010 in. (0.13 to 0.25 mm) along both the width and length. When the top is pressed in place, it should contain the strip and provide slight pressure on the components to ensure intimate contact and optimize sample flow within the strip.

It is imperative that all components of the device—including the test strip, wick, and desiccant—be assembled to the bottom half of the device and not to the top. This is because the device will be assembled right side up and all the components must be assembled to the bottom before adding the top. Whether the urine wick is placed above or below the test strip is of no consequence to the automation process, but it must be a press-fit in the bottom, with sufficient gripping ridges or "teeth" to hold it securely after assembly is complete.

Equipment Options

From small jigs, fixtures, and bench-top modules to very high-end integrated manufacturing systems, the equipment choices for strip-manufacturing processes are many. Keeping in mind the need to maintain scalability in order to minimize costly changes later on, the early development process should include evaluation of the various equipment options available to the manufacturer.

Most of the lateral-flow test strips produced today are manufactured using card stock—the membranes and backing materials are all brought to the process as small, individual pieces, usually no more than about 9 to 12 in. in length (see Figure 8). Reagent dispensing, drying, laminating, and strip-cutting can all be accomplished using stock in this format.

However, there is a trend in industry toward the use of web-based processes. Web processing can significantly increase manufacturing efficiencies because it produces less waste material and enables a number of subprocesses to be performed as parallel rather than serial activities. In addition, web processing generally increases product quality because it requires less handling of the materials and components, and because computer-vision systems can be employed for continuous process inspection.

Figure 8. Individual diagnostic components prior to lamination.

But the biggest savings brought about through the use of web-based systems is in the time required for processing. Using a web-based dispensing operation, for example, a 100-m roll of membrane can be processed through the dispensing and drying functions in approximately 20 minutes. By contrast, a card-based dispensing process would typically require well over an hour to produce an equivalent length of material, and this doesn't include drying time or the additional time needed to move individual cards into and out of the drying oven.

Lamination can also be performed much more efficiently as a web-based operation (see Figure 9). A typical lateral-flow strip comprised of four individual components (i.e., backing, nitrocellulose, conjugate, and absorbent) would require approximately 30 seconds to assemble on a typical benchtop lamination module. Assuming that 30-cm cards are used, it would take approximately 23–4 hours to laminate 100 m of material. This same 100 m could be assembled on a web laminator in about 15 minutes.

Once they have been laminated, most lateral-flow materials must be reduced to cards for further processing. Although roll-feed systems are frequently used for other types of strip products, the stiff, thick laminates used in lateral-flow devices generally cannot be rolled up without damage to the pads or membranes. Consequently, the equipment made for slitting and device assembly processes is generally designed for use with card-based stock. Cards can be fed to the cutting and assembly processes by hand, or by means of automatic card magazines.

In a wholly manual manufacturing operation, the final assembly process—bringing together the various components required to complete a finished device—typically represents as much as 70% of the total labor component of the device. It also offers the largest single opportunity for cost benefits, even if other manufacturing subprocesses remain under-automated.

An operator using card-based dispensing and laminating processes, for instance, can produce a multitude of strips in a single operation. Both functions are generally performed on contiguous groups of 50 or more strips. If it takes an average of 40 seconds to complete both functions (that is, 20 seconds to dispense the reagent, and 20 seconds to laminate the membranes), then a single operator should be able to produce approximately 75 strips per minute (50 strips/40 sec. x 60 sec.). By contrast, a manual operator may only assemble about five finished devices per minute (a rate of one every 12 seconds). And it is the difference between these two rates of production that makes the final assembly process the most tempting target for improving a manufacturer's cost/benefit ratio through automation.

Figure 9. Reel-to-reel web lamination.

In its simplest form, automated assembly includes cutting and placing the test strip into the bottom of the plastic housing, placing the top of the housing onto the bottom, and pressing the assembly closed. At the other extreme, a fully automated assembly process can include all the above operations plus automated feeding of all components (e.g., wicks, desiccants, housings, and cards or webs of the strip component), and 100% inspection of the process using a computer-vision system. Equipment that can perform this combination of functions can be expensive, and the time required to develop such custom machinery can be long. For large-volume manufacturing, however, the payback from such a system can be quite significant.

Conclusion

Although it is often believed that producing millions of products requires a high level of automation, this isn't necessarily the case in the diagnostic test-strip industry. There are many large and small manufacturing operations throughout the world, each producing tens or hundreds of millions of strips a year using only very low-technology tools—mostly bench-top modules for dispensing, laminating, and slitting their products. There are myriad assemblers sitting around tables, hand-assembling diagnostic devices. The world is flooded with these, mostly all good products, meeting a variety of needs to help us all to lead more healthy lives.

Competition in the industry is high, yet new startups are popping up every day. This is because the market is immense and has many niches. And as each company struggles to garner a larger market share, its competitors are struggling to do the same. It is through automation that the strong will survive—by planning early, by being creative, and by having vision and foresight.

David L. Carlberg is president and cofounder of Kinematic Automation Inc. (Twain Harte, CA).

First photo By Rich Miller Photography, Courtesy Kinematic Automation