Five factors to consider when designing a tablet

Tablets are still one of the most convenient methods of drug delivery and their complexity of design has increased to meet today’s market requirements. Steve Deakin, Business Development Manager and Steve Osborn, Product Design Manager, I Holland, outline the major design considerations

Good tablet design is extremely important; it has an impact upon anticounterfeiting, tooling strength, tablet coating, durability and functionality. It also helps to avoid downstream manufacturing problems such as tablet sticking, picking, lamination, capping and premature tooling failures. It is essential to consider these at the beginning of the process to ensure a problem-free, high quality end product.

Several elements need to be considered when designing a tablet; here some of the most important ones are considered:

1. Tablet shape and profile

The first thing to consider is the tablet shape and optimum profile. There are two basic tablet shapes: round and non-round; however, the complexity of non-round shapes can vary and may require specialised tool manufacturing capability.

Once the base shape has been decided, tablet size must be determined; here consideration should be given to the type of press available for tablet manufacture, as this can limit the size of the tablet.

Next follows selection of the tablet. The type of profile required is influenced by several factors; the granule, embossing requirements, coating process, packaging and company branding. Thought should also be given to the volume of the tablet and whether it will be coated. Successful coating is dependent on tablet profile. Coated tablets, whether film- or sugar-coated, present challenges for the tablet designer. The complexity of the coating process is vast. Many of the variables are within the manufacturer’s control but expert tablet design can help eliminate potential problems.

Typically, the centre of a tablet is lower in hardness, so during the coating process core erosion may take place

Typically, the centre of a tablet is lower in hardness, so during the coating process core erosion may take place. This is when the tablet comes into contact with the coating pan and other tablets, causing wear. This vulnerability, caused by mechanical stress during coating, can be reduced by avoiding very deep concaves and ensuring a robust design. For film-coated tablets, double radius profiles are the preferred choice.

For shallow tablets with hard, sharp edges, the coating process will damage the exposed edge of the tablet, resulting in chipped edges and sometimes cracks. Flat and shallow tablet profiles should therefore be avoided.

Double radius designs ensure a strong tablet edge and a balanced profile, which will roll in the coating pan. Another benefit of the double radius design is that it can accommodate most marking and branding requirements, because it increases the usable surface area available for this.

Poor marking and branding design on the tablet can lead to ‘bridging’ (the coating collects in the detail on the face of the tablet because it does not fully follow the contours of the marking on the tablet core, but bridges over leaving a void under the coating) and ‘infilling’ (when too much coating material has filled the detail making it indistinct) during film coating. Possible causes for this can include:

  • Inadequate adhesion of the film coating – the coating supplier should be consulted to improve the adhesion characteristics of the coating.
  • Inappropriate marking design where the angle may be too acute or too deep (bridging) – the marking should be redesigned in consultation with the tooling supplier. It can also be due to the stroke or section of the embossing being too wide or too shallow (in-filling).
  • Inappropriate coating procedure i.e. spray rate, drying time, etc – the coating supplier should be consulted.

Another problem is twinning: tablets sticking together during coating. This is normally caused by the flat surfaces of the tablets coming into contact and adhering to each other. To avoid this a slightly curved surface can be applied, which reduces the contact area and eliminates the potential for twinning.

2. Tablet breakability

Good tablet design will enable the tablet to be broken easily and accurately, ensuring that when it is broken, the required tolerance for dosage is achieved. Uneven breaking of a tablet may result in significant fluctuations in the administered dose. The degree of inaccuracy may be associated with breakline design, tablet hardness, and/or formulation (see Figure 1).

Figure 1: Various tablet breaklines/bisect type

The following factors should be considered when selecting a breakline:

  • Accuracy of breakage, which is important for equal dosage.
  • Holding of the tablet and ease of breakage. This relates to tablet size and hardness.
  • Inclusion of other detail such as a logo and its influence on the breakline. Product identification must be maintained to ensure brand integrity when the tablet is divided.
  • Robustness of the tablet during compression, coating and packing, because the tablet’s physical qualities are changed by adding a breakline and it may become weaker.

The breakline should penetrate into the tablet while maintaining an optimised radius and angle. A larger radius usually makes the breakline less effective.

Where a breakline is functional and present on both sides of the tablet, alignment of the breakline on the upper and lower punch tips is critical and requires the turret to have a lower key facility. Also attention should be paid to breaklines that stand above the punch tip edge. Upper and lower punches must be set correctly for effective tablet ejection and take-off. If not set correctly damage can occur to the breakline on the punch tip, resulting in catastrophic failure and damage to the tooling and the tablet press. It can also cause severe chipping of the tablet on ejection.

3. Tooling performance

Tablets are becoming more complex and exotic in both shape and profile. As the tablets become more complex so does the tooling, which increases the demand for tooling strength, durability and overall performance. This should be a major consideration when designing a tablet.

Figure 2 shows a stress and fatigue analysis of a punch tip using Finite Element Analysis, or FEA – a software-based numerical technique for calculating the strength and behaviour of engineering structures. It is used to calculate deflection, stress and strain to determine fatigue limits of both material and design.

Figure 2: A stress and fatigue analysis of a punch tip using Finite Element Analysis

One of the most important features of any tablet design is the blended land. Often, tablet manufacturers elect not to apply a land as it may not be visually acceptable on the finished tablet. Lands that are applied incorrectly – either unevenly or made too large – can present a range of issues including: flashing or lamination during compression; chipping of the land during take-off; or build-up of coating on the edge of the tablet which eventually will chip.

The answer is always to include a blended land as, when applied correctly, it will optimise tablet and tooling strength and performance. Figure 3 shows the application of a blended land to a punch tip. The correct method of applying the land is to ensure that the flat area on the tip edge is maintained while blending the intersection between the profile and the flat. This is achieved by applying a radius to the finished punch tip. A correctly selected and applied blended land provides benefits to handling, loading, setting, tooling strength, the visual appearance of the tablet and, ultimately, the brand.

Figure 3: Blended land

4. Tablet branding

When considering the visual appearance of the tablet, it is important to think about the type of font and logos used for branding. Typefaces and designs must take into account practicality of tablet manufacture.

Caution is also required during the design process when applying branding to your tablet. Failure to consult with an expert tablet design team could result in a product that looks good on paper but is not practical to produce.

For tablets with a logo, the design and placement are very important. The tablet designer should always seek to maximise the face area to avoid picking and lack of distinction. Figure 4 shows a good example of the importance of spacing on logos. The top example clearly shows the embossing within the safe zone for this particular tablet shape. The bottom example shows embossing that goes beyond the safe zone and on the side view it can be seen how the embossing protrudes.

Figure 4: The importance of spacing on logos

Occasionally, when the need to exceed the safe zone is required, the best practice is to ensure the detail is spaced far enough away from the edge of the tip. As a general guide the embossing should sit below the landed edge of the punch tip. If this guide is not followed then the embossing will be unprotected and prone to damage, causing further downstream problems during the tableting process.

The correct font style is also very important to avoid tableting problems such as ‘picking’. Picking is compressed granule that has adhered to the detail on the punch face, resulting in ‘picking out’ of parts from the tablet face. To reduce picking the best practice should be to design font styles that have large open counters and no sharp corners that could act as a trap for granule. Selection of the right font style can also help to avoid coating problems, tooling failures and lack of distinction.

When the font has been chosen it is important to ensure clarity of definition. The profile of the embossing is equally important to reduce picking, ensure good tablet coating and tooling strength. The best practice is for the stroke angle to be between 70 and 80 degrees. The stroke depth should be 50% of the width and the stroke break radius blends 30% of the stroke depth (Figure 5).

Figure 5: Stroke angle

Another technique used to minimise picking is a reduced counter. The counters, which are sometimes referred to as islands, are very vulnerable to picking and granule can easily get trapped in these areas on the punch tip face. The counter is modified to increase the surface area by reducing stroke depth, which will minimise the tendency for the product to pick. A tapered peninsula (Figure 6) can also be applied. The taper is applied to blend between the surface of the tablet face and the stroke angle. This method softens the profile, reducing the risk of powder being trapped in the shaded areas shown in the image. This maintains definition without reducing the overall stroke depth of the embossing and will assist in keeping a clear brand identity.

‘Sticking’, of course, is another major issue in the design and manufacture of tablets. Sticking differs from picking in that it is granule adherence to the punch tip face, rather than in and around the embossing. This is not normally associated with design, but when picking occurs on a tablet this in turn can result in sticking on the punch tip face by providing a key to which further granule will adhere.

Figure 6: Tapered peninsulas

5. Anti-counterfeiting techniques

As counterfeiters become more technologically advanced, basic tablet designs are more easily reproduced. There are many anticounterfeiting technologies available to manufacturers. Within tablet manufacture, this is usually achieved by applying anticounterfeit features directly to the tablet. An expert tablet designer can employ techniques to make counterfeiting more difficult. These are not always visible to the naked eye but ensure that a branded tablet can be identified as an original.

Several anticounterfeiting techniques can be used on each product to help reduce the risk

Several anticounterfeiting techniques can be used on each product to help reduce the risk – for example, altering the thickness of the embossing in places, changing the angle of the lettering, or simply having the logo on different inclines. Although hard to see with the naked eye, expert tablet designers can see the difference between the original and the counterfeit.

In conclusion, good tablet design is imperative, and it is something that should be strongly considered. It is important to consult with an expert tablet designer as early on in the process as possible, as they can ensure that tablet designs are not only unique and visually appealing, but are also robust and producible in a rigorous tablet manufacturing environment. Making just a few simple changes to a design can stop future problems, from picking and sticking to counterfeit issues.

The importance of design should not be underestimated. Punches and dies are the most critical interface with the end product, the tablet, and together everything should be measured and taken into account before tablet production commences.

References

1. Pharmaguideline.com

2. J Marriott & R Nation, Department of Pharmacy Practice, Victorian College of Pharmacy, Monash University, Melbourne.

3. Protecting your Brand with Anti-Counterfeiting Solution – John Mack. http://www.news.pharma-mkting.com/pmn46-article02.pdf

4. Eurostandard Educational Collection, 2010 – I Holland Ltd

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