When it comes to diabetes management, accurate blood glucose readings are paramount. And when it comes to generating accurate readings, the quality of the test strip is dependent upon the materials used in its design — each layer has to be able to perform its function without interference from other layers or external contamination.
There are several design considerations that play into the success or failure of a test strip. For example, most test strips are made with adhesives that bond each layer together and hydrophilic materials that promote fluid flow. If one or more materials in the strip are not selected carefully, it could lead to strip and device failure, ultimately compromising the well-being of the user.
In order to ensure optimal performance and design, it is important to understand the role of each layer and how the different materials used can react to each other and the manufacturing processes. From temperature conditions to enzyme performance and patient factors — it’s crucial to dot your I’s and cross your T’s to avoid challenges later on.
This article looks at the construction of a typical test strip and breaks down some common features, potential challenges and how to remedy them during the design and development stages.
Understanding Anatomy of a Test Strip
Each layer and material contained in the test strip performs a critical function and must be able to do its job without interference. The best way to avoid running into issues is to ensure that the right materials are chosen for the device design and application.
Some commonly seen design malfunctions that can affect a test strip’s overall success include exposure to contaminants, leeching, instability, aging, and adhesive and spacing inconsistencies. Test strips are comprised of several layers that include inside hydrophilic, adhesive, and electrode layers, as well as an outside layer that provides stability.
Let’s Face It. Similar to skin, the face of the test strip is responsible for sealing and protecting the inside layers from the outside world. The material used to construct the top facing layer on the length and width is one variable that may change from one strip to another, depending on the intended direction of fluid flow on the strip. This layer often contains branding insignia, instructions, or artwork.
Traffic Control. Although there are multiple ways to design a strip, hydrophilic materials are a consistent feature across designs. These materials are responsible for guiding the fluid as it travels through the strip to the test chamber for measurement. Hydrophilicity is an important property to consider during material selection because it will dictate the speed and amount of blood that’s brought into the chamber.
When selecting a hydrophilic material, measure flow speed to help ensure that fluids arrive in time for reaction on the strip. Measuring contact angle and wet index can also help avoid running into imperfections in this layer. Beware: forgoing measurement analysis could result in system errors or inaccurate measurements. Tip: Using selectively coated hydrophilic films can help ensure that the blood sample successfully flows to the intended location. Make sure that the chosen substrates and coatings are compatible with assembly processes and machine designs. Prioritizing surface protection will keep materials intact and readings accurate.
Adhesive and spacer layers are also common features found in blood glucose test strips. These layers adhere the hydrophilic materials to the bottom conductive layer. When selecting an adhesive, it’s critical to make sure that the chosen adhesive does not contain chemicals that will negatively react with the reagents used to measure the assay.
In addition to chemical considerations, there are several potential manufacturing challenges that could jeopardize the success of an adhesive and the overall test strip. For instance, adhesive residue can build up on machine cutting surfaces used during slitting and converting processes. For this reason, optimize process conditions for slitting, die cutting, laminating, and other operations that concern heat, speed, or pressure. Choosing materials with properties that allow efficient processing during manufacturing operations will produce clean and consistent parts, as well.
It’s also important to note that some adhesives stick better to certain materials and substrates than others. Surface energy, roughness, and process conditions influence how strong the bond is between layers. However, it’s possible to alter surface properties by using treatments to functionalize surfaces. Tip: Consult a material supplier early to ensure that the material selected will suit the intended use over the product’s lifetime. A supplier can weigh in on what type of adhesive may be strong enough to withstand the manufacturing processes the strip will be exposed to. Ask the supplier how peel force and liner release measurements can improve adhesion and bonding.
Spacer layers are responsible for providing elevation in the capillary chamber. Inconsistencies in the bonding and spacing between layers can impede fluid flow or allow some of the sample to leech into unintended areas of the strip. If the thickness of the spacing layer isn’t just right, it may also cause variable sample volume, which can result in erroneous measurements. Tip: Discuss the chamber’s overall geometry with a supplier to ensure that the required sample volume and spacer layer thickness is controlled. The adhesive supplier can make recommendations that will help mitigate coating defects, streaks, and voids in these layers and in the microfluidic channels.