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.
The enzyme layer typically covers the entire capillary channel and is the location at which an electrical charge is generated. Although manufacturers tend to have specialized recipes for the makeup of the enzymatic layer, this layer must always maintain a consistent measurement area, relative to the capillary channel, in order to reduce the potential for variation in enzymatic coverage. If there’s a change in enzyme area on the electrode, it may inhibit strip performance. Tip: Pay mind to the enzyme area and make sure the enzyme is present in excess, relative to how much glucose will be collected, to help minimize the risk of measurement errors and leakage due to construction defects.
The fluid’s next stop on its way through the test strip is the electrode layer. After signals are generated on the enzyme layer, the electrochemical response signals are transferred through the test strip to the glucose meter, which calculates how much glucose is present in the blood sample. If environmental effects, such as altitude, temperature, or meter circuitry bias occur during this phase, it may cause system and reading inaccuracies. Carefully select materials that are stable across a range of temperatures and other environmental conditions to help prevent error and ensure accuracy. It’s also important that the conductive materials housed on the electrode layer remain adhered to the supporting film layer following deposition, as well as throughout the strip’s life cycle.
It is advisable to work with a supplier that has process controls in place to minimize the risk of unintended contaminants. Engage the material supplier to discuss options for process validation, stability, and aging testing. Also consider asking them about their shop floor management and what process controls they rely on to ensure the purity of materials. Tip: Using noble metals for electrodes can help minimize variation in conductivity due to environmental effects or test strip aging. Some test strip manufacturers have also added additional circuits to their materials as a way to provide system corrections and increase accuracy.
Although they are small in size, test strips are complex products with big responsibilities. Understanding the importance of each layer’s role, effects, and reactions to different materials and environmental conditions helps provide the reliable results users rely on. It doesn’t have to be blood, sweat, and tears. Do due diligence, consult with a materials supplier up front, and learn the best practices for designing an accurate blood glucose test strip prior to beginning a test strip design project.
This article was written by David Franta, Microfluidics Global Business Manager, 3M Medical Materials and Technologies, and Kim Haglund, Advanced Product Support Engineer, 3M Health Care Business Group, St. Paul, MN. For more information, visit here .