Case study: Development of a plastic capillary blood sampling device

This brief paper describes the work undertaken by Axis-Shield PoC A/S and Carclo Technical Plastics Ltd to develop an innovative solution, thereby improving an existing product and significantly reducing cost.

Axis-Shield markets the Afinion analyser, mainly in Europe and the US, to doctors and clinics for the quantitative testing of certain complaints at the point of care. The analyser uses a disposable cartridge using plastic components manufactured by Carclo Technical Plastics Ltd. One of the tests is for HbA1c, a key test of a diabetic’s management of their condition.

In the photograph to the right, a sample of patient’s blood can be seen within the “sampling device” of the cartridge. Once this is inserted into the cartridge, the cartridge is in turn introduced into the analyser and a result is available within 3 minutes. This fast and simple test helps the healthcare professional provide rapid feedback to the patient on their condition and contributes to reduced healthcare costs.

The brief
Axis-Shield approached Carclo to help reduce the costs of production of the cartridge in the face of increasing competition and demands from the international healthcare industry.

The existing sampling device consists of a glass capillary tube which is adhesively bonded into a polystyrene moulding on a large automated assembly line. This part of the process is troublesome, beset by a high degree of scrap as a result of breakage of the fine glass capillary and the adhesive bonding process. It is costly due to the relatively high cost of the glass component and the impact on production yield. One opportunity the two companies identified was to mould the sampling device as a single unit.

Challenges encountered and overcome
In the HbA1c test, the volume of blood must be controlled accurately. The existing glass capillary tube was a 0.47x1.15x8.5mm extrusion with zero draft. To replicate this in an injection moulded component, manufactured in the multi-millions per year, required very careful consideration to component design in order to maximize the life of the mould tool. Expertise of the very best toolmakers was employed. Initially, Carclo commissioned a four cavity pilot production tool, followed by a 16 cavity production tool. Use of the very latest hot runner technology and all-electric moulding equipment was essential for good process control, minimizing waste and maximizing yield of the moulding process.

The existing glass capillary tube is hydrophilic to promote flow of blood. Replacement by a single polystyrene moulding was not initially successful since the material is naturally hydrophobic and so blood did not flow readily up the tube section. The team identified a mechanism to make the moulding hydrophilic which greatly improved capillary flow. Further optimization of the component geometry was also implemented to control the flow of blood and accurately meter the volume of sample collected into the capillary.

Concluding remarks
As a result of combining specialized component design, leading edge toolmaking and process control, with materials science, Axis-Shield have succeeded in delivering an improved component part, which has resulted in improved production yields and reduced processing costs.