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Auto-injectors are medical devices designed to deliver a dose of drug through a subcutaneous or intramuscular route. They contain a pre-filled syringe (or cartridge) which is driven by a spring system. The major benefits of these devices are easy self-administration, improved patient compliance, dosage accuracy and reduced anxiety.
An accelerated growth in global autoinjectors due to the increase in prevalence and incidence of anaphylaxis (severe and potentially life-threatening allergic reaction) and food allergies has been recorded. Therefore, immediate access to adrenaline or epinephrine is critical during the first few minutes of an anaphylactic reaction.
A client reported issues with the performance of their medical device. The auto injector was not performing to specification and thus potentially not delivering the dosage accurately. This was suspected to be related to the plunger when the device was triggered.
Utilising RSSL’s expertise and extensive experience, a feasibility study was carried out which discovered that the spring's recoil after actuation was interfering with the accuracy of the force measurements.
Activation force is the force required to actuate a self-administered medical device, which is crucial for the device's proper functionality. If the activation force is outside the specified range, the device may fail to activate or deliver an inaccurate dosage, potentially harming the patient.
Figure 1. Component parts of an Epipen®
Based on the results of the feasibility study, the appropriate equipment was identified: a GMP-compliant texture analyser. A method was then developed for accurate testing. One of the initial challenges RSSL faced was securing the device in the correct orientation for testing on this instrument. Auto-injectors, which are complex devices with many internal components as seen in Figure 1 added to the difficulty. To address this, the auto-injectors were secured in a bespoke rig created at RSSL. The activation force required was recorded by the instrument The device was then activated by pressing its top with a plate surface probe. A series of activation force tests were performed on the sample auto-injector and the resulting force-displacement profile was used to determine the actuation force, as illustrated in Figure 2.
Figure 2. Force displacement profile.
To further evaluate the device and simulate real-life scenarios, dynamic impact (such as accidental dropping) and static load (such as accidental crushing) assessments were conducted using RSSL’s Zwick equipment. Additionally, injection depth could have been assessed using equipment such as ImageJ or Python where ballistic gelatine injections of dyed solutions are analysed however this was not required for this project.
RSSL’s experienced team within the Physical Sciences laboratory identified the issue and developed a method to measure the actuation force of an auto-injector, recognizing the importance of accurately measuring the activation force—a critical parameter for the proper functionality of the self-administered medical device. The client was then equipped with all the information required to make an informed decision to rectify the manufacturing issue.