In drug delivery devices, springs may not be the largest or most complex component, but are among the most sensitive to tolerance stack-up, fatigue and in-field failure. Connecting this link between poor spring performance and high scrap rates on the production line can be difficult to decipher and solve as a result.
This article unpacks that link, offering a guide to spring specification, validation and process control strategies that can radically improve manufacturing yield and ensure repeatable actuation performance across a wide range of devices.
Rejection rates in drug delivery devices are often attributed to visible faults like dose misfires or inconsistent assembly. But in many cases, these stem from upstream spring-related issues. Understanding these failure pathways is the first step toward tackling them.
The level of precision required to minimise rejection rates and ensure consistent actuation for medical device springs depends on careful engineering of characteristics beyond basic force values. The following considerations are integral to reach repeatable, stable spring performance throughout the device lifecycle.
Specifying a nominal force value is not enough. Actuation mechanisms require predictable, repeatable force profiles across the stroke length inline with Hooke’s Law. To achieve this, you need to define spring rate tolerances at multiple points (e.g., 25%, 50%, 75% compression), not just free and working lengths. On top of that, you can use laser load testing or camera-based deformation tracking to validate spring dynamics batch-by-batch.
Unrelieved stress from coiling, forming or assembly overcompression can lead to load loss after only a few actuations. Using resilient materials like 17-7PH stainless steel or Elgiloy is important as they retain strength post-heat-treatment. Additionally, pre-set springs in manufacturing can help mitigate early stress relaxation.
Improperly specified outer diameter (OD) or coil pitch can cause side loading in guide channels, stick-slip effects in trigger mechanisms and inconsistent starting force during actuation. To combat these factors, Finite Element Analysis (FEA) and Design for Pharmaceutical Manufacturing (DfPM) modelling should be used to simulate spring housing interaction before tooling sign-off.
Selecting the correct spring material is fundamental to achieving consistent actuation and reducing in-field failure. Stainless steel, particularly 302 or 17-7PH, remains a go-to for its balance of mechanical performance, fatigue resistance and corrosion protection. For critical applications demanding higher mechanical limits, Elgiloy offers excellent fatigue resistance under cyclic loading, and retains spring properties even in chemically aggressive or high-temperature environments.
Springs are deceptively complex components in medical devices. From force delivery and fatigue life to biocompatibility and particulate generation, they play a critical role in device performance and even small deviations can jeopardise an entire batch’s compliance.
Partnering with a manufacturing expert like Advanex Medical gives medical device designers access to advanced materials knowledge, precise forming capabilities, in-house validation testing and finishing technologies tailored to the needs of high-performance drug delivery systems. From inhalers and autoinjectors to complex wearable or implantable devices, the right spring designed, tested and manufactured correctly can be the difference between launch success and regulatory failure.
To learn more about how to choose the right medical springs for performance, compliance and reliability, download our guide below.