When it comes to drug delivery devices, reliability is everything. A single mechanical failure can result in a missed dose or patient harm. In light of this, compliance testing and regulations are extremely strict, making reliability and precision exceptionally important. At the heart of these mechanisms lies one of the smallest, yet most mission-critical components; the spring.
These components, often under repeated stress cycles, must be engineered to function reliably for long-term use. Engineers need to understand how to identify fatigue risks, design effective fatigue testing protocols and implement preventative strategies to ensure long-lasting spring performance in high-cycle medical applications such as inhalers, injectors, and auto-dispensers.
Fatigue is the progressive and localised structural damage that occurs when a material is subjected to cyclic loading. For medical springs, especially those in drug delivery devices expected to operate thousands of times, fatigue failure is a key concern. Fatigue failure is particularly insidious because it can occur well below a material’s ultimate tensile strength, making it difficult to detect through static load testing alone.
The first step in mitigating fatigue is to identify where and how fatigue might occur in the device lifecycle. There are a number of key areas for evaluation that need to be considered to meet stringent medical regulations. Design engineers must map these parameters during the concept phase to guide material selection, geometry optimisation and testing protocols.
Fatigue testing simulates real-world spring operation under controlled conditions to determine its lifespan and failure mode. The goal is to quantify the number of cycles the spring can endure before losing critical functionality.
Dynamic Cycling (High Cycle Fatigue) is where springs are repeatedly loaded and unloaded at defined displacement or force values, simulating expected real-use conditions.
Step-stress testing subjects springs to incrementally increasing loads to observe the fatigue limit. This helps define design safety margins.
Accelerated life testing simulates years of usage in compressed timeframes using elevated frequencies or stress levels. Useful for design validation under worst-case conditions.
Environmental stress testing tests springs in varying humidity, temperature or corrosive environments to mimic conditions like bodily fluid exposure or sterilisation.
Preventing fatigue in medical springs begins with smart material choices and precision manufacturing. Alloys such as Nitinol offer exceptional fatigue resistance and elasticity, ideal for high-cycle drug delivery devices. Alternatives like 304 stainless steel and phosphor bronze also provide robust performance, especially when paired with corrosion-resistant surface treatments.
To maintain consistent stress distribution and extend operational life, springs must be formed to tight tolerances, often using micropressing techniques that ensure dimensional accuracy across millions of components. Equally critical is surface integrity; laser deburring and vibratory finishing eliminate burrs and micro-cracks that can initiate fatigue failure, making these processes essential in high-reliability applications.
Design optimisation and quality assurance are also key to fatigue prevention. Spring geometries must avoid stress concentrators by distributing loads evenly, with tools like Finite Element Analysis (FEA) used to simulate fatigue behaviour before production. Controlled forming and stress management strategies, including proper coil radii and transition shaping, help reduce long-term mechanical stress. Once in production, automated inspection systems ensure each spring meets the required standards, catching surface or dimensional flaws early.
Spring fatigue is a leading cause of failure in high-cycle drug delivery devices. Identifying fatigue risks early, selecting the right materials, and applying precision forming and surface treatments are key to ensuring long-term performance and reliability.
At Advanex Medical, we specialise in designing and manufacturing precision springs tailored for demanding medical applications. With decades of experience, cutting-edge fatigue testing capabilities, and a deep understanding of regulatory requirements, we help device designers integrate durable, high-performance springs from concept through to production. Our bespoke approach ensures your components are not only compliant but built to last.
Download our guide below to learn how to optimise your device for reliability, compliance, and scalable production, from component selection to final assembly.