Materials Selection for Injection Pen Components: Medical-Grade Plastics, Springs, and Seals
Introduction
Injection pens have become the cornerstone of self-administered drug delivery, serving millions of patients worldwide managing chronic conditions such as diabetes, autoimmune disorders, and hormonal deficiencies. The self-injection devices market is projected to grow from USD 5.52 billion in 2026 to USD 15.11 billion by 2034, driven by the expanding adoption of biologic therapies and the shift toward patient-centric care models. At the heart of every reliable injection pen lies a carefully engineered selection of materials—each chosen to meet exacting demands for mechanical performance, biocompatibility, chemical resistance, and long-term durability.
The material selection process for injection pen components is a multi-dimensional challenge. Engineers must balance competing requirements: dimensional stability for dose accuracy, low friction for smooth actuation, biocompatibility for patient safety, chemical compatibility with sensitive drug formulations, and sterilization resilience. As one industry analysis notes, “reliable material selection and verification are central to the safety, functionality, and regulatory acceptance of self-injection pen devices”.
This comprehensive technical guide examines the critical material families used in injection pen components—medical-grade plastics for structural and mechanical parts, spring materials for energy storage and force delivery, and elastomeric seals for container closure integrity—providing pharmaceutical manufacturers and packaging professionals with the insights needed to make informed material selection decisions.
At Vialab Pharmaceutical Packaging Co., Ltd. , we specialize in the design and manufacture of high-quality drug delivery and packaging components. From injection pens (disposable and reusable) to glass vials, sterile vials, and aluminum caps, every product is engineered to meet strict pharmaceutical standards. With advanced production lines and cleanroom facilities, we ensure consistent quality, integrity, and compliance for global healthcare partners.
1. Medical-Grade Plastics for Injection Pen Components
1.1 The Critical Role of Polymers in Pen Injectors
Polymers form the structural backbone of modern injection pens. From the outer housing and dose dial to internal gears, piston rods, and cartridge holders, medical-grade plastics enable the precision, lightweight construction, and cost-effective manufacturing that make pen injectors accessible to millions.
Research has demonstrated that “only a limited number of engineering polymers are suitable for use in a pen injector mechanism”. The selection criteria are exacting: materials must maintain dimensions under stress and across the device’s claimed lifetime to preserve dose accuracy. Sliding interfaces require stable friction characteristics, as uncontrolled changes can affect dose delivery or user actuation force.
1.2 Polyoxymethylene (POM): The Workhorse of Precision Mechanisms
Polyoxymethylene (POM) , also known as acetal, is arguably the most widely used engineering thermoplastic in injection pen mechanisms. The medical polyoxymethylene market is valued at USD 139.63 million in 2025 and is projected to grow at a CAGR of 5.7% to reach USD 236.8 million by 2034.
POM’s dominance stems from its exceptional combination of properties:
- Excellent dimensional stability and creep resistance, making it preferred for consistent actuation and torque control
- Low friction characteristics ideal for precision moving parts
- High strength and stiffness suitable for gears, ratchets, and clutch mechanisms
- Excellent wear resistance for components subject to repeated sliding contact
Medical-grade POM grades are available in both homopolymer and copolymer formulations, engineered for precision motion and dimensional stability in both single-use and durable medical devices. Manufacturers such as Celanese offer Hostaform® MT® POM with color service capabilities, while BASF’s Ultraform® N 2320 003 PRO AT is specifically developed for functional parts in devices such as insulin pens.
A notable recent development is the introduction of ultra-low-friction POM grades specifically developed for precision injection molding of medical device components. These tribologically modified materials offer exceptional sliding properties, avoiding frictional force, abrasion, and squeaking during operation.
1.3 Polybutylene Terephthalate (PBT): Strength and Chemical Resistance
Polybutylene Terephthalate (PBT) is a semicrystalline polyester that provides high strength, rigidity, low creep, and resistance to a wide range of chemicals, solvents, oils, and greases. These properties make PBT an excellent choice for:
- Housings and structural components requiring high stiffness
- Internal mechanisms exposed to lubricants or drug formulations
- Components requiring dimensional stability under load
SABIC has introduced VALOX™ HX325HP resin, a medical-grade PBT that combines outstanding processability with high chemical resistance and validated biocompatibility for high-precision applications with skin, tissue, or blood contact. The material has passed rigorous injection molding trials, showing high flow even in complex designs. Applications include insulin delivery pen components, insulin pumps, autoinjectors, and continuous glucose monitors.
Celanese offers Celanex® MT® PBT 2406MT GF20, a tribologically modified PBT+PET grade reinforced with 20% glass fiber for additional strength and stiffness. BASF has also launched Ultradur B4520 PRO, its first PBT for injection-molded applications in medical technology.
1.4 Polypropylene (PP): The Sustainable Alternative
Recent industry developments have demonstrated that polypropylene (PP) can serve as a viable alternative to POM and ABS in certain applications. Nemera, a global leader in drug delivery device manufacturing, partnered with Borealis to develop a new disposable pen injector platform using Bormed™ PP grades.
The initial design used POM and ABS—common in medical devices—but raised concerns over formaldehyde emissions from POM. Nemera selected two PP grades:
- Bormed™ HG820MO for its high stiffness, ideal for structural components like the housing and cap
- Bormed™ RF825MO for its optical clarity, important for the cartridge holder
The switch to Bormed resins reduced the device’s carbon footprint by approximately 20% compared to the POM/ABS version, while maintaining mechanical performance and regulatory compliance. The optical properties of Bormed™ RF825MO also enabled color-coded push buttons—an important feature that helps patients distinguish between different drugs.
1.5 Cyclic Olefin Copolymer (COC) and Cyclic Olefin Polymer (COP): The Primary Container Revolution
For primary drug containers (cartridges), Cyclic Olefin Copolymer (COC) and Cyclic Olefin Polymer (COP) have emerged as compelling alternatives to traditional borosilicate glass. The global cyclic olefin polymer market is valued at approximately USD 1.34 billion in 2025 and is projected to grow at a CAGR of 6.8% to reach USD 2.42 billion by 2034.
COC/COP materials offer:
- Break resistance superior to glass, reducing the risk of fracture during manufacturing, shipping, and use
- Design flexibility enabling complex, integrated geometries with tight dimensional tolerances
- Ultra-high purity with extremely low extractables and leachables
- Inert surfaces that minimize protein adsorption, making them ideal for sensitive biologics
SCHOTT Pharma has launched the industry’s first ready-to-use polymer cartridge made from COC, featuring a homogenous, cross-linked siliconization layer. The inert surface minimizes protein adsorption, provides low levels of extractables and leachables, and ensures consistent break-loose and gliding forces throughout the product’s shelf life. Produced using advanced injection molding technology, the cartridge offers exceptional dimensional accuracy and customization options. Available in 1.5 ml, 3 ml, and 5 ml formats (with a 10 ml version in development), the cartridge is compatible with all major fill-and-finish lines.
1.6 Material Selection Framework
When selecting medical-grade plastics for injection pen components, manufacturers must evaluate:
| Property | Why It Matters | Preferred Materials |
|---|---|---|
| Dimensional stability | Preserves dose accuracy over device lifetime | POM, PBT |
| Creep resistance | Maintains torque and actuation consistency | POM, PBT |
| Friction characteristics | Ensures smooth, consistent dose delivery | POM (tribological grades) |
| Chemical resistance | Prevents degradation from drug contact | PBT, COC/COP |
| Biocompatibility | Ensures patient safety (ISO 10993) | All medical grades |
| Sterilization resilience | Maintains properties after gamma, EtO, or e-beam | PP, PBT, COC/COP |
All materials contacting the patient or the drug must be assessed for biological safety per ISO 10993-1. The evaluation scope—cytotoxicity, sensitization, systemic toxicity—depends on the type and duration of contact.
2. Springs: The Energy Source of Injection Pens
2.1 The Critical Role of Springs
Springs are the heart of most injection pen mechanisms. Whether powering needle insertion, driving drug expulsion, or enabling dose setting and resetting, springs must deliver precise, repeatable force over thousands of cycles. As one industry expert observes, “springs are critical components in many medical devices, enabling precise control, consistent force, and long-lasting performance”.
2.2 Wave Springs: The Space-Saving Innovation
Wave springs have emerged as a preferred solution for autoinjectors and pen injectors, offering unique advantages over traditional helical coil springs.
A wave spring is a type of compression spring made from flat metal wire formed into a wavy ribbon-like coil. Key advantages include:
- Space-saving design: Wave springs can deliver the same force and deflection as a conventional coil spring while occupying up to 50% less axial space. This enables engineers to create slimmer, lighter devices without compromising performance.
- Precise force output: The force-versus-compression behavior is highly linear and predictable over the spring’s working range. Wave springs compress purely axially with no significant torsion or buckling, reducing side loads and friction on surrounding parts.
- Long fatigue life: Wave springs eliminate the twisting forces that can cause wear in coil springs, enhancing long-term durability. They deliver consistent force over countless cycles, making them ideal for equipment that must never fail in a clinical environment.
- Reload and reset capability: In reusable pen injectors, wave springs can be designed for easy resetting and long fatigue life. They store the energy for each injection and then reliably return to position when the pen is reloaded.
- Tolerance compensation: Wave springs can compensate for internal manufacturing tolerances, absorbing dimensional variations between parts while maintaining the required force.
Wave springs play several critical roles in pen injector design: delivering the necessary thrust to drive needle insertion and drug injection, helping regulate injection speed and flow rate, and providing consistent force for accurate dosing.
2.3 Spring Materials for Medical Applications
Not every spring material suits the medical industry. The selection of spring materials must consider corrosion resistance, biocompatibility, sterilization resilience, and fatigue life.
Common spring materials for injection pens include:
- Stainless steel alloys (including 316) : The most frequently used material for medical devices due to its excellent biocompatibility and inertness. Stainless steel offers good corrosion resistance and can withstand multiple sterilization cycles.
- Elgiloy®: A non-magnetic cobalt alloy that serves as a preferred choice for medical applications due to its superior strength and corrosion resistance. Elgiloy is biocompatible—not harmful or toxic to living tissue due to its resistance to corrosion from bodily fluids. It is often chosen for implantable applications and high-performance medical devices.
- Hastelloy: A nickel-based alloy offering exceptional corrosion resistance, suitable for demanding medical environments.
- Nitinol: A nickel-titanium alloy with shape memory and superelastic properties, used in specialized applications.
Wire diameters for medical springs can be as small as 0.10 mm (0.004 in) for extreme precision applications. Manufacturers must ensure 100% free length control and full traceability of materials.
2.4 Spring Design Considerations
When designing springs for injection pens, engineers must consider:
- Spring stiffness (k) : Determines the force generated for a given displacement
- Spring travel: Must be sufficient to deliver the full dose volume
- Fatigue life: Must withstand thousands of cycles for reusable devices
- Environmental effects: Temperature-induced changes in spring properties
- Sterilization compatibility: Springs must maintain performance after gamma, EtO, or e-beam sterilization
The predictable, linear spring rate ensures the injector delivers the intended force for needle insertion and drug delivery every time, improving dose accuracy and reliability.
3. Elastomeric Seals: Ensuring Container Closure Integrity
3.1 The Critical Role of Seals
Elastomeric seals—including cartridge plungers, rubber stoppers, and sealing discs—are critical to maintaining sterility, functionality, and patient safety in injection pens. These components must provide a leak-proof seal between the cartridge and the pen mechanism, enable smooth plunger movement, and maintain container closure integrity throughout the product’s shelf life.
3.2 Regulatory Framework for Elastomeric Components
The regulatory landscape for elastomeric components is undergoing significant change. ISO 8871-5:2025, the newly released revision of the standard for elastomeric parts used in drug delivery systems, introduces changes to terminology, test methods, and regulatory references. Key updates include:
- Renaming “aqueous solution tightness” to “dye solution tightness”
- More specific guidance on fragmentation testing, including particle size thresholds and magnification requirements
- Increased maximum pore size for microbial barrier testing from 0.5 µm to 5.0 µm, aligning with ISO 11608-3:2022
- Separation of dye tightness and self-sealing tests to reduce misinterpretation
USP <382> , scheduled to become official on December 1, 2025, marks the most consequential regulatory change to functional suitability requirements for elastomeric container closure systems. Instead of evaluating materials in isolation, USP <382> focuses on the functional performance of elastomeric components—such as stoppers, plungers, and seals—within the fully assembled system. From December 1, 2025, all new submissions for parenteral and injectable products must comply with USP <382> requirements.
ISO 13926-3 specifies requirements for primary-packaging seals (aluminum cap + elastomeric disc) whose properties can materially affect the potency, purity, stability, and safety of medicinal products.
3.3 Elastomer Materials for Injection Pen Seals
Liquid Silicone Rubber (LSR) is widely used for molded rubber stoppers, gaskets, seals, valves, and o-rings in medical devices. LSRs are elastomer systems reinforced with silica, designed for liquid injection molding (LIM) processes that can utilize single or multi-cavity molds. Key advantages include:
- Biocompatibility: Silicone materials are well-established for medical applications
- Chemical resistance: Compatible with a wide range of drug formulations
- Low extractables: When properly formulated and processed
- Precision molding: LSR can be injection molded to tight tolerances
Thermoplastic Elastomers (TPE) , such as Mediprene, have proven to be strong alternatives for plunger seals. TPEs are completely synthetic and latex-free, minimizing allergy risks.
Fluoropolymer-coated plungers offer excellent peptide compatibility with low extractables and leachables. These coatings reduce friction and prevent drug adsorption, making them ideal for sensitive biologics.
3.4 Critical Performance Requirements
Elastomeric seals in injection pens must meet several critical performance requirements:
- Break-loose force: The force required to initiate movement of the plunger
- Glide force (sustaining force): The force required to maintain movement
- Container closure integrity (CCI) : Maintaining the sterile barrier throughout shelf life
- Chemical compatibility: No interaction between elastomer and drug formulation
- Extractables and leachables: Levels must be within acceptable limits
As noted in industry guidance, “silicone materials are particularly sensitive, as they can generate leachables or particulates if not properly controlled”. Manufacturers must implement rigorous process controls to ensure consistent silicone oil application and avoid unstable injection force.
4. Biocompatibility and Regulatory Compliance
4.1 ISO 10993 Biological Evaluation
All materials used in injection pen components—plastics, springs, and seals—must be assessed for biological safety per ISO 10993-1. The evaluation scope depends on the type and duration of contact. For components that contact skin (such as the pen housing) or drug formulations (such as cartridges and seals), appropriate biocompatibility endpoints must be addressed.
Medical-grade plastics are available with ISO 10993-1 test requirements and USP Class VI compliance. Many suppliers provide biocompatibility data and Drug Master Files (DMF) to support regulatory submissions.
4.2 Extractables and Leachables (E&L) Testing
Extractables and leachables (E&L) testing is essential for any drug-contacting material—cartridges, plungers, and seals. Studies should follow risk-based E&L methodologies. Extractables studies use solvent extraction to simulate worst-case scenarios, while leachables studies monitor migration into the actual drug product under storage conditions.
4.3 Sterilization Compatibility
Materials must withstand the sterilization process—gamma, e-beam, ethylene oxide, or aseptic manufacture—without losing mechanical integrity or producing degradation by-products. Temperature and humidity cycling tests (accelerated and real-time) are recommended to verify retained function post-sterilization.
5. Practical Recommendations for Manufacturers
5.1 For Plastic Components
- Select POM or PBT for precision mechanical components requiring dimensional stability and creep resistance
- Consider ultra-low-friction grades for sliding interfaces to ensure consistent actuation force
- Evaluate PP alternatives when sustainability is a priority and formaldehyde emissions are a concern
- Choose COC/COP for primary containers when break resistance and biologics compatibility are required
5.2 For Springs
- Consider wave springs for space-saving, precise force delivery in compact devices
- Select stainless steel (316) for general medical applications
- Choose Elgiloy for high-performance applications requiring superior strength and corrosion resistance
- Validate spring performance across the full lifecycle and sterilization conditions
5.3 For Seals
- Prepare for USP <382> compliance by December 1, 2025
- Conduct system-level functional testing of elastomeric components in the fully assembled device
- Select LSR or TPE materials with proven biocompatibility and low extractables
- Implement rigorous process controls for silicone oil application to ensure consistent break-loose and glide forces
Conclusion
The selection of materials for injection pen components is a multi-faceted engineering challenge that directly impacts device performance, patient safety, and regulatory success. From the dimensional stability of POM in precision mechanisms to the space-saving efficiency of wave springs and the container closure integrity of elastomeric seals, every material choice must be justified through rigorous testing and validation.
The regulatory landscape continues to evolve, with ISO 8871-5:2025 updating testing requirements for elastomeric components, USP <382> introducing system-level functional suitability testing from December 2025, and ISO 11608-3:2022 expanding coverage to integrated fluid paths. Manufacturers must stay ahead of these changes to ensure continued compliance and market access.
At Vialab Pharmaceutical Packaging Co., Ltd. , we bring decades of expertise in pharmaceutical packaging and drug delivery components to support our partners through every stage of the material selection and component development process. From injection pens (disposable and reusable) and glass vials to sterile vials and aluminum caps, our comprehensive product portfolio and commitment to quality ensure that your drug delivery system meets the highest standards of safety, reliability, and patient satisfaction.
Whether you are developing a new pen injector, optimizing an existing design, or navigating the evolving regulatory landscape for elastomeric components, our team of packaging experts is ready to provide the technical guidance and manufacturing excellence you need to succeed in this rapidly evolving market.
Contact Vialab Pharmaceutical Packaging Co., Ltd. today to discuss your injection pen component material requirements and discover how our precision pharmaceutical packaging solutions can support your next product launch.