Developing Auto-Injectors: Designing for Small and Large Volume Drug Delivery

The auto-injector development process is far more than an engineering challenge – it’s a strategic decision that influences drug formulation, patient experience, and commercial viability.

As the pharmaceutical industry continues to develop increasingly complex biologics, the ability to deliver both small and large volume, high-viscosity formulations safely and comfortably has become a defining test of innovation.

Developing auto-injectors for modern therapies demands more than technical precision—it requires strategic foresight. At Haughton Design, we view developing auto-injectors as both an engineering and commercial challenge.

For leadership teams, understanding how design parameters – such as volume, viscosity, and injection time – interact is critical in drug delivery device development. These factors determine not only usability and patient adherence, but also regulatory approval, manufacturability, and long-term product success.

In this article, we explore how developing auto-injectors that can manage both small and large volume drug delivery influences usability, manufacturability, and market success.

Engineering Precision: The Relationship Between Force, Flow, and Viscosity in Developing Auto-Injectors

When developing auto-injectors, engineers must balance force, flow, and viscosity to achieve consistent delivery.

At the heart of developing auto-injector performance is a finely balanced equation. The Hagen-Poiseuille principle describes how flow rate depends on factors such as plunger force, needle length, internal diameter, and drug viscosity.

Increasing the applied force or widening the needle diameter increases flow rate, while higher viscosity, longer needles, or larger cartridge areas reduce it. The relationship is non-linear – small changes in these variables can dramatically alter delivery time.

For example:

• Increasing viscosity from 5 cP to 20 cP can quadruple injection time.
• Increasing needle diameter from 0.14 mm (30G) to 0.24 mm (25G) can reduce injection time by a factor of 16.

For design teams, these trade-offs must be carefully managed when developing auto-injectors. For leadership teams, they represent cost, risk, and reputation factors – impacting manufacturability, product reliability, and patient acceptance.

Successful auto-injector design relies on understanding how these factors scale from small to large-volume delivery systems.

Designing for the Patient: Injection Time and User Comfort

A key consideration when developing auto-injectors for high-viscosity biologics is maintaining patient comfort while ensuring full dose delivery.

In drug delivery, time equals tolerance. Patients typically prefer injections under 10 seconds to minimise discomfort. However, as formulations become thicker and volumes increase, maintaining short injection times demands higher forces or wider needles – both of which have usability and regulatory implications.

To address this, designers are increasingly exploring alternative architectures:

• Wearable injector technology (on-body delivery systems) allow slower delivery over minutes or hours, removing the 10-second constraint and reducing the need for extreme forces.
• Dual-spring or motor-driven mechanisms can maintain consistent force over the injection cycle, improving dose accuracy and comfort.

These innovations are not simply engineering solutions – they represent strategic shifts in how pharmaceutical companies approach human factors in self-administration, patient adherence and product differentiation.

Strategic and Commercial Implications when developing auto-injectors

For executives, developing auto-injectors is a strategic decision that shapes lifecycle value, regulatory strategy, and brand differentiation. For senior decision makers, the choice between a traditional auto-injector and a larger-volume wearable device is not just a technical matter. It affects:

• Time-to-market – Will existing platform technologies support the formulation, or is bespoke development required?
• Cost and complexity – How will higher forces and tighter tolerances affect tooling, testing, and validation?
• Regulatory pathway – Will the device meet the requirements of combination product engineering for dose accuracy and usability?
• Market positioning – Does the device align with patient expectations and brand promise?

Early cross-functional collaboration between formulation scientists, engineers, and usability experts is essential. The earlier these trade-offs are addressed, the lower the downstream cost of redesign, delay, or failure.

Conclusion

Developing auto-injectors for small and large volume drug delivery is a complex interplay of physics, physiology, and patient psychology — but ultimately, it’s a strategic business decision.

The companies that succeed will be those that integrate device, drug, and user design from the outset, recognising that comfort, safety, and manufacturability are interdependent.

At Haughton Design, our multidisciplinary team specialises in developing auto-injectors and works with pharmaceutical and medtech partners to navigate these challenges — from early feasibility to full-scale production. By combining deep engineering expertise with human-centred design and strategic insight, we help clients create delivery systems that are not only technically sound, but commercially successful and trusted by patients.

Speak to Jack Dunkley