Auto-injectors are medical devices used to self-administer medication. The design and functionality of an auto-injector depend on several factors, including the volume and viscosity of the drug being administered.

Small volume drugs are typically defined as those that require less than 1 mL of solution for a single dose, while large volume drugs require more than 1 mL. Highly viscous drugs are those that have a thick, sticky consistency.

 

What is Viscosity?

Viscosity is essentially how thick a liquid is and therefore how much force is required to make it move. Viscosity is measured in Poise (Pascal-Seconds) or commonly written as Centipoise (100 Poise) to keep the numbers sensible. Water has a viscosity of 1cP so therefore it will flow freely with little force required to make it move, whereas something like honey/ syrup has a viscosity of 3000cP therefore it requires significantly more force to move.

What does this have to do with drug delivery devices?

The flow rate through a needle with at a given force on the plunger assuming the pressure drop is zero is given by the Hagen-Poiseuille equation:

  • F=Force on Plunger (N)
  • Q=Flow Rate of Fluid (m3/s)
  • µ=Viscosity of Fluid (Pa.s or Poise)
  • L=Length of Needle (m)
  • A=Area of plunger (m2)
  • D=Internal Diameter of Needle (m)

Increasing the force, or needle internal diameter will result in a higher flow rate whereas making the fluid more viscous (thicker), making the needle longer or increasing the diameter of the cartridge (area) will decrease the flow rate.

Medicines have varying viscosities which are linked to molecule size of the drug itself. Aspirin has a molecular weight of 180Da, Insulin has a molecular weight of 5.8kDa (32x Aspirin), and Humira has a molecular weight of 148kDa (822x Apsirin).

A lot of auto-injectors use springs to provide the force which themselves have a reducing force as the plunger moves along the cartridge. For this reason, an autoinjector spring needs to be specified so that at the end of its travel it still has enough force in eject the drug.

 

However, putting more force into the cartridge isn’t consequence free. Increased force can result in broken components such as cartridge and plunger rods within the auto-injector. As well as causing shelf-life issues due to plastic clipping components creeping under prolonged loading.

Increased force also creates higher shear forces within the fluid which can damage the drug molecules themselves resulting the reduced functioning of the drug.

Typically drug delivery times from auto-injectors are sub-10 seconds. People don’t want increased delivery times due to the discomfort of holding the device against their skin.

The below graph (Figure 1) shows how increasing the drugs viscosity from 5cP to 20cP increases the time to inject linearly (4x). This can also be seen in the equation as the viscosity term is linearly proportional to the flow rate.

Figure 2 shows how changing the needle gauge from 30g (0.14mm) to 25g (0.241mm) has a significant effect on the drug delivery time. This is due to the needle internal diameter factor in the Hagen-Poiseuille equation being D4 meaning that doubling the diameter increases the flow rate by 16x.

By balancing the discussed, the effect of changes such as increased viscosity can be mitigated by varying other factors.

Alternative Options

Another approach would be to change the requirements of the device. The driving factor in limiting the drug delivery time to 10 seconds is to reduce discomfort from hold a large needle in your skin. If we used a small needle and attached it to the patient semi permanently we wouldn’t be limited to the less than 10s dose time which would mean we could allow the flowrate to be significantly higher without causing additional discomfort to the user. These wearable devices don’t have many of the complexities like high forces and therefore high strength components as the reduced flow rates don’t require them. 

To conclude, developing auto-injectors requires careful consideration of  various factors, including the volume of drug being delivered. Considering this will alter the design in ways such as the length or diameter of the needle chosen, the forces required, and therefore what components are used.

At Haughton Design we have a team of experts who have many years designing drug delivery devices, both at HD and as part of device development teams within pharma companies. This level of expertise allows us to collaborate with your team to ensure thorough engineering is considered throughout, to allow the best device for both you and the user.

Jack Dunkley CEng - Medical Device Engineering Director at Haughton Design Jack Dunkley CEng 5 July 2023

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Get in Touch with Jack Dunkley CEng

Engineering Director

Jack is a chartered engineer who holds a Master’s degree in Mechanical Engineering from Cardiff University. He has experience working with design for injection moulding, machining and sheet metal as well as design for assembly and serviceability. Prior to HD, he worked at metrology company Renishaw for 6 years where he led several complex mechanical projects from initial concepts through to production. He has also worked at electrosurgical medical device manufacturer Olympus Surgical Technologies Europe (Gyrus ACMI).

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