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.
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.
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:
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.
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| 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. As force is applied to the stopper it will expand radially due to the Poisson effect. This will apply a reaction force against the inner wall of the cartridge which will produce a resistance/ breaking force which opposes the movement of the stopper. The greater the force the greater the breaking force which means that it does become diminishing returns on the output flow rate as force is increased. This effect can somewhat be reduced by using harder materials in the stopper with a lower Poisson ratio however this could impact the function of the stopper to separate the drug from the external environment |
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.
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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.
Engineering Director
Jacks is responsible for the engineering aspects of projects to ensure technical risk is minimised. With expertise in simulation, drug delivery devices, and electronics, Jack guides your project team on all related deliverables.
Jack is a Chartered Engineer with a Master’s Degree in Mechanical Engineering. He has over 10 years’ experience working in both the medical sector and on complex machinery.
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