In pharmaceutical industries, medicines are developed to treat certain illnesses. There are several types of delivery systems with which these medicines can be administered into the human or animal body. Injectable medicines are mainly administered with the help of syringes or delivery devices. Syringes or delivery devices are used with needles to inject the medicine or drug into the body.
However, a major challenge is to develop the syringe or delivery device which can be used to deliver the drug under the skin with more comfort and less pain. Injecting the drug under the skin with the help of syringes is mostly dependent upon the fluid properties of the drug. The design of delivery devices or syringes is based on fluid properties of the medicine or drug. It becomes more inconvenient or painful to the patient if the drug or fluid is with high viscosity or semi-solid type which needs high force to inject.
So, it is very important to understand the pressure and force requirement for delivery of highly viscous fluid. Based on the force requirement to deliver the drug, the device designer can design and develop a delivery system which would be used by the patient or user with more comfort and less pain. For the measurement of force, injectable drugs or fluids are classified as Newtonian and non-Newtonian fluids. The information in this paper on a mathematical equation, experiments, and suggestions will help the pharmaceutical industries as a basis to design and develop the syringes or drug delivery devices to deliver highly viscous, semi-solid or visco-elastic type of drug or medicine. The design parameters or functional features of the drug delivery device can be collectively called as design formulation. So, for a drug-device combination product, both the things viz. design and drug formulations are equally important for its safety and efficacy.
Figure 1 shows a schematic diagram of the syringe system in which needle is pre-attached to the syringe barrel at frontend. This configuration is also called a syringe with a staked needle.
The syringe barrel or container is called a primary container as it holds the medicine and has direct contact with it. The needle has two open ends and one of it will remain inside the syringe barrel. The needle is made out of metal and comes in different gauges. The needle gauge is defined by its inner bore diameter, outer diameter, and wall thickness.
There are other syringe configurations in which needles can be attached at the front end of the syringe manually before the injection. The syringe barrel is filled with the drug fluid or medicine. The rubber stopper is placed inside the syringe barrel at the other end so that fluid will always remain inside the syringe barrel. The fluid will not come out of the needle unless there is an adequate amount of force is applied on the rubber stopper through the plunger rod to move it towards needle end. The plunger rod may be attached to the rubber stopper as shown. The material of construction (MOC) of the syringe barrel could be glass or plastic or metal. There are different types of plastics used for syringe barrel. In all cases, the inner surface of the syringe barrel should be smooth enough so that movement of rubber stopper becomes uniform and with minimum or negligible friction. The inner surface of the syringe barrel can be siliconized to make a smooth or frictionless movement of the rubber stopper inside it.
To administer the dose, the user has to insert the needle under the skin at the injection site of the body and then push the plunger rod forward to dispense the medicine. An adequate amount of force is required to push the plunger rod forward. It becomes uncomfortable and inconvenient to the patient or user if the force required to push the rubber stopper is more.
However, the needle insertion and drug delivery steps can be made automatic with the help of an auto injector delivery device. To design an auto-injector or prefilled syringe, the designer needs to understand the force or pressure required to dispense the medicine out of the needle.
To administer the medicine with comfort and less pain, it is very important to understand the design parameters or factors of a delivery device as well as fluid properties of drug which affect the force required to push the rubber stopper forward. The force required to inject medicine at a specific flow rate with a specific needle length and the gauge is called syringeability. The volumetric flow through the needle is governed by the Hagen-Poiseuille equation:
Above equation is used for Newtonian fluids. It shows that the major factors which affect the force required
to dispense the fluid or liquid medicine are as follows: