Citation: Roested J, Gundberg T, “Stability of a Wearable Injector Powered by Osmosis”. ONdrugDelivery, Issue 111 (September 2020), pp 34-36.
Jesper Roested and Tomas Gundberg present engineering test data on Subcuject’s wearable bolus injector, based on an osmotic drive system, demonstrating that the device presents a solution to the need for an inexpensive, mass-producible wearable injector.
THE FIELD OF WEARABLE BOLUS INJECTORS
The up-and-coming field of wearable injectors for injection volumes over 1.5–2 mL can presently be differentiated into two broad categories:
- High-end wearables that use electromechanical drive systems and commonly come with connectivity options, at the expense of relatively high complexity, high cost and large size.
- Low-end user-fillable wearables that are small and suitable for non-predefined injection volumes.
“Similar to the autoinjector space, the available wearable product range lacks a prefilled, simple to use and inexpensive wearable bolus injector designed to fill the role of a low-end product to be produced in bulk.”
These two categories leave a clear gap in the market. Similar to the autoinjector space, the available wearable product range lacks a prefilled, simple-to-use and inexpensive wearable bolus injector designed to fill the role of a low-end product to be produced in bulk.
Subcuject is developing such an inexpensive, prefilled wearable injector, based on osmosis as the driving force. It is now demonstrated that the base technology is indeed a feasible product, delivering consistent performance and that endures viscosities up to at least 100 cP.
THE COMPLICATIONS OF DEVELOPING A LOW-COST, PREFILLED WEARABLE INJECTOR
A wearable bolus injector is characterised as a subcutaneous injection device that is attached to the body for the duration of an injection, often several minutes, during which the drug is injected at a low flow rate in order to avoid pain. The viscosity of the injected drug is often high, and one of the challenges in developing a wearable injection device is that the delivery mechanism requires quite a high amount of energy, which must be stored for the device’s shelf-life. A further complication is that the energy must be released slowly and deliberately during injection. An ideal wearable injector must be acceptable to wear whilst attached to the body, which calls for a small size and that the product is as easy to prepare, activate and dispose of, as with a prefilled autoinjector.
In order for a wearable injector to be commercially viable in volume, it also needs to be as inexpensive as a prefilled autoinjector. Furthermore, in order not to jeopardise drug stability during shelf life, primary packaging components that come into contact with the drug must preferably be well known. Delivered dose accuracy and consistency is important, but in contrast to insulin pumps that require a very exact flow rate, the flow rate for a wearable bolus injector is less important. Thus, depending on the drug, it is less important if it takes, for example, five or seven minutes for the device to deliver the dose, as long as the injected volume is accurate.
Osmosis as a Driving Force
An actuator driven by an osmotic pump mechanism has many of the characteristics wanted for a low-cost device:
- It is inexpensive (energy is provided by means of salt and membranes)
- The energy is released slowly
- It has the ability to deliver a high pressure.
Further to these benefits, the excess water from the actuator can be used to provide a simple, compact and sophisticated hydraulic plunger pushing mechanism. However, the challenge of using osmosis as the driving force in an actuator lies in the concept being new for use in injectors and, thus, implementing the basic power concept in a practical, manufacturable solution.
Figure 1: The Subcuject wearable bolus injector using a 3 mL prefilled cartridge.
THE SUBCUJECT PRODUCT
The Subcuject wearable bolus injector is designed to be prefilled with drug from the manufacturer, of a small size and as simple to use as a prefilled autoinjector. Furthermore, it is designed to be low cost and to use standard primary packaging components (i.e. standard glass cartridge and a rubber stopper). The current moulded prototype shown in Figures 1 and 2 is based on a 3 mL cartridge.
Figure 2: The 3 mL Subcuject device attached to the body.
Performance Consistency
The Subcuject product is currently in the late stage of concept development, and it has now been demonstrated that the product can be made repeatably as moulded single-use devices, with a very consistent delivered dose of 1% standard deviation over 3 mL (Figure 3).
Figure 3: The Subcuject device’s dose delivery consistency from a 3 mL cartridge.
“The Subcuject product is currently in the late stage of concept development, and it has now been demonstrated that the product can be made repeatably as moulded single-use devices.”
The injection time is repeatable, and the injector can overcome high viscosities and high back pressures (Figure 4). The average injection time for 1 cP (water) is about four minutes, 50 cP takes five minutes and 100 cP takes about five and a half minutes. The time for injection against an extreme condition of 10 psi is about six minutes. These data demonstrate that the wearable injector can generate a high force and that high viscosity has limited impact on injection time.
Figure 4: Total injection time (seconds) under various conditions using the Subcuject device with a 3 mL cartridge.
The flow rate profile is likewise consistent during the injection. A typical flow profile of a 100 cP viscosity injection is shown in Figure 5.
Figure 5: A typical flow rate profile of a 100 cP viscosity glycerol injection using the Subcuject device.
As described, the results show that the base technology is powerful and consistent when implemented in a single-use device. All data on injection time discussed in this article were collected with the cartridge facing upwards, as when the device is attached to the abdomen in a standard-use position, but the dual membrane geometry used in the device ensures a similar injection time for all other possible positions. Tests with 1 cP were performed with a G30 needle and tests with higher viscosities use a G27 needle.
Outlook And Availability
The Subcuject wearable bolus injector will be ready for the first drug development combination programme in 2020, in partnership with one of the global top-tier contract development and manufacturing organisations. The first commercial version is now being designed for a 5 mL glass cartridge.