Mechanical and electromechanical autoinjectors
June 09 2025
Autoinjectors have evolved into advanced drug delivery systems that enhance safety, efficiency, and patient adherence. While there are various types of autoinjectors available, this article specifically explores mechanical or electromechanical autoinjectors, highlighting their mechanisms and roles in addressing the challenges of modern drug delivery.
Understanding mechanical autoinjectors
Mechanical autoinjectors operate using a spring-based mechanism to deliver a pre-defined dose of medication1. These devices are designed to be easy to use, ensuring that patients can self-administer treatments with minimal training.
Figure 1: SHL Medical’s Molly® modular platform technology—a mechanical autoinjector
A key component of a mechanical autoinjector is the compressed spring, which serves as the driving force behind the medication delivery process2. Over the years, the design of these springs has been optimized to balance material durability, spring release mechanism, and activation threshold3. When activated—typically by pressing a button or pressing against the skin—the spring releases its stored energy, driving a plunger forward through a pre-filled syringe (PFS) or a cartridge to deliver medication at a controlled rate, ensuring consistent and safe administration.
The mechanism of mechanical autoinjectors
Upon activation, the energy stored in the spring drives the plunger rod, delivering the medication through the needle into the tissue2. Variations in spring design enable flexibility in delivery force and speed to accommodate a range of medications, including high viscosity drugs. Despite their simplicity, mechanical autoinjectors require precise engineering to ensure consistent performance and patient safety.
Figure 3: Different compression springs in mechanical autoinjectors provide flexibility in the delivery force curve2
Understanding electromechanical autoinjectors
Electromechanical autoinjectors incorporate advanced features such as on-screen instructions, injection logs, skin sensors, and injection speed control, to enhance functionality, making them ideal for managing complex therapies. They vary in shape and sizes, with heights reaching up to 228 mm, compared to mechanical autoinjectors, which typically range up to 160 mm in height5.
A typical electromechanical autoinjector consists of two components: a drug cassette and a power unit.
Drug cassette
The drug cassette, a disposable component, holds a PFS or a cartridge primary container. Unlike mechanical autoinjectors, the drug cassette in electromechanical devices is smaller and lighter, as it lacks a metal spring under tension. Some models, like SHL Medical’s Elexy, integrate radio frequency identification (RFID) tags to transmit data on medication type, dosage, patient details, and usage history6.
Power unit
The power unit is a reusable component, housing the electronic motor, the microprocessor, and a rechargeable battery. It enables the device’s advanced functionality6 while supporting sustainable use for treatments requiring multiple injections. Sensors within the power unit monitor the injection process in real-time, providing feedback to the user to ensure the correct dose is administered7. The microprocessor regulates the electric motor’s speed and force, adapting to the specific requirements of the drugs and patients7. Some power units also feature Bluetooth® Low Energy (BLE) and/or cellular (LTE) connectivity for digital health integration.
Figure 5: A variant of an electronic motor with a size as small as 20 mm, offering excellent speed control, precise positioning, and repeatability of movement
The mechanism of electromechanical autoinjectors
Electromechanical autoinjectors operate through a combination of electronic and mechanical components that work together to deliver precise and consistent injections. When activated, an electronic signal is sent to the microprocessor, which regulates an electric current to power the motor. This directly controls the injection force and speed, ensuring smooth medication delivery8.
The motor drives the plunger forward, compressing the stopper and delivering the drug at a controlled speed. To maintain precision, embedded sensors continuously monitor plunger position and speed, providing real-time feedback to the microprocessor. The device’s firmware includes an algorithm that adjusts motor output to maintain the target injection time while keeping the applied force within the configured safety limits6.
Differences between mechanical and electromechanical autoinjectors
Both mechanical and electromechanical autoinjectors are widely used in drug delivery but differ significantly in functionality. Figure 6 highlights the key distinctions between the two.
| Function | Mechanical autoinjectors | Electromechanical autoinjectors |
| Adaptability to drug viscosities | Use large, compressed springs for high viscosities, increasing strain during transport and storage. | Adjust force for a wide range of viscosities, supporting thinner needles and consistent delivery. |
| Force control and stopper engagement | Upon the release of the full force of the compressed spring, the plunger strikes the stopper, creating a shock wave and stress within the syringe. | In a controlled manner, the plunger consistently engages with the stopper, building up necessary force to achieve and maintain the target injection speed. |
| Injection speed consistency | Speed varies due to spring-force decay and resistive factors, such as needle gauge and viscosity. | Maintain consistent speed by dynamically adjusting motor force during injection. |
| Error feedback | Provide minimal error feedback, leaving users uncertain about device functionality. | Sensors monitor functionality and provide real-time error alerts, preventing incorrect use. |
| Dose completion signal | Indicate dose completion with mechanical sounds, requiring users to manually hold the device for full delivery. | Use software-controlled audio and visual cues that instruct the user to “hold” the injector in place for a duration of time before removal. |
Figure 6: Functional comparison between mechanical and electromechanical autoinjectors6
Driving innovations in injectable drug delivery
Both mechanical and electromechanical autoinjectors address modern therapeutic needs for pharma and patients. Mechanical devices, like Molly and Maggie®, offer simplicity and ease of use, while electromechanical devices, such as Elexy, provide advanced functionality for complex therapies.
As a global leader in drug delivery solutions, SHL Medical combines industry expertise with advanced manufacturing to develop high-quality mechanical and electromechanical autoinjectors. Our innovations support pharma and biotech partners, ensuring patients worldwide benefit from safe, reliable, and efficient drug delivery technologies.
Learn more about the history of autoinjectors and key considerations in selecting autoinjector for your next project in our “Back to Basics” series.
References
1. Philippson, J. (2020, February 3). The interface between prefilled syringe and autoinjector – a development framework. ONdrugDelivery. https://www.ondrugdelivery.com/the-interface-between-prefilled-syringe-and-autoinjector-a-development-framework/
2. Earl, M. (2022, June 9). A checklist for autoinjector design. ONdrugDelivery. https://www.ondrugdelivery.com/a-checklist-for-autoinjector-design/
3. Dou, Z., Eshraghi, J., Guo, T., Veilleux, J., Duffy, K. H., Shi, G. H., Collins, D. S., Ardekani, A. M., & Vlachos, P. P. (2020). Performance characterization of spring actuated autoinjector devices for Emgality and Aimovig. Current Medical Research and Opinion, 36(8), 1343–1354.
https://doi.org/10.1080/03007995.2020.1783219
4. Autoinjector Spring Simulation. (n.d.). ZwickRoell. https://www.zwickroell.com/industries/medicalpharmaceutical/therapy-systems/autoinjector-spring-simulation/
5. The Adherence and Outcomes Benefits of Using a Connected, Reusable Auto-Injector for Self-Injecting Biologics: A Narrative Review. (2023, September 21). PMC PubMed Central. https://pmc.ncbi.nlm.nih.gov/articles/PMC10567963/#:~:text=Most%20recently%2C%20electromechanical%20
auto%2Dinjector,injection%20speed%20control%20%5B15%5D
6. Sørensen, B. (2023, November 13). Re-usable electronic autoinjector – flexible performance. ONdrugDelivery.
https://www.ondrugdelivery.com/re-usable-electronic-autoinjector-flexible-performance/
7. Antalfy, A., Berman, K., Everitt, C., Alten, R., Latymer, M., & Godfrey, C. M. (2023). The adherence and Outcomes benefits of using a connected, reusable Auto-Injector for Self-Injecting biologics: A Narrative review. Advances in Therapy, 40(11), 4758–4776.
https://doi.org/10.1007/s12325-023-02671-2
8. Simpson, I. (2025, January 29). High-dose drug delivery – How far can autoinjectors go? ONdrugDelivery.
https://www.ondrugdelivery.com/high-dose-drug-delivery-how-far-can-autoinjectors-go/