Biologic drugs change the way the human immune system responds to numerous diseases and conditions. These drugs have revolutionized disease treatments and offer hope to many patients who previously had no effective remedies for their illnesses. That’s good news for patients, but biologics present drug-delivery challenges for device designers.
Recently developed biologics are characterized by high molecular weights — as much as 150,000 daltons (Da). For designers of biologic drug-delivery devices, such weights create different challenges than conventional synthesized drugs. The higher molecular weights also means that biologics have high viscosities, which influence the injection profile of the drug-delivery device and, ultimately, the device’s effectiveness. These challenges make the motor system a significant element in biologic delivery device development.
Biologic Drug Administration Methods
Of the three methods for administering a drug into a patient’s body (see Figure 1), biologics use subcutaneous and intramuscular delivery methods, including:
Intravenous. This technique, often abbreviated as IV therapy, delivers the medication directly into a patient’s vein.
Subcutaneous. Drugs intended for slow absorption are often injected into the subcutaneous tissue between the fat layer just under the skin and over the top of the muscles.
Intramuscular. Because muscles have larger and more numerous blood vessels than subcutaneous tissue, intramuscular injections are preferable for faster drug absorption.
Technical Considerations for Biologic Drug-Delivery Devices
Three main types of devices are used to deliver biologic drug injections: disposable, limited-use, and reusable biologic pumps. How these devices are used— along with their technical considerations and the motors or motion components available to address them — include the following:
Disposable biologic devices. Single-use devices are designed to cost-effectively deliver a specific drug, with delivery time ranging from a few seconds to a few days. Examples include mechanical injectors and disposable patch pumps that are worn directly on the body and have a reservoir, pumping mechanism, and infusion set inside a small case.
Various motors and drives can be a good fit for this application. The most suitable motor technologies are can stack stepper and brush DC. Both motor types offer a mature technology with advantages over some alternative actuators that, although they may also be appropriate, require complex drive electronics.
The duty cycle for disposable applications is limited, ranging from a few seconds to a few hours. Based on the application requirements, a reliable, cost-optimized, motor-driven system serves these devices well.
Limited-use biologic devices. A motorized device can accept multiple biologic drug cartridges over the course of a therapy. Limited-use devices offer the advantage of having both a disposable drug unit and a separate, reusable pump unit. They also provide flexible performance as well as a lifetime that is relative to the pump price. Limited-use devices are generally designed for use with a single biologic drug or limited drug offerings, so the force and size requirements for the motors may be different than those of reusable devices.
A motor that can meet a force range of 50–80 N is typically sufficient. These devices are battery driven, so efficiency is important to the pump design. Coreless brush DC motors with precious metal commutators are well suited to meet these higher battery life requirements, providing high power density and reliability. A spur compound gearbox or custom gearhead are ideal for limited-use biologic applications.
Reusable biologic pumps. Robust, rugged, and long-lasting reusable drug-delivery pumps can handle drugs of various viscosities for a broad range of therapies. This type of delivery device presents particularly demanding motion requirements, however; the higher-viscosity biologics make a high axial force output up to and higher than 100 N necessary.
When supporting various biologics as a platform, the pump must be able to deliver the highest viscosity drug while also accurately delivering the lower viscosity ranges. Reusable pumps are portable, so their size and weight are important criteria. Outside diameters for a premium biologic device motor typically range from 10 to 12 mm (see Table 1).
A reusable pump must also be robust and reliable. Product warranties can range from two to five years, so the motors and drives have to be properly sized to deliver the same accuracy from day one to the end of the pump’s life. For a motor-driven system, the designer confirms a confidence factor of greater than 90 percent to ensure that it lasts the required lifetime.
Feedback, repeatability, and storage conditions are additional factors in drive system selection. An encoder confirms that the motor has provided the needed drug delivery, ensuring that the therapy is successful. It also ensures that every delivery increment is met each time. If the biologic requires cold storage, a motor and gearhead that can withstand lower temperatures and humidity over periods of time are necessary.
Overall, brushless DC motors (BLDC) or coreless brush DC motors with precious metal commutators are best suited for reusable biologic pump devices. A spur gearhead or a custom-designed gearhead can provide a great torque-to-force density within the smallest available size.
Current Challenges in Biologic Drug-Delivery Systems
Like biologic drugs, biologic delivery devices are relatively new to the market and are continuously improving. Because biologics are more complex with higher molecular weights and viscosities than small-molecule chemical drugs, biologic drug devices face the following challenges:
Accommodate high-viscosity drugs.
Deliver a large volume of a drug.
Cost optimization in order to reach a larger population.
Some of these challenges can be supported by motor-driven systems (see Table 2):
Brushless and brushed motors have higher power densities, which, in combination with higher reduction ratio gearheads, can handle high-viscous drugs greater than 50 cP.
Power density helps to accommodate larger drug volumes in a smaller device size, ranging from 10 to 50 mm.
Absolute encoders can help precisely monitor drug delivery at 3–5 μm resolution.
Reliable motors and gearheads provide long life for safe, home-based therapies.
Motor and Gearhead Options for Biologic Drug-Delivery Devices
Because powering the delivery mechanism is a crucial part of a drug-delivery device, design engineers must evaluate a variety of motors and gearheads to find the most appropriate component for their application. To complete this selection process effectively, engineers can consider the following list of key performance elements of drug-delivery devices and the different motor technologies that can handle them:
Torque/power. All motor technologies offer the ability to produce torque, but their internal designs present different output capabilities. BLDC slotted designs deliver higher output torque than BLDC slotless designs, based on the higher amount of copper and magnets in the motor. When considering stepper motors, hybrids provide the highest torque.
Speed. The drug dictates the duration and flow for the therapy, so the device speed will be set. BLDC stepper motors can meet higher speed requirements. Brush DC motors can satisfy medium speed requirements, whereas steppers can handle lower speed requirements. To increase output torque, you can add a gearbox and thereby increase the speed requirements on the motor by the corresponding ratio.
Efficiency. Biologic drug-delivery devices rely on battery power, so an efficient system will keep the battery size low. Brush DC coreless and BLDC slotless motors have a rotor-based design, making them the optimal choices for system efficiency.
Reliability. The device has a specific life requirement based on the number of therapies to be delivered to the patient. Motor life is determined by the commutation and bearing systems. Brush DC motors have a mechanical commutation, meaning the motor life is dictated by brush wear. BLDC and stepper motors have electrical commutation, which provides a potentially longer motor life. Bearings also affect motor life, with ball bearings providing longer life than sleeve bearings. Motor technologies offer both bearing versions, with some providing them as standard and others requiring customization.
Weight. Because biologic drug-delivery devices are portable, weight can significantly affect patient comfort. Brush DC coreless motors are the lightest option.
Cost. Each motor technology has a cost profile based on its design. The device and accompanying therapy mandate a cost profile for market acceptance, so engineers need to understand the cost drivers for each motor type.
Comparing the various motor technologies and how they function in different drug-injection devices is a necessary part of device design (see Table 3). Referencing a checklist like the one above can help design engineers assess motor options for their impact on device cost, efficiency, and user experience more effectively.
Partner With a Motor Supplier Early in the Design Process
As biologic drug treatments continue to increase in the market, each application presents its own challenge in terms of biologic therapy requirements. Connecting with a motor supplier early in the development process can help designers and manufacturers identify the best motor technology and accessories to create an optimal drug-delivery device.
This article was written by Jigar Fulia, Divisional Manager, Product Design and Development, and Kanti Vala, Research and Development Manager, Product Design and Development, Portescap, West Chester, PA. For more information, visit here .