Linear Motion Design Considerations for Medical Device OEMs

Linear actuators, in particular, electromechanical linear actuators, have become integral components of modern medical devices because of their high precision, accuracy, and ability to deliver repeatable motion control. Patient comfort, positioning and mobility, robotic surgery, imaging equipment, infusion, and pumping are just a few of the applications where the use of linear actuators has revolutionized the way medical devices are designed, improving patient outcomes and enhancing the overall quality of care.

For medical device and equipment designs using electric linear actuators for motion control, detailing exact requirements for performance and reliability will help ensure successful outcomes. In addition, an understanding of how the motion control partner meets these requirements, as well as how they engage along the development and validation process, will be critical to meeting budget and time constraints.

This article explores some of the unique product and partner considerations to ensure successful device design.

Advantages of Electric Linear Actuators

Electric linear actuators offer significant and game-changing advantages in many medical device and equipment applications over conventional manual or fluid power driven solutions:

• They are more precise and provide greater accuracy and control.
• In many cases, they can offer a smaller footprint to aid in creating more mobile, more elegant design solutions.
• Electromechanical solutions are also more reliable and less maintenance intensive, reducing downtime and costs.
• In addition, they will also be quieter to support improved patient experience.
• Finally, they can improve the efficiency and speed of medical procedures, reducing wait times and improving patient flow.

Linear Motion Design Considerations in Medical Devices

In any new design exercise, whether for a device destined for use in a procedure room or a new piece of medical equipment, an essential first phase is gaining a thorough understanding of the performance demands for the electromechanical components, balanced against the usage requirements.

When a linear actuator is designed and purpose-built in-house or through an external design/ manufacturing resource, attention to details, integration, and application considerations that reach beyond basic actuator performance will have a significant impact on the success of the overall solution. A comprehensive view of requirements can seem daunting, but organizing considerations into key categories can help simplify the design specification process:

1. Performance Requirements
2. Service Life and Reliability
3. Operation and Usage
4. Operator and Patient Safety
5. Device Serviceability
6. Manufacturability
7. Comparison and Selection

Performance Requirements

Typically, the initial application considerations and requirement set to be evaluated are the mass and forces to be managed. Linear actuators must meet the specific performance requirements for each aspect of device movement and often with multiple interactions, including target accuracy, motion precision, speed, force, and linear travel.

For example, in infusion pumps the speed and force of the linear motion is needed to ensure system can provide the proper the flow rate for the procedure. Delivery must be precise and reliable to ensure the correct dosage is delivered to the patient. Accuracy and repeatability ensure that the dosage from procedure to procedure is in alignment with dosage accuracy. In the cause of surgical robots, the accuracy and precision of the motion control system determine the accuracy of the surgical tool’s placement, which is critical for achieving surgical objectives while minimizing trauma to surrounding tissue.

Linear Actuator Types

As is most often the case in medical devices, designers may choose to utilize efficient electromechanical actuators over less-practical solutions with pneumatic, or in select cases, hydraulic cylinders. With the selection of an electric linear actuator, the more detailed the understanding of the motion profile and loading profiles, the more refined a solution can become relative to the overall objectives of the project.

Rod-style or Rodless Actuators. An application’s requirements will most often guide the selection of a rod-style or rodless actuator. Rod style actuators are ideal for situations where linear thrust only is required. The pushing and pulling action of an electric rod actuator works well in many applications like fluid compression, dispensing or injecting procedures. Rod style actuators offer limited stroke length but support higher forces designed for pushing, pulling, or pressing with up to 50,000 lbf force. However, they do not provide support to a load. In the event that the load is cantilevered or overhung, it can deflect the rod, causing wear on seals and bearings and even triggering major positioning problems.

In contrast, rodless actuators have an inherent advantage over rod-style actuators for being able to support and carry loads. This reduces costs and design time by eliminating the need for additional load-bearing and guiding elements. In contrast to rod-style actuators, a rodless actuator’s stroke lies completely within the length of its body, resulting in a smaller working footprint and a more efficient design that supports frequent cleaning, aesthetic requirements or even decontamination processes. Rodless actuators commonly use one of two main drive train types to convert a motor’s rotary motion to linear motion: a power screw drive or a timing belt. While both drive trains offer efficiency, reliability, and long life, each has characteristics that may influence selection.

Operation and Usage Environments

Medical environments require low-noise operation to ensure patient comfort and minimize interference with sensitive equipment. Noise and vibrations can also affect the accuracy and precision of the linear actuator technology. Actuators should be designed to operate quietly and produce minimal vibrations, particularly in devices used during patient examinations or procedures. Intangible aspects like noise, vibration, or other aspects including visual appearance can at times be the most challenging and hardest to quantify and manage throughout a design exercise.

From a usage experience standpoint, designers may only know what they are trying to avoid, rather than what they are seeking to achieve, prior to a working prototype. At times, these more challenging and intangible aspects can drive over-specification in an effort to avoid undesirable unknowns. Here, it’s most valuable to have strong collaborative partners in their given areas of expertise to help mitigate undesirable outcomes through design experience and know-how, rather than through over- specification.

The size and weight of the linear motion and actuator technologies are also vital design considerations for medical devices. In many cases, medical devices are designed to be compact and lightweight for portability, function, operational environments, and ease of use, while other applications may be fixed installations, quite large, heavy, and demand utmost levels of stability for accuracy and movement precision. Even in the largest, heavy-duty applications, there are often underlying objectives to produce desired performance outcomes while minimizing the physical footprint and weight to deliver a more practical and functional execution.

Beyond superior functionality, other top-of-mind considerations for the finished product package throughout the design process include creating a compact, aesthetically clean, and quiet solution. In cases where external partners are to be engaged, finding a partner that can employ technical flexibility on mechanical travel, mounting, motor selection/integration, or if needed, complete custom capabilities, can provide the designer greater latitude in creating a prioritization and balance between design criteria that optimize the design outcomes.

Conclusions

The trends for integration of advanced motion control utilizing electric linear actuators in medical devices will continue to surge. The continued development of emerging technologies ranging from surgical, procedural, and diagnostic devices to more advanced iterations of patient positioning, ergonomics, and mobility assistance, will propel the use of an enlarging scope of electric linear actuators to provide both simple and complex controlled movements.

Medical device designers must consider several factors when selecting the appropriate technologies and evaluating their development partners. By considering the factors that reach beyond performance specifications to address the larger impacts of the operating environments, human factors, and long-term serviceability and maintenance, designers can ensure that their medical devices are safe, reliable, and effective in meeting the growing needs of patients and healthcare professionals.

This article was written by Ryan Klemetson, Business Development Manager, and John Fenske, Director of Marketing, Tolomatic, Inc., Hamel, MN. For more information, visit here .

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