Optics and medical devices have a long history and partnership. Since the invention of the microscope in the late 1500s to mid-1600s, the interest and desire to “see” into the human body to diagnose illness and subsequently repair it has only intensified.

Optics in modern medical devices fall into two categories: those used for diagnosis of illness, and those which are therapeutic or used in a variety of treatments. Devices within these categories include endoscopy, optical coherence tomography (non-invasive imaging test), surgical guidance cameras, intraoral cameras, ophthalmological imaging, 3D dental scanners, short-pulse laser surgical systems, and much, much more.

What Differentiates Optics Used in Medical Devices from Other Industries?

What distinguishes optical medical devices from other photonic products is not necessarily the use of extreme tolerances or unique manufacturing processes. It is more of a mindset — being clear about the unique functional requirements of medical devices before the design even begins. A few of the considerations include: Will the device touch the patient? Does it need to be autoclavable? Will it be reliable and perform consistently in a medical environment, meeting specifications that remain consistent over time? Does it use materials that are compatible with medical use and the manufacturing processes, and they must be clean as well. High-quality materials, such as non-leaded glasses, medically qualified epoxies, and stainless steel are some of the best choices.

In addition to applications-driven product requirements, government regulations and demanding health and safety standards guide the development process for optical medical devices from beginning to end. Unlike other industries where engineers can correct an optical design misstep after the prototype phase, governmental compliance, schedules, and budgets leave little room for changes or improvements after product introduction. There may be only one chance to design it, so the design must be right. These high stakes are in part due to the expensive medical qualification process that governs prototypes. After this, changes will mean requalifying, which is expensive and time-consuming. This can be a double hit to the project affecting both scheduling and costs.

Planning the End Result from the Beginning

Defining an optical device encompasses the creation of the design specifications, development of a realistic budget, avoidance of mechanical interferences while minimizing the packaging volume, manufacturability, test protocols for every stage, and ensuring that the desired features do not overshadow the critical function of the device. Any misstep of these functions adds time and cost and can dramatically impact the device’s utility.

To achieve the desired device performance, the optical design engineer defines the tolerances in concert with the optomechanical design engineer, balancing labor and material cost with performance requirements. Some of these parameters include surface figure, thickness, tilt, and centering, just to name a few. This is no small feat considering that many medical devices include flat, spherical, and aspherical optics as part of the design, and must operate over a wide temperature range. It is not surprising then that some medical device original equipment manufacturers (OEMS) encounter difficulties right from the start.

Insufficient tolerance analysis can affect the entire project. If the tolerances do not support the end use of the device, all the remaining functions will be impacted, including sourcing materials, inspecting incoming components, selecting a manufacturing partner, determining how each component will be tested, and even a shipping method that will not misalign or crash delicate optics. The tolerance analysis must be as complete and accurate as possible. This can eliminate many of the errors that occur further down the line.

Avoiding Pitfalls

Waiting until the end to test will not work. Trying to develop specifications without the right expertise of application knowledge will not work. Testing and validation of components before assembly is critical to achieving system performance. One of the first considerations is the level of precision required. Optical precision is measured in microns and nanometers. The instruments best suited to this are 3D profilers or laser interferometers.

Final system testing can include a test such as modulation transfer function (MTF) or encircled energy. An easier and more robust test is wavefront error, but it still needs to be interpreted. At this level of precision, the equipment and knowledge to accurately measure the optics may not be in-house.

This is where an experienced optics supplier can help. This group will understand the properties of the optical materials and how one should handle them. Grinding, polishing, and finishing techniques, assembly methods, and component and system testing will be part of their daily operation. In addition to the knowledge in these areas, they will already have the necessary equipment and staff.

Looking Ahead

With medical device optics, planning is key. This means creating a strategy from the earliest design concept through the final acceptance and testing. This starts with a complete understanding of the functional requirements necessary for good clinical results and then proceeds with a design that includes a complete and thorough tolerance analysis. Attention to government compliance requirements will also minimize missteps that can extend the schedule. One must choose a highly qualified business partner that brings unmatched knowledge and experience to achieve cost-effective yet robust medical devices.