Slit lamp biomicrography is paramount to the practice of ophthalmology, allowing for an inside look at various areas of the eye with a range of magnification levels and light sources. The development of medical devices, such as the ophthalmologic slit lamp, brings about a number of design restrictions. The equipment must be able to perform the necessary tasks with precision to allow for patient diagnosis and proper treatment, and must maintain a high level of cleanliness, low levels of noise during operation, and, due to the typical high cost of medical equipment, have a high service life with as little required maintenance as possible. With these design requirements in mind, the development team from A.R.C. Laser GmbH, Nürnberg, Germany, was tasked with creating a new slit lamp for an already established marketplace.
Prior to the development of their new slit lamp, A.R.C. produced laser systems to treat various optical conditions, including laser systems for photocoagulation, which is used to treat retinal hemorrhages, and systems to help remove cataracts. These separate systems were a perfect segue into a complete system for the diagnosis and surgical treatment for ophthalmologic conditions.
The complete development of the new device was a challenge for the engineers at A.R.C. According to Thomas Herberger, a design engineer with the company: “A critical point for the system was the movement of the microscope arm. The slit lamp has to move easily, and the axes have to be guided with extreme precision, as every irregularity and all the play in the system is transferred through to the doctor at 40x magnification. The play in the system needs to be absolutely minimized if, for example, an ophthalmologist is looking for or attempting to treat a hemorrhage on a retina. At the same time, the microscope arm needs to be able to be securely fixed.” (See Figure 1)
Typically, a clinical biomicroscope, or slit lamp, is made up of a binocular viewing system coupled with illumination to allow for viewing of various areas and angles of the eye. Most are equipped with two lighting sources with variable flash intensity, adjustable magnification, and angles of illumination.
In their testing of existing slit lamps available on the market, A.R.C. saw room for improvement with their new device. “We had tested [slit lamps] that placed mechanical emphasis on either smooth operation or high precision, but we wanted to be able to offer both,” said Herberger. “This was all the more difficult to achieve, as the microscope is approximately 250 mm away from the bearing on the moveable arm, forming a lever which makes it tough to achieve precise support with little to no play.”
While searching for a means to achieve both extreme precision and smooth operation, A.R.C. design engineers discovered self-lubricating plastic bearings from igus Inc., East Providence, RI, which were already in use in a number of medical technologies, including laboratory and cleanroom equipment. Typically, a tolerance of E 10 is sufficient for infeed applications, however, for this particular application, a tighter than usual tolerance was required to meet the high demands of the new slit lamp design. “Here,” said Herberger, “we found ourselves at the limits of what we thought was mechanically available.”
From Semi-Finished Product to Perfectly Fit Plain Bearings
To fulfill all of A.R.C.’s system requirements, igus worked to create custom plastic bearings from semi-finished components made of its “iglide J” plastic material. These custom solutions were machined to extreme accuracy in order to achieve the desired fit and clearance. When designing the custom bearings, the goal was to create the thinnest possible bearing wall thickness in order to ensure a high level of dimensional stability. This was even further complicated by A.R.C.’s plan to include apertures on the microscope arms to carry cables through the slit lamp. (See Figure 2)
The custom bearings are comprised of a polymer material base, homogenously blended with strengthening fibers and solid lubricants, and are characterized by their extremely low coefficient of friction while operating dry and with minimal stick-slip effect. The self-lubricating plastic material bearings successfully met all of the requirements set forth by A.R.C.’s engineering team.
“In combination with the stainless steel and brass materials used against the iglide bearings, we achieved perfect results,” explained Herberger. “It was very important that the material blend was homogenous, so that the sliding properties, even when we machined custom components, are always the same.”
Also critical for the A.R.C. design engineering team was the silent operation and freedom from lubrication that the bearings provide. Without external lubricant, the slit lamp does not attract dust or other debris, which is crucial in any piece of medical equipment. The freedom from lubrication also eliminates required maintenance to the bearing points of the slit lamp. The extensive service life of the bearing materials, which has been extensively tested in true-to-life applications, is also a critical factor, as slit lamps, like many other pieces of medical equipment, are often used day in and day out for decades.
The use of the custom components allowed the development team at A.R.C. to create a new slit lamp with all their desired qualities. The system components are extraordinarily long lasting, and maintain their ability to self-lubricate and run silently throughout. Intensive durability tests of the A.R.C. lamp was carried out over months, proving the longevity of the full system, and the long-term precision of the bearing systems.
This article was written by Nicole Lang, Product Manager for iglide plastic bearings at igus Inc., East Providence, RI. For more information, Click Here