Choosing the Right Medical Grade Fiber Optic Cable Part 2: Pistoning

Choosing the Right Medical Grade Fiber Optic Cable Part 2: Pistoning


Solving Problems with Fiber Optic Cable Pistoning   

The pace of innovation in the medical market is demanding, and no more so than in areas that require advances in diagnostic techniques. Timbercon has expertise in taking customer ideas or initial designs and providing accurate testing, rapid prototyping and the ability to move to large scale, low-cost production. Here we present an interesting case where Timbercon applied our expertise in these areas to realize a new type of fiber optic probe that will enable more accurate measurement of cell densities for biologics production.

Current solutions for monitoring cellular growth processes use one of several methods. Most of these methods (such as measuring the accumulated mass of organisms) are indirect, e.g. they infer the number of organisms based on another measurement. These current methods are subject to errors and often carry lower accuracy and are subject to data artifacts. One common method depends on the capacitance of the medium to measure cell density – while it allows for real-time monitoring, capacitance measurements are often very noisy due to the influence of bubbles generated during the agitation of bioreactors/fermenters.1,2 Our customer had invented an optical measurement method based on turbidity, which utilized novel processing techniques to eliminate the disruption in measurements caused by bubbles and other artifacts found within a bioreactor. This system solves problems with errors and off-line measurements in bioreactors, creating a probe that allows real-time, highly accurate monitoring of cell growth under a variety of conditions. The goal was to improve performance over existing electrical solutions and allow for greater accuracy. They worked with Timbercon to create an optical probe that would meet the following key requirements:

  • Custom hardware interface to match measurement hardware
  • Stable: Ability to endure repeated autoclave sterilization
  • Efficient: >99% transmission per meter
  • Tight Tolerance: Probe tip to have fiber protrusion by 30 microns +/- 10 microns
  • Probe materials to meet ASME BPE materials requirements
  • Probe materials to meet FDA requirements for biocompatibility
  • Hybrid cable: Both power connections and optical fibers packaged in the same probe

Timbercon created two variants of this probe design for initial testing. Note that the tolerance for the probe tip is extremely tight, and is a critical parameter for proper functioning of the system. To put this requirement in perspective, this tolerance is less than 1/20 the diameter of many common multimode fibers. Perhaps the most important requirement was that the probe be suitable for sterilization by autoclave; sterility is paramount in any system involving defined cell cultures. During transmission testing of the prototypes after repeated autoclave treatments, a problem with the initial design became apparent.

After repeated autoclaving of prototypes, system performance began to degrade to the point where data was unusable. The failure analysis determined that the loss of transmission was due to fiber pistoning. Fiber pistoning is often seen when damp heat (such as the 100% humidity found in the autoclave environment) is applied to fiber optic cables. It refers to the axial movement of an optical fiber within a connector or connector ferrule. It is thought to result due to differential expansion and contraction of the materials and/or epoxy used in construction. Fiber pistoning will typically result in decreased transmission due to increased pointing error or changes in fiber height. In this specific case, the cause was tracked down to separation between the fiber core from the buffer (rather than the entire fiber from the ferrule, which is the textbook definition).

The challenge was to keep the optical fiber type used (which was selected for biocompatibility and ability to maintain transmission after repeated autoclave cycles) while maintaining the extremely tight tolerance in the protrusion of these fibers from the tip. The engineers on the project hit upon a novel solution by utilizing a sapphire insert to maintain the position of the fibers within the probe assemblies. Sapphire provided the required biocompatibility, as well as providing a thermal expansion profile much more similar to the fused silica core. Timbercon process engineering used their expertise in fiber processing to remove the buffer and jacket in the probe tip and replace this material with the sapphire insert; this arrangement required a new process developed for assembling and polishing this unique arrangement. Testing validated that this new design overcame the limitations of the original, allowing for the product design to move forward.

New designs almost always encounter unforeseen problems. The difference between whether these problems are overcome in a timely fashion is determined by the creativity and expertise of the people involved. Timbercon has a long history of success in helping customers overcome even the most difficult problems in fiber optic cable designs. We value the partnership that we have with our customers and stand ready to help you tackle the unforeseen (and critical) issues that one inevitably encounters with innovative medical fiber optic designs. Call us to get a quote and see how we can help you make your product design a reality today!

  1. Carvell JP, Dowd JE. On-Line Measurement and Control of Viable Cell Density in Cell Culture Manufacturing Processes Using Radio-Frequency Impedance. 50 (1–3) 2006: 35–48; doi: 10.1007/s10616-005-3974-x.
  2. Carvell, John, et al. Monitoring Live Biomass in Disposable Bioreactors. 14 Mar. 2016,
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