The Devitrification Cascade -- Blast Shield Disease & Contagion August 15 2016
Initiation, Progression and Transmission of Holmium Laser Blast Shield Failures
My business partner has repeatedly told me not to assume that others know what I know – “Get the information out there, Stephen. Much that you take for granted is new and valuable to others.” It’s hard to know what these nuggets of value are. I am usually enlightened by a customer question, complaint or concern. That is the case for this blog post examining blast shield and fiber failures as an outbreak of disease. While the outbreak described progressed rapidly due to sharing reusable fibers between laser generators, small outbreaks may also occur with a single laser and single use fibers.
Many of our longtime customers lease lasers and provide fibers to hospitals -- they're sometimes called "mobilizers" -- and one of them recently came to us with an urgent concern. He’d had an acute rash of blast shield failures: half a dozen in as many weeks. (There were even more fibers damaged than blast shields, but it was the blast shields that got his attention.) We eagerly agreed to analyze the problem because every problem is a learning opportunity, at minimum. Our customer provided us with more than two dozen fibers -- a couple each from three vendors, and two dozen of our own ProFlex™ LLF holmium fibers. All the fibers had been used three times according to the customer. He also sent four of the damaged blast shields.
I picked the least damaged blast shield to examine for clues: I found a clue right away. The red arrow points to that clue: a brown dollop stuck to the blast shield. With this clue in hand I knew what to look for and where to look among the fibers to find it.
Stage 1 of Blast Shield Disease: Contamination
The fingerprint on this blast shield prompted a separate conversation with our customer, the gist of which is found near the end of this blog post, but the brownish haze and brown dollop told me that at least one fiber in the provided pile would show evidence of glue creep, aka adhesive cold flow with vaporization or spatter of that glue. Knowing the designs used in the other fiber brands presented allowed me to find such a fiber in just seconds: this one (below).
Possible Patient Zero
There were only two of this brand in the sample and both had what I was looking for, so finding what I sought on the first try is not evidence of my formidable forensics skills (they are formidable though), but rather it is a testament to the competence of this brand's design. This particular brand's construction is just poorly conceived and it invites this very failure mode (which is why I chose it first), as well as other failure modes that advertising for the patented design purports are minimized rather than aggravated.
The patent states that “the fiber is surrounded by at least one diffuser through which radiant energy that fails to couple to the fiber passes, wherein said diffuser is a member that surrounds the fiber, said diffuser being arranged to dissipate said radiant energy as it passes through said member” – from claim 1 of U.S. Patent 7,090,411. You can see the ‘diffuser’ around the fiber in the photo above. It’s actually just a quartz tube that’s been saw cut with the ends left rough. That little thought and effort is made in producing this 'diffuser' surface is evident by the presence of a huge chip in the ferrule edge at 9 o’clock. According to the patent, this is a critical element...
But this brand's problems goes much deeper -- using a diffuser scatters laser energy in a completely indiscriminate manner. Diffusers send energy in unspecified directions, basically filling the whole of the quartz ferrule (51 in the drawing below) or at least potentially filling the whole thing. That's a fundamental problem: you don't know where the energy will go. This ferrule is glued in place and the fiber is glued within the ferrule. Filling the ferrule with laser energy guarantees some interaction will occur -- that laser energy will come into contact with some glue, at some point.
In a drawing that I lifted from the patent (below) I’ve added red where the relevant adhesive is typically found. Now imagine laser energy (from the left) being scattered at the diffusing face (52) and continuing into the connector along the ferrule (51) to the right. Some rays are sure to cross the fiber itself and couple with the core at unpredictable and high angles, launching the very 'cladding modes' that this invention is supposed to minimize.
As described in a subordinate claim, "A fiber optic connector as claimed in claim 1, wherein a cladding of the fiber is partially removed from a predetermined length of the fiber at a termination end to reduce coupling of radiant energy into the cladding." Removing fiber cladding does not stop 'cladding modes' because the term is a misnomer: there are no modes that exist as described in the patent for rationalization of this feature. It's sort of like an egg-free and cream-free egg cream: it's meaningless.
Patented Holmium Fiber Design, taken from the patent (7,090,411, Fig. 3)
Side note: Just because something is patented doesn't mean it works. In fact, it may not be possible to ever build some things that are patented. The only inventions that the US Patent Office requires be reduced to practice before accepting a patent application are inventions that purport to violate the Second Law of Thermodynamics, e.g. perpetual motion machines. This requirement has cut down the wacky workload substantially, but definitely not entirely.
Some adhesive is used in almost all fiber connectors and some glue is present in every single small core connector I've dissected. A critical design imperative is to make darned sure that the adhesive is not in a place where it can be hit with holmium laser energy: there is no adhesive that can survive that. If you can’t predict where the laser energy is going to wind up, or if it is destined to be everywhere, this is an unsolvable problem. The red arrows on the "Possible Patient Zero" photo make this perfectly clear: they're pointing at glue where glue is not supposed to be.
There is no evidence of glue on the fiber itself and very little appears immediately around the fiber; it was likely burned away by the laser. There is some on the metal of the connector face, appearing as brownish spots and there's plenty on the ‘diffuser’ face. With this much glue visible, some has definitely hit a blast shield. I can’t say for certain that this very fiber started this spate of blast shield failures, but it is a prime suspect owing to the abundant evidence of glue creeping (arrow at 3 o’clock and others pointing at the glass to steel gap) and the sheer amount of glue present. And of course there is the thing that led me to look at this brand first: the incomprehensible design.
Did I just say that the design was bad? I am usually not that harsh, but yeah, it’s pretty bad: it’s laziness masquerading as high technology. That's just my personal opinion, of course. So here’s the other one of that brand; judge for yourself.
Red Arrows Indicate Adhesive
This one didn’t start the outbreak of blast shield disease, but it likely will if it’s ever connected to a laser again, and it would have had the case gone a bit longer. The glue was heated sufficiently to flow to the front of the ferrule (red arrows) and what saved this fiber was that it continued flowing in the connector bore rather than spread across the diffuser surface (the ‘diffuser’ ferrule is recessed about a millimeter in the steel connector ferrule bore -- see drawing, above). This brand is zero for two.... as I said, it's an incomprehensible design.
There were a couple of fibers from another manufacturer that showed evidence of adhesive cold flow and adhesive vaporization: those made by a laser OEM (two out of the three provided, in fact) and one of these is definitely a contender for patient zero. Of course it is also very possible that there was more than one patient zero initiating this outbreak. Yes, glue from one fiber can contaminate a blast shield and that blast shield can then contaminate a different fiber -- one fiber can initiate a cascade of fiber and blast shield failures. If a fresh fiber is contaminated enough to get hot enough, it will also suffer glue creep, vaporization and burning, adding its own quiddity to the brewing cauldron of goo. The third fiber of this brand showed contamination, but it came off fairly easily with alcohol and no catastrophic damage was evident. I'll give this brand one out of three.
Here’s the first of those three fibers of brand 2. It is pretty nasty and would be a prime suspect if there were more glue and evidence of glue combustion/vaporization. But like rivulets on Mars, it shows only evidence of flow. You can even see that most of the adhesive flowed from the central bore but it stopped being used just in time to avoid the fate of its brother (below, next).
This is what happens when the adhesive flows but does not get into the laser focus spot. This fiber may have contaminated a blast shield, initiating the blast shield disease, but the next fiber in our menagerie is a far more likely candidate.
Laser OEM fiber showing adhesive cold flow from ferrule bore
Side Note: It's called cold flow because glue flows at much lower temperatures than where it melts or burns, but for most adhesives, that’s still pretty hot.
Here's the second fiber from the laser OEM (brand 2). This little beauty definitely took out a blast shield (as in contamination, not shattering it, at least not initially).
Classic Image of Burnt Adhesive After Overheating and Cold Flow
The obvious damage is, well, obvious. More subtle are the adhesive vapor deposits on the metal of the connector, most clearly seen above the glass ferrule. That's about the concentration I'd expect to see on the blast shield.
The third laser OEM fiber showed no substantial damage -- a bit of contamination but it cleaned up fairly nicely. There was nothing more than contamination to the third fiber brand (two fibers provided). One problem with the third brand is that it is a well-type connector (see my blog series, "All Holmium Fibers are the Same, Right?") and, as such, it is almost impossible to clean the fiber face when it gets contaminated. The jury remains out on the fiber design but I did not see the damage that I've typically seen in the past, so it is possible they worked out a fix. I hope so.
When I write things like that I can hear some of you moan or snicker in disbelief. But it is true. I'd rather there not be a patient zero out there to start an outbreak that catches some of my fibers up in it. While ProFlex LLF is the only holmium laser fiber ever designed with this type of contamination in mind -- ProFlex LLF is immunized against this failure mode, if you'll permit me stretch the metaphor a bit further -- even my fibers can be irreparably damaged by cross-contamination from bad fibers.
ProFlex LLF does get contaminated by dirty blast shields like any other fiber, but it does not outgas adhesive vapor or cold flow onto the fiber face when it's heated. I'd tell you how this works, but I can't give everything away. Suffice it to say that the ProFlex adhesive can only cold flow one direction: away from the fiber face.
The forth fiber brand was made by InnovaQuartz and our fibers were well represented in the sample with about two dozen provided. That's not surprising since ours are the primary fibers the customer uses. I dare say this event should convince the customer to drop at least two of the other fiber brands from his inventory. Oh, by the way,not a single one of our fibers was damaged. I am not surprised: they are ProFlex LLF fibers. Here’s a photo. I carry one in my wallet ;-)
ProFlex LLF Contaminated by Outbreak and Cleaned w/ a Q-tip
There is slight evidence of exposure that can be still be seen if you look really carefully and know what to look for; a slight discoloration spot that is larger than the fiber core and off center shifted upwards a bit. That's where adhesive vapor from a contaminated blast shield deposited on the fiber and was burned off by the laser focus spot. Even still, this fiber could be used again and again without contaminating a fresh blast shield: ProFlex is immunized against blast shield disease.
So what is the cause and progression of blast shield disease?
To understand that, we have to know what a blast shield is made of...
Blast shields are made of low [OH] fused silica plate that is AR coated. Low [OH] fused silica is used because it is low in water (the [OH] stands for ‘low water concentration’ or ‘low hydroxide concentration’) and even the tiny amounts of water (80 to 120 ppm) that are naturally occurring absorb infrared energy very strongly. I suspect fused natural quartz would work OK at the thickness of a blast shield and they’d be somewhat less prone to this type of failure if they were fused quartz, but I don’t know for sure and I don't need to replace blast shields very often so I have no incentive to look for a cheaper option. Perhaps if I start testing more competitors fibers I'll investigate it.
Hydrochloric acid gas is used to dry silica soot that is formed from hydrolysis of SiCl4 -- World War II naval smoke screen stuff (they also used TiCl4) -- before it’s melted to form the glass from which blast shields are made. The result is silica with a very low [OH], at under 1 ppm, but it's quite high in [Cl] (chlorine concentration): around 1500 ppm. Chlorine contamination has no adverse effect on infrared optical properties, but it has some effect on the stability of the glassy state of the fused silica plate material.
While chemical bonding is the same in all silica and quartz (these are chemically identical bulk materials with different production methods and impurities where this industry refers to “silica” for synthetic materials and “quartz” for natural materials that are just melted quartz sand), ‘fused’ silica and ‘fused’ quartz are glasses where naturally occurring quartz is crystalline; fused silica's atoms are bound to one another the same way, but the repeat patterns or stacks that make crystals unique are not there: fused silica is amorphous.
Amorphous is good because the crystalline form is birefringent, meaning the refractive index is dependent upon the polarization and direction that light strikes the material. A circularly polarized laser beam would be split into two focal spots by a birefringent blast shield. (As far as I know, Trimedyne OnmiPulse 80 watt lasers are the only circularly polarized holmium lasers in the US, but there are now some of the new Chinese lasers coming on the market that likely use polarization beam combining just as Trimedyne does.)
An interesting thing about glassy silica (aka vitreous silica, fused silica, amorphous silica) is that it is metastable. It’s not unstable or stable, but stuck in a state that is in-between. Crystalline forms are more stable so, given a chance, fused silica will rearrange itself into a crystalline form. The only thing that stops fused silica from rearranging spontaneously is that it is too cold and too viscous to rearrange itself at room temperature. But give it a helping hand and fused silica will reorder its atoms into a crystalline form every time, losing its ‘glassiness’ in the process. This is called devitrification: the loss of vitreous nature.
Helping hands come in several guises but all of them relate to the mobility of the atoms or the viscosity of the glass. Elevated temperature lowers viscosity so heating the glass enough is sufficient to start devitrification – but we’re talking very hot @ about 1000°C (~1800°F). But there are other non-thermal ways to lower the viscosity of the glass, at least on the very surface, and that’s all it takes. Like rust, the surface is just a toe-hold for catastrophic failure. Here’s one of the blast shields that is just beginning to show devitrification. The bluish stuff is actually brown in color: its the adhesive vapor deposits.
Stage 2 of Blast Shield Disease Progression: Initiation of Devitrification
You might remember from high school science that the melting points of pure materials are lowered when impurities are introduced, like adding salt to ice to make homemade ice cream. The same is true of fused silica, even though the melting point is significantly higher than that for ice. Viscosity and melt point generally track: lower melting silica is less viscous than higher melting silica. This is where the 1500 ppm chlorine comes in to play…it’s a substantial contaminant. Parts per million may seem like a trace level, but 1500 ppm is really 1.5 ppt or 0.15%. That’s not that tiny. Personally, when someone puts nasty things terms of ppm, I get suspicious of their motives: Flint, MI suspicious.
Of all contaminants, alkali metals like sodium and lithium are known to decrease fused silica’s viscosity the most, followed by halogens like chlorine and fluorine. That’s why you have to wear gloves when replacing a halogen light bulb. The envelope is made of fused quartz and your fingerprints have salt in them, from sweat. Salt is sodium chloride so it has two of the worst contaminates for lowering fused quartz viscosity. Fingerprints on halogen bulbs quickly burn and, within a few minutes time (if you let the bulb cool) you can see the fingerprint on the glass bulb envelope as a faint white pattern of ridges and troughs that are actually tiny crystals.
Halogen bulbs crack much sooner that they would otherwise if you fail to take the proper precautions in handling them. Fingerprints also have moisture and oils in them that absorb infrared light strongly and burn, heating the fused silica, which also lowers the viscosity… A fingerprint on a blast shield may shorten its lifespan by 90% or more.
If a blast shield is contaminated with other things that absorb infrared strongly, the chlorine that is already in the glass is enough to start devitrification, especially once the AR coating is damaged. AR coating is, as you surely know, anti-reflective coating. This coating is very thin and it’s on both sides of the blast shield. The materials used are typically damaged by heat at far lower temperatures than the fused silica substrate, the blast shield proper. Once damaged, the less efficient AR coatings actually start absorbing some of the laser beam, creating more heat and damaging the AR coating more in a cascade of failure.
Adhesives typically absorb infrared energy very strongly. Where ever you see a brown film on a fiber, or any color for that matter, and on a connector or a blast shield, glue was burned somewhere nearby, usually just inside the connector, but it can creep onto the connector face as well. Glue does not need to splatter onto the blast shield to destroy it. Only the vapor of nearby burning adhesive is needed: it deposits as a thin film on the blast shield. The combustion products and adhesive vapor deposits absorb laser energy and the blast shield heats up. If it’s not already gone, the AR coatings are damaged and the blast shield heats even more until the atoms in the fused silica plate start to rearrange into cristobalite (a crystalline form of silica that is birefringent and has a different density that fused silica). At IQ we’ve shown that this devitrification can occur at temperatures as low as 120°C, from fingerprints.
Once the cristobalite starts to form, it separates from the bulk glass because it has a different density. You can see these tiny crystals (they look white because they are all oriented in different directions and have different refractive indices depending upon the direction of the light and its polarization; you know, birefringent). You can plainly see various levels of progression in the photos that accompany this blog and this final photo taken at higher magnification to show the crystalline nature of the damage where the blast shield got so hot that it re-melted some crystals.
Stage 3 of Blast Shield Disease: Mass of Devitrification and Melting
The last blast shield that was provided to us was not photogenic – it was the next stage after this one, where the blast shield cracks into pieces, exposing the focusing lens to damage. There were but a few fragments left.
Devitrification looks white to the laser beam, too, so the laser beam gets scattered by devitrification like that shown above. The scatter causes more heating, more damage to the fiber connector or a new fiber connector, etc. – like a snowball rolling downhill, this “devitrification failure cascade” leads to catastrophe. The “devitrification cascade” or “devitrification failure cascade” is InnovaQuartz’ term for this phenomenon, first coined in 1994 and used to describe pitting in side fire fiber caps. I’m pretty sure this is what causes burn back in stone fibers, too.
Keep in mind that a contaminated blast shield may still work but in using it you can contaminate or damage a perfectly good fiber, causing it to overheat and outgas adhesive vapor. Unless all affected blast shields and fibers are purged, you’ll suffer an outbreak of blast shield disease like our good customer did. Like many a disease, blast shield disease can easily spread between lasers that share fibers. That’s why it is better to just discard a suspect fiber or blast shield. In the long run, the cost is trivial compared to all the fibers and blast shields that could ultimately be affected.
Of course, the simplest precaution is to switch to ProFlex LLF. Give me a call or drop me a line to discuss all of the advantages InnovaQuartz and ProFlex bring to the table.
In closing, I cannot resist one last foray into the disease metaphor. InnovaQuartz does not yet make fibers for all surgeries using a holmium or a thulium laser. We will do so, eventually, but proper product design and FDA approvals take time and considerable funds. In the meanwhile it would pay to vaccinate your fiber inventory to the highest herd immunity possible: use ProFlex LLF wherever possible.
As with any vaccine, some can't be vaccinated so susceptible individuals must rely upon the herd immunity for their protection in suppressing outbreaks. ProFlex LLF does just that, but there is a minimum herd immunity that was required to protect everyone. the customer who's outbreak inspired this blog post chose to use around 25% nonimmune fibers so he suffered a considerable outbreak. It would have been worse had he not use ProFlex at all, but if he stops using the two brands that we found to be patient zero candidates and replaced those fibers with ProFlex, his next outbreak will be much smaller and likely self-extinguishing.
You need not be a mobilizer or share reusable fibers between lasers to benefit from ProFlex LLF's immunity to blast shield disease. A single laser may still serve as a vector of transmission to otherwise safe fibers, damaging them before the disease state is diagnosed. Blast shield disease is progressive; it does not display signs and symptoms in the early stages where it is contagious. So immunize your inventory, today.
Use only as directed. Only available by prescription. Side effects may include less time in the OR, reduced need for laser maintenance, lower costs per case, lower liability risk, peace of mind and more time with family and friends.
Many thanks to Dr. Jason Guth for his skilled photography and for authoring the complaint report that formed the basis of this blog.
Stephen Griffin (firstname.lastname@example.org)
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