Waterjetting Technology 17d - Some medical applications - don't blink!

Our discussion on the ability of high-pressure waterjets to remove some materials without damaging others has, as I noted in the last post, found some applications in the medical field. The example that I provided in that post dealt with the use of waterjets to dislodge material in the cleaning of wounds, or to carefully excise thin layers of burnt flesh without doing damage to the underlying tissue. I also pointed to the ability of a waterjet to remove weaker diseased tissue, such as brain and skin tumors, without damaging the surrounding tissue, which is healthy and thus able to resist the pressure of the cleansing waterjet.

There are a number of other applications in the medical field which have developed from these advantages. The first is in surgery on the liver and kidneys in particular. In these organs there are generally a large number of blood vessels that carry blood through the organ so that the blood can be cleaned of impurities. The difficulty that this creates usually occurs in cases where the organ becomes diseased. Medical treatment recommends that the diseased volume of the organ be removed, and the historic method for doing this has been to take a scalpel and carefully cut around the diseased region, so that it can be lifted out. This is often referred to as liver resection.

The problem that this creates is that, in the process of cutting out the diseased part of the organ, the surgeon must also cut through the blood vessels in that part of the organ. Because they are buried in the organ, it is not clear where these vessels are, and they are conventionally difficult to isolate. Early work in finding an answer was carried out in Japan.

However blood vessels are relatively tough (and when full of blood require pressures of around 2,000 psi or more to penetrate) whereas the surrounding tissue is much softer. As a result – using the same conceptual approach that I described last time for the removal of skin cancer, a surgeon can remove the tissue from around the blood vessels and along the projected line of dissection, without cutting through the vessels. Tests during brain operations have shown that as long as the pressure is kept below 300 psi there is no damage to any of the vessels in the brain, and similar conclusions are likely to hold for other organs in the body. The jet was, however able to remove diseased tissue, while leaving healthy tissue.(In these surgeries the jet is on the order of 100 microns in diameter, and as a result uses very little water during the operation.)

Figure 1. Japanese surgical removal of liver tissue around blood vessels of the liver, exposing them so that they can be tied off, before being cut, thereby significantly reducing blood loss.

Clinical trials rapidly spread around the world, Papachristou had published on the technique in 1982 when, after 75 trials with dogs it was tried in four male human patients. At the time it was noted that there was significant reduction in blood loss. This is fairly critical in older patients (who are more likely to need these operations) since large losses of blood can induce shock, and can be fatal. Blood loss is the most common complication of the surgery and consequent cause of death (which now runs around 5%, but was higher prior to waterjet introduction).

It also impacts long-term survival and post-operative complications. (Historically the problem was addressed using what is known as the Pringle manoeuvre in which all the blood flow to the liver is halted by clamping for the period of the operation. However this can only be applied for limited amounts of time and is of limited effectiveness.) Using a waterjet approach has the advantage, over ultrasonic and cavitating techniques, that the path cut through the liver is narrower and the vessels are more clearly delineated. At the same time, as figure 1 shows, the sides of the cut are relatively clean and well defined. The jet is able to handle the tougher tissue in a cirrhotic liver either through a longer residence time, or by raising the pressure of the jet by about 150 psi.

The very narrow cut achieved by the waterjet has another useful feature in that the jet will, in fibrous tissue, push apart the fibers rather than fusing them, as would be the case with laser cutting. This has advantage in eye surgery where any such fuse points can cause scarring that interferes with future vision, while the separation of the tissues with the jet does not carry that problem.

Figure 2. Precision cutting across the face of the eye at a jet pressure of 20,000 psi and with a jet diameter of 10 to 100 micron.

The technique was first announced in 1994 and animal tests had been successfully completed by 2001. Because the jet cuts so fast (less than a second per cut) there is no tissue loss. (The operation uses a device that fits to the eye to hold the lens in the right position to make the cut).

The technique has not, however, been that successful in the field as has another application, that of removing herniated disks in the spine. The technique uses a procedure known as Hydrocision. It is interesting to read a quote from an article last November on the technique:

"It's basically a high-velocity water jet eroding system," Kevin Staid said about the medical device that his North Billerica, Mass.-based company makes. "And this is our first entry into the area."

With HydroCision, a jet of saline solution comes out of a nozzle that is 0.005 inches in diameter -- "slightly larger than a hair" -- and can cut away protruding disc tissue that can cause the back and leg pain without an actual blade.

"Just the energy of the jet would be doing the cutting," said Staid, an engineer. "In our case, the water is going about 600 miles an hour and has the ability to cut quite effectively."

The advantages of the 20-minute outpatient procedure are: No hospitalization, quicker recovery times, less pain, no surgical trauma to the back muscles and no general anesthesia.

"There is no muscle damage, no bone removal, no nerve root manipulation ... and the size of the wound is approximately 4 mm," Kowalkowski said.

The use of the tool is sufficiently popular that in 2009 the company (Hydrocision) announced that the tool had been used in more than 40,000 procedures.

I’ll give other medical examples in the next post.

Waterjetting 8d - Choosing angles

How times change! I was reading a column in the British Farmer’s Weekly, and came upon this, where the author is discussing the need for a generator.:

It will also be vital to keep the fuel flowing into the tractors, and power the pressure washer, and light the security lights, and all the other essentials of an average arable farm.

It is an indication of how far the use of pressurized water has come, that it is now seen, at the lower end of its application, as a vital farming tool. Which is a good introduction to talk a little further about the use of cleaning streams, and how to interact with differing target materials.

There was an initial first step, when someone would send the lab a mystery block of material and asked – how do I cut it? Generally the samples were small, but we would find a flat surface on the material, and carefully point a jet nozzle perpendicular to this surface. (In the early stages this was hand-held). When a jet strikes a surface, but can’t penetrate it, then it will flow out laterally around the impact point, under the driving force of the following water.

The test began with the jet at low pressure, and this was slowly raised, until the point was reached when the pressure was high enough to just start cutting into the material. At this point the jet had made a small hole in the target, and so the water flowing into that hole had to get out of the way of the water following. The sides of the hole stop it flowing laterally, and so it now shoots back along the original jet path. This spray can hit the lance operator if the nozzle is hand-held, but it is a fairly graphic way of determining the threshold pressure at which the material starts to cut. (and I’ll get into what happens as the pressure continues to go up in a future series of posts).

But for the purpose of cleaning, the jet has to move over the surface, once it has made that initial hole, at pressure. But, in many materials, if the jet comes vertically down onto the target, then only the material directly under the jet will be removed. And so the jet has to be played on every square inch of the surface in order to ensure that it is cleaned, or that the coating/layer is removed. In some sandstones, for example, two jet paths could be laid down, almost touching one another, and yet the rib of material between them would remain standing.


Figure 1. Adjacent jet passes in sandstone, the cuts are about an inch deep, but note that even though the narrowest rib is about 1/8th of an inch wide, it is only when the cuts touch that the intervening material is removed.

Yet that rib of material was, in that case, so weak that it was easy to break it off with a finger. (This turns out to be a weakness in making delicate sculptures out of rock). To use the full pressure of the water can be a waste of energy, if the material is very thick, since it all must be eroded with such a direct attack.

Yet the minimum amount of material that needs to be removed is that that attaches the layer to the underlying material (the substrate concrete, steel etc) and this can be quite thin. Thus, in attacking a softer material, particularly one that can be cut with a fan jet, a shallow angle directed at the edge of the substrate can be more effective.


Figure 2. Round v fan cleaning from Hughes (2nd US Waterjet Conference)

Because there is a balance between cutting down through the material to be removed, and cutting along the edge to grow the separation crack between the materials, some practice is needed to find, for a given condition, what that angle would be.


Figure 3. Choice of angle from Hughes (2nd Waterjet Conference)

The more brittle the material, then the greater the angle to the surface, since rather than just erode the material, the jet may also shatter the layer into fragments that extend beyond the cut path. But otherwise using an angled jet to the surface can be more effective. Hughes (from whose paper at the 2nd Waterjet Conference I took these illustrations) has a simple test for orifice choice.


Figure 4. How target response influences nozzle selection. (Hughes 2nd Waterjet Conference)

Some of the more advanced cutting heads use a series of nozzles that spin within an outer protective cover, as they remove anything from layers of damaged concrete to thin layers of paint from ship hulls. Increasingly these are connected to vacuum systems that will draw away the spent water and debris from within the contained space, without it entering the work space, and creating problems for the worker.

In order to reduce any collateral damage to the surroundings these jets are often made very small (thousandths of an inch in diameter) so that their range is short, and they are inclined outward to cut to the edges of the confining shield.

We have had some success in turning those angles the other way, so that they cut into the shield, rather than away from the center, and also so that each jet is directed towards the path of the next jet around the circumference. The intent in this case is to allow the use of a slightly larger jet, with a greater cutting range. In this case the individual cleaning/cutting path is a little larger, but because the jet at then end of the cut moves into the range of the adjacent jet, then any remaining energy that it and the dislodged debris still have, will not be enough to get through this second jet.


Figure 5. Inclined jet and shroud design.

The action of each jet then becomes not only to cut into and remove material, but also to contain the spent material from the other jets dispersed around the cutting arm, and to hold the debris in the center of the confinement for the very short time needed for it to be caught up in the vacuum line.

In all cases the choice of pressure, nozzle size, and operational factors such as angle of attack, come down to the target materials, those that have to be removed, and those that need to be left undamaged. And it is why it is useful, at the start of any new job, to take the time to do a little testing first, to make sure that the right choices of nozzle and angle have been made to get the job done quickly and efficiently.

Incidentally the idea behind the test of effective pressure, that the jet flows laterally when it hits something it can’t cut, can help, for example in easing the meat from the bone when a jet cuts a deer leg.


Figure 6. Cut across a deer leg, note how the jet has cleaned off the meat from the bone, undercutting the flesh.