Magnet Selection

There are many factors affecting the appropriateness of a magnet for implantation. The material a magnet is made of and its size, shape, and orientation determine the field and thus how effective it is at either producing sensation or lifting objects. Although lifting objects is a fun way to demonstrate an implant the focus of this section will be on magnets optimal for sensation. Far more important than field though is getting a magnet coated with a bio-compatible material. A person can’t fell or lift with a magnet that rejects.

Magnet Material


There are many materials used to make magnets but most are far too weak for our purpose. The most common and oldest type of manufactured magnets are Ferrite and Alnico. Most fridge magnets are made of Ferrite. The old horseshoe shaped magnets sold as children’s toys are made of Alnico. Both of these types get their magnetism from the alignment of the iron within. Although inexpensive and easy to work with these iron based magnets are nowhere near strong enough for our purposes. In terms of strength, iron based magnets have been superseded by rare earth magnets such as Samarium Cobalt (SmCo) and Neodymium Iron Boron (NdFeB). SmCo is very strong compared to Iron based magnets and has some desirable qualities such as the ability to withstand high temperatures. It’s strength though pales in comparison with NdFeB.

 

Ferrite Ferrite magnets are also known as ceramic magnets as they consist of Iron (III) Oxide in a ceramic base. They are cheap, weak, and brittle.

Modern ferrite magnets may provide a Br Gauss of around 2000.

Alnico Alnico magnets are around twice as powerful as the very strongest ferrite. The name Alnico comes from the three metals which they are made of: Aluminum (Al), Nickel

(Ni), and Cobalt (Co). Alnico is unique in that it can be heated up to 1000 °F and still maintain its magnetic strength.

Alnico magnets may provide as much as 7,100 Br Gauss.

Samarium
Cobalt

SmCo

Samarium Cobalt is a rare earth magnet that is considerably more powerful than Alnico. It’s used when one needs a very powerful magnet that can operate in a relatively wide range of temperatures. It isn’t as powerful as Neodymium but can withstand over twice as high a temperature.

Some very high grade Samarium Cobalt magnets have a Br Gauss of 11,000.

Neodymium Iron Boron

NdFeB

Neodymium Iron Boron is the only magnet worthy of consideration for implantation. It is nearly twice as powerful as Samarium Cobalt depending on grade. One limitation is that if heated above300 °F, it permanently loses magnetic strength. It’s also relatively brittle and prone to cracking.

An N52 grade Neodymium Iron Boron magnet has a Br Gauss of 15,000.

 

 

 

The strongest type of permanent magnet type is NdFeB. It’s the optimal choice for an implant. This shouldn’t be misunderstood as claiming that a more powerful field is always better. Some grinders have experimented with placing very large powerful magnets. Although these are obviously able to lift more, they are also strong enough to cause crush injury to the tissue between the magnet and whatever is being lifted. The reason NdFeB is optimal is that allows for a smaller magnet size; this in turn allows for a smaller and thinner implant which is less invasive, less likely to be rejected, and produces better results in terms of sensing the electromagnetic spectrum.

Amongst NdFeB magnets, there is a spectrum of grades available. The higher the grade, the stronger the magnet. The grade is determined by the number which follows the indicator “N.” This N number is the maximum energy product of the magnet using the unit Mega Gauss Oersted. This N scale is linear so an N42 would be twice as strong as an N21 grade magnet. The highest grade of NdFeB magnet commercially available is an N52.11.The strength of these N52 grade magnets is phenomenal. Consider that a four inch square with a depth of .5 inches can suspend over 300 pounds.

Shape


There are only two shapes commonly chosen for implants: discs and cylinders. Because these implants function by displacement of the densely innervated fingertips, hands, and less often tragi, discs are the most common. A disc shape can be seated unobtrusively and yet provides a broad area of displacement in response to nearby fields. Cylinders are becoming more popular and have promise in terms of providing an easy means of implantation. An appropriately sized model can be implanted with use of an RFID injector. Many people are intimidated by the idea of opening up and undermining their own flesh. The injection method would be undeniably simpler. As of this time, there are no injectable magnets being sold prepackaged and sterilized but both Dangerous Things and Cyberise are working to make this a reality.

A cylinder implant comes with a significant downside. The field shape produced is less desirable and the total field strength is significantly reduced. One way to demonstrate this is by comparing the pull forces in relation to a magnets volume. 2x12mm Cylinder has a volume of 38 cubic mm and a pull M36 - 10 Minutesforce of 0.46 lb whereas a 1x3mm Disc has a volume of 7 cubic mm, and a pull force of .30 lb. The cylinder magnet has more than 5 times the volume of the disc but only provides just over a 50% increase to pull force. This doesn’t mean that cylinder magnets aren’t worth considering. The ability of a shape or design to elicit the maximum sensitivity to the electromagnetic might not end up being determined by any of these characteristics at all. I’ve spoken with a number of grinders that self-inserted cylinder shaped magnets that report contentment with their implants.

There are a number of ways that magnet shape could be improved in future models.  For example, a disc magnet produces symmetrical field lines resulting in about half of the magnetic field being directed deep to the implant rather than aimed at the surface. Arc magnets like the ones used in brushless motors produce a more desirable asymmetrical field. The problem with an arc shape is that it’s nearly the exact opposite of the contour of the body. The large outward facing arc creates a pocket which the body would struggle to fill and two pressure ridges that will inevitably break down skin. Sphere shaped magnets are an interesting shape in that they produce a symmetrical field but have particularly focused flux near each pole. Similar issues exist however in terms of using a sphere for an implant. A sphere under the skin produces a very undesirable pressure point.

A particularly promising shape is that of a parabaloid. This shape is commonly used to project or collect different forms of electromagnetic energy. For example, a satellite dish collects signals from a broad area and reflects it upon a small receiver positioned directly at the dish’s focal point. A paraboloid shaped magnet also projects its field towards a focus but has similar disadvantages as an arc magnet. Paraboloid magnets aren’t currently being produced and NdFeB magnets are notorious brittle and thus hard to shape. There is a lot of room for improvement in terms of shape being used and this is something a number of grinders have been working on.

During the 2016 Grindfest, a ring shaped magnet was removed from Grinder Ben Beezy. This unit had been implanted for as little as four months and yet significant tissue growth occured through the hole in the center of the magnet. This shows that a ring shaped magnet or any implant for that matter might be useful if a person wants the unit to be fixtured into a set position.

Size


Magnet size is of course another factor warranting considerable attention. It may be obvious that a larger magnet is more powerful but the increase in magnetic field strength isn’t necessarily proportional to the increase in volume.  Regarding permanent magnets, size is important but must be analyzed in context of its shape. I’ll begin with the claim that the optimal size of disc magnet for implant is 3mm X 1mm and I’ll explain why along the way. We’ll begin by comparing the effects of small size changes away from our 3mm X 1mm disc.

Thickness Diameter Gauss Pull Force Volume in Cubic mm
1mm 3.00mm 128 0.3 7.07
1.25mm 3.00mm 148 0.39 8.84
1.50mm 3.00mm 188 0.48 10.61
1.75mm 3.00mm 218 0.54 12.37
>2.00mm 3.00mm 269 0.6 14.14
1.00mm 3.25mm 137 0.34 8.3
1.00mm 3.50mm 155 0.38 9.62
1.00mm 3.75mm 183 0.41 11.05
1.00mm 4.00mm 193 0.45 12.57

As diameter is increased, the volume increases linearly.

The results of the graphs above may initially seem skewed. As one can see, doubling the thickness of the magnet doubles the volume and the pull strength. Increasing the diameter though doesn’t follow the same trend. By extending the diameter out to 4mm, we have a 78.8% increase in volume but only a 50 percent increase in pull strength. An increase of diameter provides diminishing returns. Furthermore, Neodymium Iron Boron is notoriously brittle. Some strength is provided by the coating chosen but anything larger than a 1mm thin broad disc comes with a risk of shattering.

Increasing the thickness of the magnet though does show a proportional increase in field strength. I considered increasing the dimensions to 3mm X 2mm such as provided by the leading magnet implant distributor. The reason I began with a preference for a 1mm depth has to do with how well it fits under the skin without acting as a pressure point but I’d be willing to give these advantages up for a magnet with twice the performance. Unfortunately, the 3mm X 2mm magnet doesn’t live up to the “twice as good” I’d hoped for. In terms of pull strength and Gauss at the surface of the magnet, the 2mm is twice as strong; a promising start. But analyzing field strength at various distances showed little substantial gain. Both magnets have a field strength below what’s palpable at a point between 0.7 and 0.8 inches. The 1mm model seems to dip below utility at 0.76 inches and the 2mm lasts to around 0.79. The increase in useful field is negligible. Some might argue that the increase of maximum lifting power of 0.6 would make the increase in size worthwhile despite having nearly no gain in total field size. The reality is actually the opposite.

Studies performed in the early eighties demonstrated that as little as 35mmhg can cause pressure ulcers over a time frame of 8 hours. This level of pressure prevents capillary refill. A pressure of 70mmhg over 2 hour can cause pathological changes to canine skin as this exceeds the pressure needed to occlude veins. A pressure of 500mmhg can cause pressure sores and muscle damage in pigs within 1 hour through a combination of arterial occlusion and some local tissue damage. It makes sense to analyze how much pressure can be generated by our 3mm X 1mm magnet. A 3mm disc has an area of 0.01 in2. which is exposed to a maximum 0.3lbs of pressure in the form of a metal object being attracted to the magnet unit. This equates to 30lbs of pressure per square inch or 1551 mmhg. These numbers are useful to help us approximate how much pressure our skin can withstand without damage but no solid conclusion can be drawn. Based on these numbers, I suppose that a 3mm X 1mm Neodymium Iron Boron could safely carry its maximum load for as long as twenty minutes without worry of skin damage occurring. If a person places another magnet over the implant, this safe time would likely be less than ten minutes. Doubling the strength of the magnet by increasing the thickness to 2mm doesn’t increase the functionality of the implant, it limits it as it provides no increase in range, no increase in ability to sense the electromagnetic spectrum, and it decreases the allowable contact time with a ferrous object.

For those of you who already have a 2mm X 3mm disc magnet implanted, your in luck. We’ve performed testing on the leading suppliers 2mm X 3mm silicone coated magnet and found a Gauss rating of 1850 from the surface of silicone and once cut open to expose the surface of the magnet is rated at 3500 Gauss , far lower than the calculated 5837 that an N52 of that size should exhibit. An implant must be assessed according to it’s whole size. Thus, a rating of 1850 Gauss at surface is closer to the field one would expect from an Alnico magnet rather than Neodymium Iron Boron. On the other hand, I’ve known grinders who state they can pick up objects weighing more than would be expected from the units we tested. We very well might have been sent a number of flawed units. If this is the case, it’s advisable for those with these implants to be cautious as to how long they allow contact in order to prevent injury.

Searching online will demonstrate the plethora of different magnets. There are so many shapes, materials, and features available. The vast majority are completely unsuitable though in that while getting a Neodymium magnet is easy; getting a magnet appropriately coated in a biologically inert material is far more difficult and effective bio-proofing should be your number one consideration. There’s more to this process than slicing open a hole and dropping in a specimen of foreign rare-earth metal.

Biocompatible Coatings


There are scores of biocompatible materials. We have hips of titanium with hydroxylapatite surfaces adhering to bone. Nylon catheters feed us through perforations in the abdomen while Latex tubes in the urethra collect waste. Silicone wraps sexual organs when having intercourse or collects menstrual fluid when not. Teflon coats both pacemakers and artificial heart valves serving plumbers and electricians alike. Despite the variety, finding a coating appropriate for our application is rather difficult as Neodymium Iron Boron has one major downside: its very low Curie temperature.The Curie temperature is that point where the magnetic moments of a substance are able to rotate and move freely. If an implant is heated above 100C it loses a considerable amount of strength. Further heating to 310C and the unit will cease to be magnetic. It will not regain strength upon cooling. It can be remagnetized if one locates a facility able and willing to do this. The units must be arranged in the same direction they faced during sintering for the remagnetization to provide similar performance. If heated much higher than the Curie point, the units can be rendered both irreversibly and irrecoverably damaged. The alloy itself has changed and no amount of external field can cause remagnetization.

 

Teflon

Teflon MagnetPTFE, more commonly known by the trade name Teflon is one option for biocoating. It was one of the earliest materials identified as safe for implantation. It’s still used extensively in grafts to repair blood vessels. One can even buy non-stick cookware repair kits and while using it to coat an implant would be ill advised, the directions on these sprays reveals the shortcoming of all such products: Bake at 500°F. Teflon application generally requires temperatures that will destroy a magnet. Variants of PTFE have been developed that can be applied at much lower temperatures but there is another reason Teflon is inappropriate. It has a Young’s Modulus of 0.5 GPa and a yield strength of 23 MPa. This means it’s slightly more difficult to tear than an equally sized sheet of aluminum foil. If applied thickly enough, PTFE is an excellent biocompatible coating. I’ve never seen someone tear a sheet of 3mm thick aluminum with their hands; however, magnetic field strength diminishes rapidly with distance which necessitates a very thin coating. Teflon is a great option for many implants but for magnets it’s less than optimal because it’s thickness decreases the overall performance of the magnet.

Silicone

HaworthA very thin coating on  magnetic implants is  important. Adding even a millimeter of coating can cut the strength and palpable range of an implant in half. This requirement for thinness is why silicone is a poor choice as a coating. Silicone has a Young’s Modulus between 0.001 and 0.05 GPa and a yield strength of 2.4 MPa. Because it’s such a weak material a coating less than 1mm would be worthless. Silicone tears easily and responds poorly to fatigue. Breast implants made with this material are notorious for rupturing with even the newest models exhibiting a failure rate as high as 33 percent. Keep in mind that is not the silicone gel within that is the issue but rather the silicone bag in which it rests. Earlier models on average began leaking by year ten and ruptured by year 13. Discontentment with silicone is one of the divides between the grinder world and the body modification world. Body modification artists pioneered magnet implants. As such, they designed the early implants using a material that they’re familiar with. Silicone can be shaped into nearly any shape of interest and so has been used for decades as when implanted beneath the skin it results in totally aesthetic changes of bodily form. Solid silicone serves this purpose well but leaves a lot to be desired in terms of sequestering a moderately toxic metal. Silicone allows the passage of fluid and while it will slow the degradation of NdFeB, it doesn’t stop it. Silicone will also swell as it takes in fluid so a unit using silicone actually ends up being larger after being implanted for a period representing a further decrease in performance. To prevent breakdown and rejection there needs to be another coating beneath the silicone. This is most often a coating of gold. Although having two types of coatings seems like a good fail safe, silicone leaves too much to be desired in terms of a coating for magnets. Like Teflon, Silicone is an excellent material for many types of implants but isn’t a good option for magnet implants.

Parylene C

Parylene MagnetGrinders have approached the design of implants without a bias towards any particular substance and the most prevalent coating found in grinders implants is a substance called Parylene C. This brings us to V&P Scientific. V&P is a company which produces coated stir rods for chemistry application. They are significantly more capable than the preceding Teflon coated models as they use neodymium rather than Alnico or Samarium Cobalt. They can do this because they use Parylene which is applied via vapor deposition in a vacuum chamber. High temperatures aren’t required so the temperature limit is circumvented. Its Young’s modulus is 2.8 GPa and it has a yield strength of 55.2 MPa, significantly better than Teflon or Silicone. For a period of time, the most commonly chosen magnet for implant by grinders was the V&P scientific VP782N-3 magnet. This model was only available in sets of 100 at a price upwards of $250. This did wonders in terms of drawing the grinders together as a community as a number of group buys were made. This was an important first collaboration; however these magnets are not optimal as implants. Parylene is a biologically stable corrosion resistant coating which can be applied in layers as thin as a couple of microns. Therein lies the shortcoming. Though these coatings are as thin as might be desired they are also somewhat prone to failure particularly at edges. While perfect for many applications, they also provide little resistance to mechanical stresses such as those we place on our fingertips daily. Thicker coatings aren’t an option either. In pursuit of a better magnet, I ordered a run of multiple deposition Parylene coated magnets against the advisement of the coating companies and as advised it resulted in a coating which easily peels away from the magnet surface. Were the coating to chip or peel after implantation rejection is inevitable. Despite this, there have been many successful implants using Parylene. Parylene is a far better option than silicone or teflon but there remains the potential for failure due to mechanical stress.

Biocompatible Resins

ResinA number of grinders have found methods to overcome the limitations of Parylene using durable biocompatible resin coatings. I’ve tested a number of these options. There are excellent glass ionomer compounds used in dentistry that provide a coating similar to glass. There are also a number of biocompatible resins available from Atom Adhesives which have been successfully implanted without rejection or effect for more than six months. Some early Grinders trialed far less desirable materials such as Sugru and hot glue but these substances break down far too easily for a good coating. Although there are strong and biocompatible resins available, they are less than optimal as magnet coatings for a few reasons. These resins are somewhat difficult to work with. They require sanding and polishing in order to achieve a thing coating and if one fails to achieve a glass like smoothness the surface promotes fouling. These resins have great potential for the grinding community in that we can easily coat prototypes for testing; however, there are better options for magnets.

Titanium Nitride

m31-adjustedThere is another material which holds promise. Titanium Nitride in theory provides the best of all worlds. TiN has a Young’s modulus between 350 and 600 GPa and a yield strength of around 400 MPa. This is two orders of magnitude greater than the best material discussed previously. Where Parylene failed because of brittleness, a TiN coated magnet requires a hammer to break integrity. TiN also exhibits significantly lower bioreactivity than any of the other materials that have been used. It’s preferred in orthopedic implants related to it non-reactivity in the body as well as for the coating of machine tools such as drill bits due to its incredible resistance to wear. TiN can be applied via a few different methods. The most common are physical vapor deposition and chemical vapor deposition. Both of these methods require very high temperatures far outside the range allowable for Neodymium Iron Boron. However, if one searches long enough and hard enough there are alternative methods available which can provide a TiN coating at an adequately low temperature. Unfortunately, applying TiN to magnets has proven to be hit or miss. The magnets being used are so small that they are difficult to fixture in a way that allows for full coverage. Furthermore, PVD and CVD application methods can result in pinholing. Because the metal is essentially being sprayed on in a molecular state occasionally very very small gaps remain leaving a passage by which water can degrade the underlying material. This isn’t common but it’s nearly impossible to detect because the holes are far smaller than could be observed under a microscope. Even tests devised to show gaps in a TiN coating aren’t 100% dependable as surface tension can prevent fluids from moving into the gap for long periods. The result is that if you are one of the unlucky few that get a bad magnet you’ll have no way of knowing. The magnet will perform perfectly until the day it doesn’t and it begins to reject. This isn’t super common but it was enough of a concern to pull the M31 model off the market. Since then, over 8 different TiN coating companies have been trialed in the hopes of devising a method by which to use this otherwise perfect material. As of the writing of this page, a solution consisting solely of TiN hasn’t been devised.

The Blood Diamond

img2During Grindfest II, a number of participants received a type of magnet implant with a coating consisting of an inner layer of titanium and an outer layer of diamond that we’ve developed at Auglim. Someone came up with the name Blood Diamond and it stuck. Prior to these trials, the blood diamond magnet successfully passed a slue of tests and I’ve had one implanted in my hand for three months without adverse effect. Depending on the results of these trial implantation, the Blood Diamond may become available for purchase through a vendor. The coating on the blood diamond is comparable in thickness to TiN coated models. Diamond is downright unreactive in the body and the dual coating process eliminates any potential for pinholing. The shape of the magnets themselves have also been altered to decrease the edge. Although not yet available, the blood diamond represents the best implant magnet so far produced.