Shannon MicroCoil Engineering Resources

Medical Engineering Resources

Medical Engineers at the preliminary design stage need to evaluate the various parameters and limitations of wire forming and coiling. The key considerations for this process include:

Some general formulas and material descriptions are also provided for your reference. This medical engineering section will be constantly updated and please feel free to email us with relevant content suggestions that you/others may find useful.

Coiling

There are two main methods of forming a wire coil through medical engineering; – By wrapping it over a mandrel and then removing the mandrel – By deflecting the wire against one or more coiling points. The deflection coiling method is the one we use at Shannon MicroCoil. There are few limitations to what can be achieved using this method but one key parameter to bear in mind is the coil index. Coil Index is defined as follows: Index = D/d Where D is the coil outside diameter and d is the wire diameter. Some people prefer to use the Mean Coil Index which utilises the wire centreline and is given as follows: Mean Index = D/d -1 Lower Limit For coil forming, the lower Index limit is 4 (Mean index 3). Although with certain materials it may be possible to go slightly below, in general it’s not possible. So, for example, if you wish to make your coil at an O.D of 0.016” then the maximum wire size you can use is 0.004”. If you require tight tolerance on the OD or ID then we would suggest a 0.0035” wire to allow adequate headroom. Upper Limit  Coil Indices greater than 15 can have their own particular problems in that the stability now becomes the issue. While a 0.010” wire can be taken to an index of 30, a 0.002” will be completely unstable at this index and cannot even be handled without damage. In general, flat wire profiles are more stable at these indices. Droop  Droop is usually specified for guidewire coils and endoscopic coils and the deflection coiling method can control droop over a wide range of values by setting the initial tension of the coil. Initial tension may be thought of as a virtual negative pitch. However, too much initial tension for stiff droops can cause climb out of the coils and while this might not be visible at the production stage, any exposure of the coil to low temperatures or high temperatures can trigger climb out. This may present as intermittent roughness or sometimes a sinusoidal wave along the coil. If really high droops are required then switching to flat wire profiles is the best option. Tolerances A common problem in specification of wire coils and forms is specifying too tight a tolerance or using tolerances more suited to machining operations. Wire is “live” and its spring quality from foot to foot is variable. All forms of wire forming and coiling that use wire in the spring condition compensate for “springback”. While very tight tolerances in the micron range can be achieved it comes at very high cost since the wire specification tightens and the production speed slows and tooling costs go up.

Materials

Stainless Steels  The most commonly used material for medical wire forms and coils is without doubt 304 Stainless Steel and in particular the Vacuum Arc Remelted version 304V which contains few impurities. It is also among the cheapest materials available. Surfaces for small diameter wire are bright and shiny and roughness can be below 5 RMS if the material supplier is drawing the wire through Single Crystal Natural Diamond dies. We can also use 316, 316Ti and 302 if required. CoCr/MP35N  Common in some applications requiring long term implant – coiled stents, tissue screws. The material is moderately difficult to process in low index coils since the surface can tend to gall under high contact stresses on the coiling points. We have developed special tooling for this wire to overcome the problem. In wire forming, these materials cannot withstand a 90 degree bend over a tight radius as they are too brittle. Material cost is quite high. Titanium Ti6Al -4V  Spring grade Titanium is not a common material in coiling and wireforming and its size range is limited. Surface condition of the wire is very important and there must be no imperfections. This material can explosively fracture undergoing coiling if surface imperfections exist. Precious Metals We regularly use PtW and PtIr in coil forming, particularly for marker coils. A good alternative is Palladium. These materials are difficult to coil since their mechanical properties are poor. The addition of Tungsten or Iridium is intended to improve the properties of these metals and impart a degree of hardness to them. Typical sizes are 0.002” to 0.004”. Material is extremely expensive and depending on the geometry of the coil and tolerances, scrap rates can be high. These materials may be thermally shape set for products such as embolization coils. Au Plated Tungsten  This is acceptable at a high index but below an index of 6 the wire can tend to split along its centreline when cold formed. It needs to be hot-formed at low index. The material itself is relatively inexpensive.