Surface Modification

I am not an expert, but, in this case, I'd like to find a material that is compatible to the media where it's to be installed rather than find a coating over a substrate. Can you guarantee that DLC coating is flexible enough to absorb a deformation like silicone? If anything different from a polymer is used, it's more likely to crack or become loose that bend enough. Just an idea.

Dear bhescobar,

That's the problem, I am not an expert as well and at this stage I cannot guarantee that the DLC is flexible enough. However, it can be achieved with some mixture of silicone (DLC-Si). Do you know which approach to follow to design an optimal thickness for my application? DLC is highly compatible for my application and it has very good hydrophobic characteristics, and this is an outlayer coating to reinforce the safety of the device and delay as much as possible water absorption, by the medical silicone substrate.

Regards,

Leonardo Sarmento

Why coat? if it is to protect a non bio compatible material, why not find a bio compatible material?

You do not adequately specify the devices form, fit, or function, or substrate but 100 degree angle suggests application ofshape memory alloys- with no need to coat.

milo

Dear Milo,

The material I am using is biocompatible, it's a medical silicone grade. However, I would like to delay as much as possible water absorption and reinforce the device's safety, which is a class III implantable medical device on the neurological system. I can tell you that its form is something close to a catheter and it will be placed like a L-shape connector. Shape memory alloys have no place in this application. The device is in total 114 mm length by 3 mm outter diameter. Would you have any suggestion of which approach I should follow to design the thickness of the DLC coating?

Greetings,

Leosarmento

Ok. In that case, why not laminate or sandwich your primary material with an additional layer of silicone for redundancy of barrier. I agree with the other posters that putting a brittle coating on flexible substrate is not an approach likely to pass muster with the FDA.

Milo

Hum... I think I finally understood your question...

I still would like to look for a material that would have fully the characteristics you want, to avoid failures due to surface cracking, disbonding, etc... Take surface protection of metals as an example. In harsh environments, or when wear is predicted, you should always take a look if it is not better to choose a base material resistant to the media than look for a coating.

But, if you have to look for a coating, or better, you're looking for a methodology to define an optimum coating thickness (or to check if any coating thickness is actually applicable), I'd do the following calculation:

1. Take the geometry and the amount of bend you need to allow (you already did it). Include some safety margin, just in case...

2. Consider the catheter like a bending beam. Use beam equations to approximate the amount of surface deformation you'll expect in the operation.

3. Use this deformation value and apply to the DLC material, in a typical thickness value you'd like to use. This is a "back of the envelope" calculation only, so, don't be ashamed to state some values and see what happens.

4. If there's a valid solution (I mean, if the numbers does not indicate that a thickness of 1E-48m is required... something a little hard to achieve and control...) then you could model the cateter in a more near to reality environment using FEA, for example. Take the results and repeat calculation. The back of the envelope calculation will assure you modeled it right. Then, You could use some composite material library in the FEA software to model the surface coating itself, and check everything again. I would expect a really thin coating, because we're talking about two materials that has elastic modulus really really different...

I assumed that you had a somewhat amorphous implant (like breast implant, I mean), but a tube geometry with some limits for deformation do simplify the problem somewhat.

Good luck, keep us informed. It sounds like a really challenging and fun project.

Perhaps I don't understand your question properly. You have a very flexible (?) silicone substrate and you wish to apply a very rigid diamond-like coating onto the silicone substrate and still have the ability to make a 100-degree curve? I would think that you'd need to sputter the diamond-like carbon film onto the silicone, and you would need to make the bends before applying the DLC film. What are you trying to achieve? Does the film require arterial flexability? Chemical/environmental resistance? A non-stick surface? What about some other biodurable films with a fluoro-polymer as a co-pol or a surface film? There are a number of biodurable polymers available for applications such as CardioPass(tm) synthetic arteries or specialty polyurethanes for drug-eluting stents. There are also ways to make stainless or platinum flexible films for medical applications.

Hi Cardio07,

Yes, I do have a very flexible substrate and the coating (sorry for that) is Diamond like amorphous carbon, a-C:H, which can be modificated to achieve the required properties. It would be nice to achieve arterial flexibility (but it is not necessary), it is indeed required to be chemical/environmental resistant and, non-stick. For my application I have to take into account the temperature of deposition; no more than 150 °C. I have chosen this coating by its excellent mechanical, thermal and electrical properties, in addition to the hydrophobic properties. At the moment I just want to design an optimal material thickness to achieve such flexibility and to keep certain properties of the material and later compare with other different coating materials (polymers and polyurethanes) to decide which one is suitable for my application. Thanks for the comments. Greetings,

leosarmento

I assume you know that a main difference between diamond and graphite is that diamond has sp3 bonding in 3D, while graphite has sp2 bonding in 2D layers. Elemental silicon has the same sp3 hybridized structure as diamond. One research direction would be to try and marry the sp2 - 2D structure of graphite with the sp3-3D structure of silicon. That just might give you the properties you need, plus perhaps a Nobel Prize. Good luck! I'd be interested in your progress and the problems you encounter in your development. There is always a ton of expertise and help here at CR4!