Laser machining has long been used in medical device manufacturing to create small sub-components with extremely fine features. More advanced manufacturers are now using their experience with CNC laser systems to refine techniques and processes for laser surface texturing. Laser texturing provides significant improvements over micro-abrasive blasting, but requires specific equipment, experience, and process development expertise to maximize its effectiveness.
Many medical devices consist of two or more subcomponents fabricated from different materials, often a metal and a polymer, which are bonded using UVcure adhesives. Because bonding strength is affected by surface roughness, machined metal subcomponents are mechanically treated to produce a roughened surface and improve adhesion. Micro-abrasive blasting is the most common technique used.
The Drawbacks of Micro-Abrasive Blasting
In micro-abrasive blasting, compressed air is uniformly mixed with a fine grit abrasive media such as aluminum oxide or crushed glass. The mixture is forced at high velocity through a small nozzle, and the fine stream of abrasive particles is used to etch the surface of a component.
Most micro-abrasive blasting is performed manually by moving a hand-held nozzle across the surface of the component. Every part is textured differently, and there is a high variability in bond strength as a result. It is possible to add CNC control to the micro-abrasive blasting nozzles. However, this increases equipment cost by a factor of 15 and does not solve the issue of variability in the supply pressure and flow rate.
The variability of the textured surface, and the resulting variability in bond strength, means that micro-abrasive blasting contributes to higher costs and longer cycles for testing and validation, as a large number of parts must be run to obtain a confidence level.
Micro-abrasive blasting also leaves a residue of abrasive media in the textured surface of the component, even after cleaning. Medical device manufacturers will use only biocompatible abrasive media such as alumina or crushed glass. However, some concerns have been raised about the use of alumina in brain tissue, and even biocompatible particles become potentially damaging irritants when loose in the body and should be avoided.
In medical manufacturing, the abrasive media cannot be used more than once, as particles from the machined piece mix with the spent blast media and will alter its performance. The inability to recycle blast media makes micro-abrasive blasting more expensive for medical device manufacturing than for other machining applications.
Laser Surface Texturing Solves Problems
Using CNC-controlled lasers to texture a surface for bonding eliminates the drawbacks of abrasive blasting. It has been demonstrated to improve bond strength and reduce variability in devices for vascular, cardiac, and other applications.
Industrial lasers have been used in component manufacturing for more than 40 years, for creating finely detailed parts at the very low sub-component level. Most contract manufacturers specializing in medical device manufacturing have some laser machining capabilities, and use them to produce simple cuts and machined parts.
It is more rare to find a contract manufacturer with more advanced laser machining capabilities including the ability to laser-cut small tubes, maintain tight tolerances, and make multi-axis cuts. Laser texturing, which requires surface cuts that do not go all the way through the material, is one of these more advanced operations as the light can’t simply be stopped at a specified depth.
These operations require a deeper understanding of laser behavior and its interactions with different materials, more in-depth experience in programming the tools, and expertise in creating fixtures to achieve the desired result.
Outsourcing to a component manufacturer that has specific expertise in laser texturing can benefit both large and small medical device OEMs. Smaller medical device designers gain access to advanced manufacturing techniques without the need to invest in CNC laser equipment suitable for surface texturing, which runs approximately $250K plus substantial costs for programming and training. Larger OEMs may take advantage of a contract manufacturer’s more specialized experience or, if they have this expertise inhouse, they may benefit from having an equally sophisticated manufacturing partner to overcome capacity limitations or speed production.
Optimizing Techniques, Controls, and Processes
Because process is key, OEMs should look for a partner with an experienced engineering services group that can design experiments and optimize the laser texturing process for the OEM’s device. For example, a designer may specify the component materials, the adhesive to be used, and the required bond strength. The outsourcing partner’s engineering services group can devise experiments, develop test plans, and evaluate multiple texturing patterns to deliver optimal results. Alternatively, an OEM may want to evaluate different adhesives to balance performance and cost. The partner can evaluate variations in both laser pattern and adhesive type, and create a matrix of results.
In one such process development effort, our engineering services team worked with a major cardiac device manufacturer to increase the strength and reduce the variability of bonds between a polymer cannula and a metallic subcomponent. The goal was to double the lifetime for this acute device, to create a competitive advantage, and improve patient outcomes through extended healing time. Specific objectives were to:
- Increase bond strength to meet higher requirements for longterm fatigue life.
- Increase mean bond strength to diminish the impact of variability.
- Develop advanced controls to improve repeatability.
- Reduce operator influence on adhesion strength.
Developing a CNC-controlled laser texturing process for this device was integral to meeting all of the objectives. A key outcome was the development of a laser pattern optimized for bond strength, and development of process-capable techniques.
Developing the Optimized Surface Texture for Bonding
The test methodology involved producing a variety of textured surfaces using CNC-controlled laser etching, benchmarking laser texturing versus micro-abrasive blasting, and optimizing surface textures for increased reliability. The device to be tested consisted of a stainless steel subcomponent, textured and bonded to a silicone polymer cannula.
The laser textured surfaces include a pattern of deep lasercut holes with regular rows and hole spacing, and a similar pattern but with surface pits rather than through-holes. (See Figures 1 a and b)
Laser texturing pattern 1, with through-holes, produced bonds with statistically insignificant increase in normalized pull strength over bonds with micro-abrasive blasted materials. Laser texturing pattern 2, with surface pits instead of holes, performed slightly better. Patterns with surface pits displayed microscopic spur-like features around the edge of the pits, so we created additional patterns to increase this effect by varying the spacing between pits, the spacing between rows of pits, and the size and depths of the pits.
This achieved an optimal pattern that not only produced a roughened surface but also a micro-barb fitting effect. This pattern resulted in bonds with normalized pull strength three times greater than bonds with a micro-abrasive blasted surface (See Figure 2).
While the pattern of surface pits requires significantly more experience and skill to achieve, it delivers significant benefit.
Laser texturing processes can be developed and optimized for specific materials and adhesives. This is applicable to other areas, including bonding electrodes or marker bands to a silicone tubes and bonding handles onto orthopedic tools. It offers results superior to micro-abrasion in a number of important areas:
- Bond strength – Normalized pull strength is up to 3X higher.
- Process capability – Patterns created with CNC-controlled lasers are highly repeatable for a given material, reducing variability in bond strength and removing operator influence from the texturing process.
- Cost-effectiveness – A repeatable process with controls, laser texturing reduces the time and cost required for device validation.
While the capital equipment investment in CNC-controlled lasers is significant, manufacturers with modern laser machining equipment can, with training and experience, extend its use to laser texturing as well as manufacturing.
When evaluating an outsourcing partner’s laser texturing capabilities, an OEM should consider the length of the manufacturer’s experience with laser machining in general, and with laser texturing in particular. They should evaluate what controls are in place on inputs, for example, power and gas flow, and whether the effect of inputs on the outputs has been demonstrated. Finally, they should consider the scope of their partner’s engineering services for process development with customers. An engineering services group that will allow the OEM to participate in and observe optimization studies, own the outcome of the optimization, and take the process with them for manufacturing at their own facility or at another provider of their choice will provide the greatest benefit to the OEM’s business.
This article was written by Andrew Fisk, PhD, Director of Technology and Innovation, and Julie Fetch, Manufacturing Engineer, Pulse Technologies, Inc., Quakertown, PA. For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/49746-163.