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Rediscovering the Benefits of Microabrasive Blasting Technology

Mon, 10/05/2009 - 6:19am
Colin Weightman
Microabrasive blasting offers medical device manufacturers an effective way to treat the surface of a variety of materials for a wide array of applications. However, many may not be aware of how extensive its benefits are. This article reviews the various applications for which it is ideally suited, while also highlighting some areas for which this process has more recently been indicated.


Colin Weightman is the director of technology at Comco Inc. He is responsible for developing Microblasting applications in the medical industry. Weightman can be reached at 818-841-5500 or colinw@comcoinc.com.

Microabrasive blasting has been used in various medical device processing and finishing areas for years. Recent advances in equipment engineering have resulted in microblasters that are able to flow smaller abrasives at more consistent rates. The growth of automated systems that can accurately position multiple abrasive nozzles, manipulate parts under nozzles, and monitor process variables (e.g., media flow rates) also offer expanded options for device processing.

The unique properties of high-energy abrasive particles make microblasting ideal for medical applications. They cut without heat or vibration and can selectively remove blemishes or coatings without damaging underlying layers or surfaces.

Growth in Standard Applications
To reliably bond multiple strain gauges to the sensing surface of load cells, many companies are conformally texturing the bonding area using microblasting.
The ability of microabrasive blasting to deburr and texture surfaces without causing dimensional changes to the part make it ideal for finishing critical medical devices from cannulae to medial molds. Propelling a very fine, dry-abrasive powder mixed with clean, compressed air through nozzles with openings as small as 0.018" to 0.06", the pinpointed abrasive blast can be as tight or as broad as the application requires.

Deburring Needles–When the point is ground into the end of a hypodermic tube to create the cannulae (needle), a fine burr is created. Using a small nozzle, microabrasive blasting easily deburrs both OD and ID without affecting tip sharpness. Glass bead media is often used because the nature of a spherical bead will both deburr the needle and round the heel region. Using custom automation, needle arrays are presented to the nozzles. Some manufacturers have combined the grinding and blasting clean-up processing into single, customized automated systems incorporating subsequent steps.

Texturing for Stronger Bonding–When a device is to be bonded to a delivery system or hub, bond strength is dramatically improved by texturing the finish on the blunt end of a tube, catheter, guidewire, or needle. Coatings are often applied to allow easier insertion into subdural areas. It's fairly common to see guidewires coated with a Teflon or PTFE material. This material must be removed to expose the bare wire before bonding to other components.

Catheters are commonly inserted with guidewires. Removing a coating from a guidewire that is only 0.005" to 0.02" in diameter is a very delicate task. Guidewires differ from tubes in that they are usually much finer and can have a variety of configurations. They can be made from solid wire with a fair degree of rigidity, yet still be flexible. Multiple nozzle microblasting systems are typically used to abrade selected areas along the length of the coated wire. Sodium bicarbonate, glass beads, or crushed glass are commonly used.

Microblasting can precisely target the threaded portion of a dental implant where the surface needs to be roughened, without affecting the surface finish of the apex. (Before microblasting above, after below)

An example of some of the small sizes of medical devices created by the molding process.
Coating Removal–Conformal coating removal has always been a prime application in all electronics, including medical. Spot-removal is done for various reasons such as repair or to expose a test point. Microabrasive blasting can pinpoint areas within extremely small, high density medical electronic assemblies and remove coatings without damaging surrounding devices, traces, or leads.

Medical Sensors–The latest medical monitoring, ultrasonic, physical therapy, and resuscitation devices require the integration of custom designed load cells to read pressures, strain, and temperature, and transmit readings to an integrated control system. These products are typically made out of precision machined metals with strain gauges bonded to key surfaces. To reliably bond multiple strain gauges, many companies are conformally texturing the bonding area using microblasting.

Medical Molds–Medical device molds and micro-molds are critical to a rising family of medical electronic devices. Molds are typically tool steel, created using electrical-discharge machining or laser process to cut mold cavities. Polymer implantable devices with delicate enclosures for electronics, such as miniature cochlear implants are created in molds. Micromolds can be exceptionally small with very delicate internal geometries. Retaining exact mold geometry is essential and yet both technologies leave residue that must be removed. Microabrasive blasting easily removes this residue without damaging the cavity surface or altering cavity dimensions.

Applications on the Leading Edge
NiTiNOL–Processing NiTiNOL with microblasting began in the early 1990s for the peripheral stent market. With its memory shaping feature, NiTiNOL offers the most flexibility for stent size and shape development. The process of laser-cutting the design into a tube leaves an oxide layer on the surface of the stent and remelt on the sides of the struts.

Microblasting removes the oxide layer, the remelt, and the heat effected zones on stents and implants prior to electropolishing. The process is often controlled by measuring the amount of weight in thousandths of a gram for removal. The action of microabrasive blasting is also effective at peening the surface and reducing stress concentrations so that the devices are less likely to crack in the expansion process.

The complexity of NiTiNOL devices has increased significantly. What started out as simple tubes are now longer, smaller, conic, and asymmetrical. These changes have created significant challenges for microabrasive blasting. Additionally, device manufacturers looking for techniques to reduce operator-induced variability have spurred advancements in automation. Microblasting systems using multiple nozzles to automatically process stents held on a mandrill are common today.

Pacemakers–There are several aspects of pacemaker production that benefit from microabrasive blasting. Currently, the three main uses are blemish removal on titanium cans, epoxy removal from the header, and coating removal on pacing leads. After pacemaker cans have been drawn or welded, they are blasted with glass or ceramic beads to remove discoloration or blemishes in the surface of the can.

Once the header has been bonded to the can with epoxy, sodium bicarbonate is used to remove excess epoxy. This abrasive quickly cuts through the epoxy without damaging the titanium surface. Microabrasive blasting is focused, enabling the operator to carefully trace the edge of the can.

Finally, the blasting process is used in the lead manufacturing process. Leads are typically encapsulated with silicone to improve biocompatibility. But, this silicone needs to be removed from the platinum coils to allow electrical contact with the heart muscle once the device is implanted. The elastic nature of silicone makes it difficult to remove without damaging the coils. Here, sodium bicarbonate makes an effective cutting abrasive. This needle-like abrasive cuts into the silicone layer, leaving a uniform surface finish.

PEEK–The use of exotic plastics has increased in the manufacture of implants because they can be effectively machined and/or molded. Materials like PEEK are commonly used for spinal implants. However, a common problem when machining PEEK is the creation of very fine feather burrs. Scraping with knives or filing is labor intensive and can also actually lift up new burrs. Using the correct abrasive and psi will gently strip off these burrs without damaging or burning the surface finish and will significantly reduce deburring time. The same process can also be used on molded parts to minimize the appearance of parting lines from the mold cavity.

A microscopic view (corner) laid over a microfluidic assembly shows the repeatability of drilling holes that are only .005" diameter in size using microabrasive blasting. Intricate patterns can be achieved with holes go all the way through the top layer.
Heart Valves–Pyrilytic carbon is widely used to manufacture replacement heart valves. To manufacture the valve, carbon is normally applied around a negative made from graphite. Once the valve has been formed, the graphite needs to be removed without damaging the carbon structure. Sodium bicarbonate is effective at cutting through the graphite without etching into the carbon. Due to the volumes produced, this microabrasive application is typically accomplished by a fully automated system.

Microfluidics–Advances in micro-fabrication of structures has created a new industry within the medical diagnostic industry. Companies are now creating lab-on-chip devices, such as blood testing chips. These devices introduce blood and a reagent into a chip, forcing them to mix together. The chips are fabricated by etching the mixing pattern on two pieces of glass. The glass pieces are then fused together. Holes that introduce the blood and reagents are drilled using a microabrasive process. The location of these ports can be accurately controlled and their shape makes filling the chip easier.

Implants–Whether bone screws used to strengthen a damaged bone, prosthetic hip and ball-joint replacements, or dental implants, these devices all face the same environment within the body and need help to reduce rejection and enhance surrounding tissue growth. When implant devices are manufactured, their surfaces are often too smooth for proper tissue adhesion. For this reason, they are processed with abrasive media to roughen the surface, producing a finish that is conducive to tissue growth.

Over the past 5 years, the dental implant market has grown significantly along with advancements in restorative dentistry. In order to optimize osseointegration, implant manufacturers require a specific surface finish with a tight tolerance. The need to provide a controlled surface finish to a specific region of a part is what makes microblasting an ideal process. Aluminum oxide is typically used, while automated systems enable sharp delineation without masking.

Implant manufacturers are also increasingly working with exotic abrasives which have improved biocompatibility characteristics. These abrasives are very expensive and the accuracy and highly adjustable abrasive flow make these processes more cost-effective.

Echogenics–Echogenicity is the ability to create an echo (i.e. return a signal in ultrasound examinations allowing medical professionals to better "see" a device when it is inside the human body). Because modern ultrasounds are unable to easily pick up shiny surfaces, matting the surface of the needle or probe allows the doctor to follow it on-screen while it travels through the body. The surface altering ability of microabrasive blasting creates a finish that is very visible in the body using ultrasound equipment.

Conclusion
The microabrasive process is often difficult to quantify. Unlike some other production tools, there is a degree of art behind understanding abrasive technology. The process is forgiving, but does need careful control to ensure consistent results on a production scale. Therefore, the medical device and instrumentation industry relies on microabrasive equipment manufacturers to work with them to solve problems by demonstrating the capabilities of the process.

The medical manufacturing market is showing a significant opportunity for growth outside of traditional uses such as cutting, cleaning, and texturing. The latest research is taking the capability of the microabrasive process to effectively meter fine materials into an air stream to even higher levels. New tests show these systems can be used to meter fine powders or to apply coatings onto the surfaces of medical devices. These advances will help solve the next generation of challenges for microabrasive blasting within the medical industry.

Online
For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or Comco Inc. at www.comcoinc.com.
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