Ultrafast lasers in medical device manufacturing

ERIC MOTTAY

TAKING ON AN INCREASING ROLE

The first generation of ultrafast lasers, using titanium doped sapphire as the active laser material, was commercially introduced in the early 1990s. In 1999, the Nobel Prize for Chemistry was awarded to Prof. Ahmed Zewail for his work on the analysis of chemical reactions at ultrashort time scales. Moving from a new technology to a Nobel Prize on an application of this technology in less than a decade demonstrates the revolution ultrafast lasers brought to the scientific community.

During this decade, the potential of ultrafast lasers for new industrial or medical applications was identified, although the lasers available at the time could not answer industries’ requirements in terms of performance, cost, size and reliability.

A new generation of diode-pumped ultrafast lasers, using Ytterbium doped laser materials and telecom-class semiconductors, was brought into the market around the year 2000. These compact, high power, high reliability, and cost effective ultrafast laser sources became the answer to developing industrial applications in markets that expanded rapidly. As a result, installations doubled annually in the decade.

Today, industrial ultrafast lasers are commercially available with a wide range of pulse durations, from femtosecond to picosecond, with average powers in the tens of watts range, and with the ability to operate in demanding industrial and medical environments.

Applications background

Ultrafast lasers concentrate the pulse energy in an extremely short time period, leading to an extremely high power density. The power accessible from compact, desktop ultrafast lasers exceeds that of a nuclear power plant. Because of this high power, laser ablation will occur on virtually any type of material, including materials traditionally difficult to process such as metals, ceramics, and glass. Additionally, because of the extremely short pulse duration, little or no heat dissipation occurs during the interaction process, leading to a virtually a thermal ablation and therefore a very high quality process. Micromachining occurs without melting, cracking, vaporizing, or other detrimental heat dissipation effects.

Ultrafast lasers are now being used in industrial applications where the quality of the process is a key factor, such as:

• Selective ablation of thin films in the semiconductor,
display or photovoltaics industry
• Stress-free internal engraving for anti-counterfeiting applications in the pharmaceutical and luxury
industries
• Refractive eye surgery, including vision correction and cataract surgery
• High quality micromachining applications in the microelectronics industry
• Medical device manufacturing

Medical device manufacturing

Medical devices are high value added products that have stringent requirements in terms of quality and very often require challenging industrial manufacturing processes. For these reasons, ultrafast lasers are gaining significant implementation in medical device manufacturing.

The most well-known application is stent manufacturing. A stent is a prosthesis made either of metal or polymers. Stents are used in percutaneous angioplasty to allow the blood to flow into a closed artery after a stenosis or occlusion has occurred. Laser cutting of stent materials offers both quality and versatility and today is the main manufacturing process for stents and its associated tools.

A typical stent is a small tube with profi les cut by a laser beam such that the resulting tube behaves like a spring in order to prevent an artery from shrinking after surgery. Depending on the models and the manufacturers, diameters vary from 1.2 to 3.5 mm, and the thickness of the stent’s wall from 0.10 to 0.25 mm. Three different kinds of stents can be considered:
• Simple stents made of metals, stainless steel (80%) or Nitinol (shape memory alloy made of Nickel and Titanium, 20%)
• Metal stents plus active substances to prevent the stenosis reappearing. The active substance is added by a “several step elution” to increase durability effects. This elution is made by using little reservoirs or a coating on the stent. These stents represent the main share of the actual stent procedures annually (>75%).
• Bio-absorbable stents, recently being manufactured, are typically made from a PLLA polymer. These also accept the active substance adjunction. When used, the stent slowly degrades and gradually disappears into the blood after the artery has healed, a process that happens over several months or up to a year or two. Recently, bio-absorbable polymer stents have received CE accreditation for European use.

Because bio-absorbable stents are made of polymer, a material that is extremely sensitive to thermal effects, they cannot be machined with suffi cient quality by long pulse lasers, where the ablation process is of a thermal nature, and therefore these stents require an ultrafast laser process for efficient, high quality manufacturing.

Metal stents, on the other hand, are processed today by long pulse lasers (typically μs or ns pulse duration).

Laser cutting, starting from a tube, represents only a part of a stent manufacturing process. Other processes include deburring, mechanical extension and thermal treatment, electro- polishing, disinfection and sterilization, and packaging. Depending on the laser use, wet cutting is performed, using a water flow in the tube during laser cutting. Assist gasses may also improve the overall cut quality. Cut widths of 10 to 20 μm, accuracy of 5 μm and cutting speed of 5 mm/s are typical.

The post-processing steps represent a signifi cant portion of the total manufacturing costs. Since ultrafast lasers beams can cut metal with a higher quality than long pulse lasers, the post-processing stage is signifi cantly reduced.

The total manufacturing cost includes amortization of laser investment, laser operating cost, and post processing costs. Ultrafast lasers typically have higher investment costs than other technology. However, they have low operating costs, and they signifi cantly reduce the post-processing costs. In addition, continuous improvements in laser power and repetition rate mean signifi cant increases in process productivity.

All these factors lead to an increase in the use of ultrafast lasers in stent manufacturing, which will continue over the coming years. More generally, an increasing number of industrial processes in medical device manufacturing benefit from the high quality enabled by ultrafast lasers, including laser cutting for vascular devices (valves, neurostents); laser micro-hole drilling for catheters or needles; and surface micro-texturing for implantable, bio-compatible components.

Emerging applications

Emerging applications will gradually find their way to broader useage. For instance, direct laser printing with ultrafast lasers allow the precise deposition of living cells on a biological substrate with a high precision and low cell mortality – an exciting outlook for tissue engineering or reconstruction.

Ultrafast lasers are also used as microtomes to precisely cut or section tissue or biological samples. These techniques are currently being extended to the nanometer scale, allowing for intracellular nanodissection.

In other fields, ultrafast lasers are becoming a key tool for the manufacturing of biochips with integrated mechanical, optical and micro-fluidic functions. The Femtoprint project (www. femtoprint.eu), sponsored by the European Union, is a multipartners collaborative project aiming to develop a micro- and nano-scale printer for the manufacturing of micro-systems using advanced compact ultrafast lasers.

Conclusion and outlook

Industrial ultrafast lasers enable high quality processing and are ideally suited for applications such as medical device manufacturing. A dynamic scientific and industrial community gives rise to an increasing number of new industrial applications. Further development in ultrafast laser technology will lead to higher average power, higher repetition rates, and smaller sizes, meaning that more and more of these new applications will be economically competitive. Ultrafast lasers will take on an increasing role in future manufacturing processes. ✺

ERIC MOTTAY (emottay@amplitude-systemes.com) is the president and CEO of Amplitude Systemes, Pessac, France.

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