A significant amount of research has been done on the manufacture of both positive and negative micro-molds with lasers, where their feature sizes are submillimeter with a relatively small amount of material to be removed. In general, these molds are metal-based, requiring short pulse lasers—and competing technologies such as wire electrical discharge machining (wire EDM) can give lasers a run for their money.
However, an area where lasers are the only choice is the production of negative polymer molds for use in the medical device and pharmaceutical industries. In this application, the laser is used to machine the polymer material from which the mold is made. Polymer machining is based on breaking interatomic bonds, so it is ideally suited for nanosecond and picosecond lasers.
All that a polymer mold must be able to do is accept the fill material and not distort. If the fill material has a lower melting temperature than the mold, then there is no issue with using polymer molds. In certain medial applications, this is exactly the case and a large amount of work is carried out using polymer molds.
One specific application is manufacturing microneedle patches used for drug delivery. A microneedle patch consists of an array of small needles, where each needle is conical in shape and measures approximately 600μm high. Because of the size and shape of the needles, the body does not feel any pain when these needles penetrate the skin. Microneedle patches are therefore an alternative to the inch-long hypodermic needle commonly found and feared in most doctor's surgeries.
Microneedle patches are similar to small bandages and each needle is formed from a chemical that will dissolve when in contact with the body's natural heat. The needles are made from sugar, which is itself doped with a medical active ingredient such as a painkiller. On application, the microneedle array pushes through the skin, creating lots of tiny holes—and the sugar loaded with painkiller then melts and is absorbed into the body.
|An example of a microneedle patch.|
The microneedle mold must have three characteristics:
1. To reduce the force necessary for insertion, the microneedles need to be very sharp and the tip radius must be less than 2μm;
2. The mold must be flexible so that it can be peeled from the microneedle array without causing damage; and
3. The mold manufacturing process must have a high speed of production at a low cost.
As the minimum feature of the laser beam is approximated by the wavelength, the ability to form a sharp tip in the mold is made easier using laser light. Tests have found that microneedles formed from a mold made by laser micromachining at Blueacre Technology (Dundalk, Louth, Ireland) are sharper and require less force than other manufacturing techniques.
|The laser-machined mold produces sharp, conical needles.|
In general, molds are made from polymers such as polydimethylsiloxane (PDMS) that can be easily machined, but have a relatively high melting temperature. This polymer is also relatively cheap, lowering production costs.
Blueacre Technology has investigated a number of methods to speed up production of such molds, and galvanometer-based processing provides the fastest and most economical method. Each negative needle can be machined with multiple pulses and can produce needles ranging in height from 50 to 1500μm. The company currently works with research groups around the globe, and has recently developed a process that allows an individual needle cavity to be produced by a single laser pulse, speeding up production rates and improving industrial scalability.
Although microneedles have been around for almost two decades, they have yet to make it into the mainstream drug delivery market. Questions remain over drug uptake within the body, and pharmaceutical studies are ongoing to obtain market approval. However, when they do become available, lasers are poised to offer a cost-effective and scalable manufacturing process.
For more information, please visit www.blueacretechnology.com.