Rhyme and resin

They were made famous as gifts brought by the three wise men in the Christmas nativity story. Frankincense and myrrh were two of the gifts carried by the three kings of Orient who traversed afar across field and fountain, moor and mountain, according to the famous hymn. Both items are essentially resins, coming from the sap of trees found in northern Africa and the Middle East. Resins were also used to help mummify the pharaohs in ancient Egypt, providing glue for the linen bandages and a waterproof coating to the wrapped bodies. It was a vital element of the process designed to prepare the dead for their journey into the afterlife.

While resins have had many uses in the history of humankind, in modern times they are adapted to the development of medical devices. These days, resins, or epoxies as they are also known, are often made into polymer plastics or adhesives, and they are classified in a group called biomaterials. Synthetic biomaterials have been used for a wide range of medical and dental applications, with polymeric and ceramic substrates playing an important role since the 1930s. With contemporary technology, the products from resins are now extremely useful in disposable and reusable medical devices.

Multiple uses

Biomaterials are highly flexible and adaptable due to components such as polymers made from resins. They are also non-toxic and biocompatible, so they don’t interact with the human body, causing irritation or harm, and don’t cause physiological or immune system reactions.

This makes biomaterials an ideal substance for the production of medical devices. According to ‘Biomaterials and their Applications’, a 2015 academic work from Hamid Reza Rezaie, a professor of biomaterials at the Iran Institute of Technology, the uses of resin include:

• cardiovascular medical devices, such as stents and grafts
• orthopaedic and dental applications, such as implants and tissueengineered ‘scaffolds’
• ophthalmologic applications, such as contact lenses and retinal prostheses
• bioelectrodes and biosensors
• burn dressings and skin substitutes
• sutures
• drug delivery systems, such as excipients
• other dental applications, such as tooth crowns and root replacements.

In ophthalmic treatments, biomaterials are used in contact lenses, inlays or onlays, viscosurgical devices, glaucoma shunt surgery and vitreous replacements. As Rezaie says, “Heart valves, endovascular stents, vascular grafts, stent grafts and other cardiovascular grafts are common medical devices in cardiovascular applications.”

There are various types of ‘scaffold fabrication’ methods used in tissue engineering, with biodegradable polymers helping to structure natural growth and healing after the tissue has been damaged. Highly porous biomaterials combine with the body’s cells to assist regeneration of the tissue. This process avoids the need to graft tissue from one part of the same body to another, or from another person to the patient.

Behaviour problems

As a group of academics from universities in Singapore and South Korea said in a research review published in 2016, science has improved quickly in this area. In the paper, ‘Polymeric biomaterials for medical implants and devices’, the researchers say polymeric materials are used because of their fabrication, flexibility and biocompatible nature, as well as their wide range of mechanical, electrical, chemical and thermal behaviours, especially when combined with different materials as composites.

These are “biocompatible and biostable”, according to the paper by Adrian JT Teo, from the school of mechanical and aerospace engineering at Singapore’s Nanyang Technological University. Polymers are extensively used to package implanted devices. The important qualities of this packaging are gas and water permeability from the polymer, which protects the electronic circuit of the device from moisture and ions inside the human body.

“Polymeric materials must also have considerable tensile strength and should be able to contain the device over the envisioned lifetime of the implant. For substrates, structural properties and, at times, electrical properties would be of greater concern,” the paper states.

Tim Duncan, global colour technology manager at US polymer engineering firm RTP, says, “Biomaterials are used in disposable applications such as packaging, but with modification, they can be used in more durable applications. For example, impact-modified PLA polymer blends can rival polycarbonate/acrylonitrile butadiene styrene alloys for applications such as instrument housings.

“Primary biomaterials are nylons and polyesters, and a little bio-polyethylene. These resins are used in much the same way as the fossil fuel (FF)-derived comparable polymers. They can be injection-moulded or extruded, again, much in the same way as the FF polymers.”

RTP’s polymer engineers use more than 60 different resins with additives to produce thermoplastic compounds with bespoke qualities that have many uses, including medical devices. Typical applications in medical engineering are bonding and sealing of two-piece transparent polycarbonate or acrylic housings; for example, for dialysis filters and blood oxygenators, or bonding of blood-bag plastics components.

Epoxy bonding

Epoxy resins can be developed as onepart or two-part compounds depending on the use of the product. One-part resins contain a curing agent that only acts when heat is applied, with curing time ranging from a few minutes to several hours. If the epoxy has a longer curing time, it usually has enhanced qualities with better electrical insulation and strength characteristics.

One-part epoxies are resistant to chemicals and high temperatures, and adhere well to metals, plastics and glass. They can be found in MRI machines and ultrasound devices. For instance, the assembly for a catheter and guidewire, used for catheterisation and vein puncture, could be made using one-part epoxy adhesives that can cure in fractions of a second. Adhesives made from one-part epoxies can be used to bond the parts of a breathing circuit, such as anaesthesia masks, breathing bags, tubing and connectors, and laryngeal mask airways. These adhesives work on substances such as acrylic, PVC, polyethylene, polycarbonate and styrene.

The adhesives can also be adapted for hypodermic and biopsy needles, syringes and bonding to steel cannula. These adhesives can come into contact with liquids at the bonded joints. So, in addition to being health-compatible and organically safe, they must be resistant to liquids and common sterilisation methods. Also, adhesives used in tube systems need to have the required flexibility and strength. Capillary-action adhesives are especially well suited for application in the tight fits of tubes and connectors.

Valves of various sizes can also be simply and quickly bonded with epoxy adhesives. The recommended adhesives are certified to meet biocompability standards and are available in various viscosities. This allows rapid filling and bonding of joints with different gap dimensions to create transparent joints between plastic components.

Delicate medical devices, such as optoelectronics, are made using one-part epoxies. ‘Glob tops’ made from this type of epoxy can be applied in the electronic components of medical equipment. Films for medical uses, such as lidsealing or attaching substrates in microelectronic packages, can be made from one-part epoxies. These films have a uniform thickness that makes them particularly useful.

Two-part epoxies are made through the polymerisation of a resin and curing agent using a specific mix ratio. The process can take up to two weeks. They are usually used in industrial settings. Cost factors will dictate the future uses of resins in the medical world, according to Duncan. “The goal for the industry is to improve the performance of biopolymers and the efficiency of manufacturing to drive costs downward, allowing them to compete with standard FF polymers,” he says.

“As these properties are improved, they will more rapidly grow in applications and volumes. Certainly, broader use in packaging is expected and depending on the successful modification of the base polymer, their use for replacing engineering grades is expected to rise.”

The future is bright

Biomaterials are now being used in a host of different medical devices that range from super-strong sutures to artificial hearts. The complete list is too long to mention, but they can be found in innovative drug delivery systems and in coatings that make instruments easier to implant.

With this huge array of options opening up, manufacturers and researchers are setting their sights on life-changing new treatments, such as regenerative medicine. It is hoped that resins will be able to create easy-to-produce, comprehensive medical devices that can really improve patient care.

In countries where it is hard to maintain hygiene, the use of disposable medical packaging and devices will become more prevalent. In these developing countries, cost and ease of production will be determining factors in how successful the devices will be. As science develops other uses for this adaptable material, there are a growing number of modern-day applications for one of the most famous materials in history.

As a link from ancient to modern times, resins have proved they are an invaluable part of medical treatments for the human body. They are likely to play an increasingly important role as science strives to find more adaptable materials or gifts to assist in advanced medical care.

Printed biomaterials that ‘degrade on demand’

As reported on the website 3Ders.com, a group of engineers at Brown University, in Rhode Island in the US, are following the trend for changeable materials, having developed a technique for making 3D-printed biomaterials that are capable of something akin to “degrade on demand”. The materials could be used to fabricate intricately patterned microfluidic devices or to make dynamic cell cultures.

This degradation can be brought about with a special chemical trigger, and could be useful in the fabrication of microfluidic devices, artificial tissue, and biomaterials that need to respond dynamically to stimuli.

Study co-author and assistant professor at Brown’s school of engineering, Ian Wong, says it is like Lego. “We can attach polymers together to build 3D structures, and then gently detach them again under biocompatible conditions,” he says.

The researchers have used stereolithography – the light-based resin printing method – to make 3D-printed structures with potentially reversible ionic bonds. Wong says that nothing like this has ever been done before on a stereolithography machine, so the team had to figure out how to make the biomaterials. Then, to carry out the novel procedure, the scientists made solutions with sodium alginate, a compound derived from seaweed that is capable of ionic cross-linking.

By using different combinations of ionic salts, including magnesium, barium and calcium, Wong and the other researchers were able to make 3D-printed objects with varying stiffness levels, a factor that affected how quickly the structures dissolved.

“The idea is that the attachments between polymers should come apart when the ions are removed, which we can do by adding a chelating agent that grabs all the ions,” Wong explains. “This way we can pattern transient structures that dissolve away when we want them to.”

So how can a 3D-printed structure that dissolves on cue be useful? The researchers already have a few ideas, such as using the alginate as a template for making lab-on-a-chip devices with complex microfluidic channels.

“We can print the shape of the channel using alginate, then print a permanent structure around it using a second biomaterial,” says Thomas M Valentin, a PhD student in Wong’s lab and the study’s lead author. “Then we simply dissolve away the alginate and we have a hollow channel.”