About the author
Polymer Chemistry and Biomaterials Group
As promised, our journey into the fascinating world of polymers starts here…
Commonly used household items are often manufactured from the same materials as specialised medical devices. The latter are only possible because of the versatility of polymers, as they can be fine-tuned for a variety of applications by applying appropriate processing techniques or through chemical/physical alterations. Believe it or not, ‘plastic’ windows and contact lenses are manufactured from the same material called poly(methyl methacrylate) or PMMA.
PMMA was discovered in 1877 and patented in 1933. It is a thermoplastic polymer which is often used as a glass substitute. Referred to as ‘acrylic glass’ or ‘plexiglass’, PMMA has one important feature in common with glass, i.e. transparency – although they are completely different materials from a chemical perspective. Glass mainly consists of SiO2, rendering it an inorganic material, whereas PMMA is an organic material as it is built up of carbon, hydrogen and oxygen (see structure below). PMMA can be selected over glass because of its higher shatter resistance, its ease in processing and shaping and its low cost and weight. Conversely, this plastic is less scratch-resistant and more flammable than glass.
The material properties characteristic of this polymer include its glass transition temperature, which is around 110°C, and its hydrophobic nature (ie . insoluble in water). Examples of solvents which are able to dissolve PMMA include chloroform and tetrahydrofuran.
Chemical structure of PMMA. x denotes a repetition of its building units, called monomers
Because of their advantages, plexiglass windows can be found in aquariums, caravans, delineation of children playground as well as in police shields, car lights, etc…
Medical discovery: lucky strike or pure observation?
Actually, the first major application of PMMA was during World War II when it was used for aircraft windows, submarine periscopes, gun turrets for airplanes, etc. It was during these times that the biocompatibility of PMMA was discovered by the English ophthalmologist Harold Ridley. Pilots often suffered from eye injuries caused by PMMA splinters coming from the windows of their airplanes. Dr. Ridley concluded that compared to glass splinters, almost no rejection occurred in these cases. As a consequence, in 1949, Dr. Ridley implanted the first intra-ocular lens, in an attempt to cure cataract. In the same timeframe, hard plastic PMMA lenses started to replace the first glass contact lenses.
“Only man who has invented his own operation” – Harold Ridley
Customising polymer properties based on future application
Apart from ophthalmology, in the world of orthopedics, PMMA is well-known as bone cement which is used as ‘glue’ to fix prostheses in bone. In the 1950s, it was mainly used for head prostheses, but nowadays it is used to fixate hip and knee replacements. Apart from these examples, in the medical world, PMMA is also used in dentures or biochips and as microspheres which can be used as a drug carrier.
Applications of PMMA in the medical world include bone cement, an intra-ocular lens and a contact lens
Where PMMA is used as bone cement, you can imagine that its hard and strong nature is very desirable, as the plastic fulfils a load-bearing function. On the other hand, if we want to implant something subcutaneously (i.e. under the skin), a certain softness and flexibility of the material will be required to assure the patient’s comfort.
Each polymer intended to function as an implant should fulfill certain requirements specific for the implantation site in the body (subcutaneous, heart, knee, bone, etc.), its surrounding environment and on the physiological role that it has to play (skin graft, bone defect filler, heart valve replacement, etc.).
My research = a small piece of the future puzzle
Recently, PMMA has been investigated as a potential packaging material for implantable medical devices including glucose sensors. To ensure sufficient flexibility, the bulk properties of the hard and brittle PMMA need to be adjusted. The latter can occur by introducing long side chain oligomers in the basic structure of PMMA in order to create copolymers which are characterised by an improved flexibility and softness.
As already stated in the previous preliminary post, an implant interacts with the human body through its surface. As a result, the surface properties of the packaging material will have to be altered as well. In case of a glucose sensor, it would be advantageous to grow blood vessels in the vicinity of the implant, as glucose needs to reach the sensor. Since blood vessels are built up of endothelial cells, this will be the cell type we want to attract towards the sensor. To realise this, biological compounds can be immobilized on the packaging outer surface.
This all sounds great, but how will we create such a polymer packaging?
A film casting technique is one possibility that is often used in industry, but also in academia (including the PBM research group), due to its straightforward approach and concomitant results. As shown in the figure below, a monomer solution is injected between two glass plates and irradiated with UV-light (hν). This light will activate an initiator (present in the solution) which results in the formation of radicals (i.e. reactive species). These radicals will then trigger the initiation of the different monomers in the solution and, as a result, polymer chains will start to form and further grow (i.e. propagation). In the end, a solid material will be obtained. The shape of the material will be determined by the shape of the central cavity (in this case rectangular). This technique differs from other processing techniques such as bioplotting or electrospinning, in the sense that the polymer production and moulding are combined in one step, whereas for the other techniques the polymer needs to be produced prior to processing.
Film casting technique: the central hole in the silicone spacer (blue rectangle) will determine the shape of the plastic created.
A contribution by Elke Van De Walle,
Polymer Chemistry & Biomaterials Group, Ghent University, Belgium
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