The material Bioactive glass was invented by American Professor Larry Hench during the Vietnam War. Tasked by the US Government to develop a material which could be used to repair large bone injuries suffered by Servicemen during the war, Professor Hench used silica (glass) as a carried or host material which could be combined with other ingredients such as calcium in a powdered form to pack between bone fragments to fuse shattered bones.
Dr Ian Thompson has taken Bioglass© a stage further in the casting of molten Bioglass© into shapes up to 5cm long which can then be carved to a required shape and implanted into the face of a patient in need of repair of the nose or eye socket to name but two.
When Paddy and Ian first met, Ian was using a casting method that involved carving the negative shape of the implant into a block of graphite, melting the Bioglass in a platinum crucible and pouring the fiercely hot glass into the pre-heated graphite. The quickly cooling implant had to be immediately transferred to an annealing kiln to be slow cooled over two days. For very basic shapes this method worked well but some of the implants required were incredibly delicate and needed to be of an exact shape to repair patient specific injuries. This required a complete rethink of the casting process that utilised a combination of Paddy’s sculpture casting knowledge and Ian’s expertise in the performance characteristics of the Bioglass©. A hybrid casting process was devised combining mould-making materials traditionally used by glass artists for lost wax casting and vacuum suction. Paddy would take a wax model of the implant to be made and invest it into a mould mix of plaster and Investrite. The wax model would then be melted out of the mould to leave a cavity for the hot glass to be poured into. Initially, Paddy and Ian attempted to fill the mould with powdered Bioglass©, place the mould in a furnace and fire it up to the melting point of the glass. They quickly discovered that the melting point of the mould was lower than the glass when the mould itself became molten liquid.
The one drawback to casting bioactive glass is its high melting point of 13600C. Usually if a silica or glass material needs to have its melting point lowered, additional materials are added. This however cannot be done with Bioglass©, as it is an exact glass composition that the human body responds favourably to. Change the recipe and the Bioglass© implant will be rejected by the body’s immune system. Paddy and Ian were keen to continue working with the plaster/Investrite mix because of the soft quality of the mould. Some of the implants to be made were very delicate and this required the mould to crumble away from the cast glass. Ian suggested that the hot glass could be sucked through the pre-heated mould so the mould would not be exposed to these high temperatures for anything but a few seconds.
Initial trials of this method are very encouraging and testing has proved that with further refinement of this technique will prove to be the method of production of the Bioglass© implants.