biomedical devices, including internal prostheses, have improved the
quality of life for countless patients around the world. Cochlear
implants for the deaf are just one example of such aids, transmitting
electrical signals directly into the human nervous system through
the inner ear. While the enormity of these technological leaps are
mind-boggling, certain practical aspects such as how to fix the implants
in place can still be problematic. A Brite-Euram project set out to
overcome one problem by developing an improved biomedical cement,
combining advanced testing and production methodologies in a worthy
challenge to American domination in this sector.
Current glass-based dental cements formed by
reacting powdered fluoro-alumino-silicate glasses with weak polyacrylic
acid have attractive properties for use with implants, but they
lack toughness and wear resistance and tend to break down over time.
In cochlear implants, this can lead to loosening and ultimately
to hearing-aid failure. In addition, volatile silicon tetrafluoride
and hydroflouric acid are often formed during the glass-melting
Diagnosing the problem
Launched in 1993, the objective of the project
consortium was to develop an improved glass polyalkenoate cement
for biomedical applications. The new cement was based on novel glass
compositions and was designed specifically for use in cochlea implants
and other prosthetic devices.
First, the mechanism of failure of existing cements was determined
through ion release studies. Results demonstrated that breakdown
is due to ion exchange whereby a fluoride ion in the cement is exchanged
for a hydroxyl ion in the aqueous medium. This discovery contradicted
the previously accepted model which had alkali cations and flouride
ions being released simultaneously from the set cement. These and
other findings pointed to the use of ion leachable fluoro-alumino-silicate
glass as a possible solution. The term 'ion leachable' refers to
the ability to remove cations from the material before it sets.
By combining ion leachable fluoro-alumino-silicate
glass with polyacrylic acid a more stable, hydrophilic cement was
produced which was ideally suited for use in the aqueous environment
of the human body. The aluminium to silicon ratio of the glass was
found to be less significant than previously thought, and toxic
silicon tetraflouride production was shown to be preventable in
the presence of sufficient oxygen in relation to silicon and sufficient
aluminium in relation to fluorine.
Thus, a strong correlation was established between glass composition
and the setting properties, the mechanical properties and life span,
allowing the production of an extremely hard, ceramic-like, water-based
adhesive displaying a very low risk of failure.
Biocompatibility and Exploitation
In-vivo biocompatibility studies proved
positive, both in animals and humans, and the new cement has now
received medical certification and carries the 'CE' mark of European
A licensing agreement with the project consortium allows Corinthian
Medical Limited to produce and market glass polyalkenoate cements
under the trade name Biocem for the ear, nose and throat market
and especially for use with cochlea implants. The consortium has
been earning royalties since 1997 and is expecting increased gains
as further non-medical applications are developed and exploited.
The global market for cochlear implant cement alone has been estimated
at around 8 million euro. This is a new and exciting development
for European producers in a market traditionally dominated by US
From a medical perspective, the new cement allows for simpler and
safer cochlear implant procedures. Improved fixation means reduced
risk of hearing-aid failure and consequent improved quality of life
for the patient. The new material can also be used in dental applications
as an ultra-hard filling material that can bond to dentine and enamel.
Finally, biomedical applications are envisaged, such as joint replacement
prostheses or mouldable bone substitutes.
Credit for the project's success goes to excellent technical expertise
and Cupertino between partners and industrial sponsors, as well
as commercial contacts in the medical device industry.