Most babies born in 1900 died before the age of 50;
100 years later life expectancy in the UK now exceeds 80 years, with the number
of over-65s expected to double by 2030. This trend is radically changing the
age demographics of the population and creating a new set of challenges for
engineers. One of the most significant of these is to give people a higher
quality of life in their old age.
Significant progress has been made; 300,000 hip
replacements are now performed annually worldwide, releasing people from pain,
and extending the active period of their lives by 20 years or more. The success
of these implants has led scientists to develop a new type of biomaterial that
is promising to do for medicine what silicon did for computing.
Historically the function of biomaterials has been to
replace diseased or damaged tissues. These biomaterials were selected to be as
inert as possible while fulfilling mechanical roles such as teeth filling and
hip replacement. Metals such as titanium and mercury amalgam have been
remarkably successful in repairing hard tissues like bones and teeth
respectively because they are chemically inert and so don’t decay inside the
body, and are strong enough to last tens of years. Applying this approach to
softer tissues has proved less easy because designing soft materials with the
required flexibility that can also maintain their integrity is tricky. The
proposed solution is to create implants that grow and repair themselves.
Take the knee for instance, its function relies on
cartilage, a soft material in which elasticity is vital to its role of
transmitting forces that are produced when we walk, jump and generally lark
about. It needs to do this while being hard enough to allow the knee joint to
smoothly rotate and twist as you change direction. Being hard but elastic is
not an easy combination for a material, and cartilage performs this by having
cells living within it, the job of which is to continually maintain a
three-dimensional internal skeleton of collagen fibres that give the material
its properties. These cells are called chondroblast cells, and cartilage is
their habitat. It is possible to grow chondroblast cells from a patient’s own
stem cells. However, the injection of these into an existing joint does not
result in the repair of the cartilage because it is not just that the cells are
missing, but that their habitat is damaged or destroyed.
What is required is the erection of a temporary
structure within the joint that mimics some of the basic internal architecture
of cartilage but also protects the cells from mechanical stress. Introducing
chondroblast cells into such a scaffold, as it is called, allows them to grow
and divide, to increase their population, and in doing so gives them time and
space to rebuild their habitat, and so regrow cartilage. The neat thing about
this is that the scaffold can be designed to dissolve once the cells finish
rebuilding their habitat.
This idea was pioneered in the 1960s by Professor
Larry Hench in response to the huge number of amputees produced by the Vietnam
war. Hench and his team discovered a material called hydroxylapatite, a mineral
that occurs in the body and bonds very strongly to bone. They experimented with
many formulations and in the end found that when it was made in the form of a
glass, it had extraordinary properties: bone cells, called osteoblasts, like to
live on it, and when they did that they created new healthy bone. When this
bioactive glass was made in a porous form it had tiny channels into which the
osteoblast cell could grow and by doing so they replaced it with fully
functioning bone.
Such tissue engineering has been very successfully
used in clinical practice to provide synthetic bone grafts and to rebuild the
bones of the skull and face. It is not yet in clinical use for cartilage
regeneration, but is being successfully used in laboratories. In this case the
cells are nurtured in a bioreactor that mimics the temperature and humidity of
the human body while also providing the cells with nutrients.
The potential of scaffolding technology has opened up
the future possibility of building replacement organs for the human body such
as livers and kidneys. The first steps in this direction have already been
taken with the development of a human windpipe grown in a laboratory and
implanted into a patient. One of the major problems for such tissue engineering
is creating and maintaining a blood supply to the artificially grown tissues so
that they can survive and function when they are implanted inside the body. If
synthetic organs become a reality, they will radically change the world of
medicine. Such a fundamental change is going to be needed to allow an aging
population to work for longer before taking their pensions, and to live to be a
centenarian while being fit and healthy. I certainly hope to be one of these.
