Biomaterials - Introduction to Biomedical Engineering - page 269

FIGURE 5.19 The primary advantage of local
delivery is a reduction of the systemic blood levels
of the drug to below toxic levels. High local levels
at the site of injection or implantation allow for
increased drug efficacy and reduced side effects.
This technology is particularly applicable for che-
motherapy drugs and therapeutic hormones.

Intravenous injection

Toxic limit

Local delivery on a controlled
release drug delivery vehicle

Time

Collagen is commonly used for the delivery of bone growth factors. The protein growth
factors are typically not covalently immobilized on the collagen surface, leading to rapid
release. Hydroxyapatite materials also are used for delivery of bone growth factors. The
drug or protein often attaches quite strongly to the hydroxyapatite surface, and release is
greatly delayed as compared to a collagen drug delivery vehicle. Depending on the drug
and its mode of action, longer release times may or may not be desired.
Rather than attempting to design a controlled-release drug delivery vehicle with the per-
fect release profile, drug delivery chips that can be programmed to open up drug compart-
ments by an external signal are being developed. The ultimate smart drug delivery vehicle
is the cell. Unlike a passive drug delivery device that acts independently, cells produce
cytokines, growth factors, and extracellular matrix materials based on the signals from
the in vivo environment. Attempts are being made to exploit this with the implantation
of encapsulated xenograft pancreatic islet cells. Pancreatic islet cells produce insulin in
response to the circulating blood levels. The xenograft cells need to be encapsulated within
a biomaterial (typically alginate) to evade immune surveillance activity that can be toxic to
the cells. Current research in this area is focused on varying the properties of the alginate
to maintain sufficient permeability to keep the cells vital yet protected from immune cell
toxins, while still allowing diffusion of insulin out of the device—a tall order for one
material. Materials selection for medical devices will always involve this type of balancing
act between properties.

5.8. EXERCISES
1. Explain briefly how the idea and role of a biomaterial has changed since their first use.
2. List the four classes of materials used in medical devices, along with three advantages and
disadvantages of each.
3. What advantage does a composite material afford over the singular components?
4. Explain the importance of the extracellular matrix in controlling cell activity and tissue
maintenance.

Continued