Biomedical Engineering Reference
In-Depth Information
light (e.g., lasers) in order to change its properties. Fundamentally, when you excite a quan-
tum dot with one wavelength, depending on the size of the dot, it will emit (fluoresce) at a
narrow-band longer wavelength. The size-dependent behavior allows them to be used for
multiplexing, since a single wavelength can be used to excite different-sized quantum dots
to produce multiple wavelengths within a sample.
Although quantum dots are nanometer-scale crystals that were developed in the mid-
1980s for optoelectronic applications, one application in the biomedical area has been to
probe living cells in full color over extended periods of time. Such a technique could
reveal the complex processes that take place in all living organisms in unprecedented
detail, such as the development of embryos. Existing imaging techniques use natural
molecules that fluoresce, as just discussed, such as organic dyes and proteins that are
found in jellyfish and fireflies. However, each dye emits light over a wide range of wave-
lengths, which means that their spectra overlap, and this makes it difficult to use more
than three dyes at a time in order to tag and image different biological molecules simulta-
neously. The fluorescence of dyes also tends to fade away quickly over time, whereas
semiconductor nanocrystals—quantum dots—can get around these problems. In addition
to being brighter and lasting longer than organic fluorophores, quantum dots have a
broader excitation spectrum and thus, as mentioned, a mixture of quantum dots of differ-
ent sizes can be excited by a light source with a single wavelength, allowing simultaneous
detection and imaging in color.
Although the preceding example has focused on biomedical sensing using fluorescent
quantum dots, these micro- and nanoparticles can also be made of various materials and
used with all of the preceding light propagation methods. For instance, metal nanoparticles
such as gold or silver can be used with Raman spectroscopy to produce an effect known
as surface-enhanced Raman spectroscopy (SERS), which gives rise to signals a million times
more sensitive than regular Raman signals. These same types of metal nano- or micro-
particles can be injected into cancerous tissue and used as absorbers that when hit with
infrared light will absorb the energy, cook the cancerous tumor, and kill it. Since nano-
particle development is still in its infancy, where they go in the body, their toxicology,
whether they can be functionalized to go to specific organs or cells like cancer, and what
other biomedical applications are to come from the combination of these particles with light
remain to be seen.
17.5 FUNDAMENTALS OF THE PHOTOTHERMAL THERAPEUTIC
EFFECTS OF LIGHT SOURCES
The therapeutic application of light, including lasers, is mediated by conversion of
photonic energy to absorbed energy within the material phase of the tissue. The primary
mode of this energy conversion manifests itself as a nonuniform temperature rise that leads
to a series of thermodynamic processes. These thermodynamic processes can then be
exploited as a means to affect therapeutic actions such as photothermal coagulation and
ablation of tissue. Another mode of interaction is the utilization of the absorbed energy in
activation of endogeneous or exogenous photosensitizing agents in a photochemical process
known as photodynamic therapy.
