Biophysics:
Biological & Medical
Faculty in this area
of research:
Applications of lasers to biology and medicine
are pursued with studies in theoretical and experimental biological
physics. Reflectance, fluorescence, and near-IR Raman spectroscopy
are being used for biochemical/biophysical analysis of tissues and
blood, and for diagnosis of dysplasia, cancer, Alzheimer's, atherosclerosis,
and other diseases. A novel system for acquiring near-IR Raman spectra
of microscopic samples has been developed and is being used for
detailed studies of cancerous or otherwise diseased tissue of many
organs. Clinical studies are being pursued with researchers from
the Cleveland Clinic Foundation, Brigham and Women's Hospital, Metrowest
Hospital, and New England Medical Center in colon, Barretts' esophagus,
bladder, breast, coronary and peripheral arteries, and brain studies.
Quantitative analysis of blood analytes through the skin using Raman
spectroscopy is under development. UV resonance Raman spectroscopy
is being explored to characterize dysplasia. A new technique based
on observation of the nuclear signatures in reflectance spectra
is being developed for measuring nuclear size distribution in biological
tissues. Photon migration using a novel picosecond-pulse optical
tomographic system is being used to image small fluorescent lesions
imbedded in turbid biological tissue, and to study paths of early
arriving photons. The mechanism of pulsed laser ablation of soft
and hard biological tissues has been shown to be thermoelastic in
origin. The experimental and theoretical work being conducted in
this program is advancing new laser diagnostic technologies in the
field of medicine.
The phase transition and aggregation of several
biological molecules are studied. These phenomena are of great interest
because the factors which govern phase separation and aggregation
of biological molecules are believed to play a role in several human
diseases such as cataracts, Alzheimer's disease, and cholesterol
gallstones. A combination of experimental work, theoretical anaylysis,
and computer simulations is used to understand how the interactions
between such molecules govern their separation into coexisting phases.
Phase transitions and critical phenomena
of gels are studied. Gels are a jello-like material made of a cross-linked
network of polymers that contains a fluid. They undergo an abrupt
change between collapsed and swollen states in response to a small
change in temperature, solvent, acidity, light, electric field,
or some molecules. The volume change is reversible and can be as
large as many thousand times. The phase transition is universally
found in synthetic and natural gels and is classified by four fundamental
interactions: van der Waals, hydrophobic, electrostatic, and hydrogen
bonding. Recent research involves theories and experiments on heteropolymers
and heteropolymer gels in which monomers interact with different
fundamental interactions. Heteropolymers are also a model for proteins.
Theoretical analysis predicts that heteropolymers have four phases:
swollen, collapsed and rubbery, frozen in a degenerate conformation,
and frozen in a designed conformation. Experiments are under way
to search for these phases and also to develop gels that can memorize
conformation and recognize target molecules.
Heteropolymers composed of long, seemingly
random, sequences of monomers, are ubiquitous in living matter.
Understanding their global conformation, given the basic interactions
between monomers, is a challenging problem of many-body physics.
Focusing on the role of the long-range Coulomb interactions, theoretical
studies reveal "necklace" phases composed of compact beads connected
by stretched strands for charged heteropolymers and elevated freezing
transitions for neutral heteropolymers. Polymer statistical mechanics
is conducted by renormalization-group techniques. This method simultaneously
yields both long-range properties, such as the scaling of end-to-end
separation as a function of chain length, and local properties,
such as the fraction of segmental bends. Thus, phase diagrams are
obtained, exhibiting coil, rod, and globule phases. The approach
is now being extended to heterogeneous systems and proteins.
The first steps in recognition of an image
from the input to the eyes take place in the primary visual cortex
(V1). Experiments have revealed that cells in V1 respond preferentially
to inputs from one eye (ocular dominance) and to lines of particular
direction (orientation preference). On the larger scale of the whole
cortex, the patterns of response are organized into textures reminiscent
of many two dimensional non-equilibrium motifs. Inspired by their
similarity to quench patterns in symmetry-breaking phase transitions,
Landau-Ginzburg equations are constructed for the evolution of macroscopic
fields: Ocular dominance is represented by a scalar (Ising-like)
order parameter, while orientation preference has a complex (XY-like)
character. The natural defects in the two fields, domain walls and
vortices respectively, are observed to repel each other. This requires
a coupling between the two fields, which has a natural interpretation
in the cortex's limited computational capacity.
The theory of chemotactic dynamics of bacteria
has been developed and used to study pattern formation in bacterial
colonies. The structures predicted by the mathematical model, such
as travelling bands and rings, and collapsing aggregates, are observed
in recent experiments on chemotactically interacting E. Coli.
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