Biomedical Engineering Reference
In-Depth Information
phospholipid bilayer of human cell membranes
[4]
, RNA
[5]
, and DNA complexes
[6]
. The deter-
gent surfactant molecules exhibit self-assembly phenomenon due to their amphiphilic properties. The
molecular building mechanisms underlying the formation of bacteriophage and viral particles are all
based on self-assembly.
13.2
MECHANISM OF MOLECULAR SELF-ASSEMBLY
Molecular self-assembly is a powerful phenomenon borrowed from nature by scientists for fabricat-
ing novel supramolecular architectures. Molecular self-assembly is the assembly of molecules without
guidance from an outside source. Molecular self-assembly is, by definition, the spontaneous organiza-
tion of molecules under near-thermodynamic equilibrium conditions into structurally well-defined
and stable arrangements through non-covalent interactions
[7]
. This is mainly governed by weak non-
covalent bonds like electrostatic interactions (ionic bonds), hydrogen bonds, hydrophobic and
hydrophilic interactions, water-mediated hydrogen bonds, and van der Waals interactions
[7,8]
.
Although these forces are weak, their collective interactions can produce structurally and chemically
stable structures. Frequently, molecular self-assembly relies on chemical complementarity and struc-
tural compatibility
[7,9]
. The molecular components need complementary properties such as specific
surface characteristics, surface charge, polarizability, mass, and surface functionalities to self-assemble
into different physiological forms
[9]
. Biological molecules like proteins, peptides, nucleic acids, lip-
ids, and other cellular components with complementary properties self-assemble to form the basic
biological unit, the cell. Cellular events like amyloid fibril formation, antigen-antibody recognition,
chromatin assembly, and phospholipid membrane self-assembly are excellent examples of molecular
self-assembly.
13.3
CLASSIFICATION OF SELF-ASSEMBLY
Self-assembly is a native process. It can be classified into two types: static and dynamic
[10]
. Most
research studies done in self-assembly have been focused on static type while the study of dynamic
self-assembly is still in its infancy. Static self-assembly involves systems that are at global or local
equilibrium and do not dissipate energy
[10]
. In static self-assembly, formation of the ordered struc-
ture may require energy, but once it is formed, it is stable. A few examples of static self-assembly
phenomenon tailored by nature are lipid molecules forming oil droplets in water, four hemoglobin
polypeptides forming a functional hemoglobin protein, and the combination of RNA and ribosomal
proteins to form a functional ribosome. The other common examples of static self-assembled struc-
tures have been illustrated in
Figure 13.1
. Dynamic self-assembly occurs when the formation of an
ordered state of equilibrium requires dissipation of energy. In other words, the interactions responsi-
ble for the formation of structures or patterns between components occur only if the system dissipates
energy
[10]
. The examples of dynamic self-assembled structures have been illustrated in
Figure 13.2
.
Self-assembly takes place at molecular, mesoscopic, and macroscopic scales. Based on this cri-
teria, self-assembly has been classified as molecular and nanoscale self-assembly (classical form
of self-assembly in chemistry involving atoms, molecules, and crystal formation) and meso- and
macroscopic self-assembly (e.g. engineered microparts). Molecular and nanoscale self-assembly
can be further classified as intramolecular and intermolecular self-assembly
[8]
. In intramolecular
