Pharmaceutical Manufacturing Handbook: Production and

eplicated on the substrate, with nanoscale features usually patterned by X - ray or
electron beam exposure. The mask is then placed on the surface of the substrate
and exposed to near - fi eld light, the intensity of which leads to replication of the
pattern on the mask on to the substrate. Complex geometries, shapes, and features
can be produced on the substrate which can subsequently be removed to give free -
standing particles or objects [170, 171] .
Molecular Imprinting Molecular Imprinting is a process used to imprint or copy
recognition sites from desired molecules on to polymer structures [172, 173] . The
recognition sites can be produced on organic or inorganic polymers and inorganic
materials such as silica and biomaterials such as proteins. A template molecule is
dissolved in solvent with polymerizable monomers which undergo bond formation
with the template molecule forming either noncovalent bonds through electrostatic
interactions, hydrogen bonds or hydrophobic interactions, or reversible covalent
bonds. The monomers are then polymerized to form a cast or semirigid polymeric
structure which maintains the steric conformation of the molecule template and its
recognition site upon removal of the template molecule. As a result, the molecular
template affi nity for molecules and analyte is mimicked by the “ imprinted ” polymer
[174] . This has applications in chromatography and drug discovery and potential
applications in targeted drug delivery.
Layer -by -Layer Electrostatic Deposition Electrostatic deposition utilizes the
electrostatic bonding affi nities of materials imparted by their surface charge to build
highly ordered multilayered fi lms or structures on a substrate. The process involves
the successive deposition of oppositely charged polyions, exploiting the Coulombic
long - range electrostatic interactions between the oppositely charged molecules,
allowing formation of multilayers over a large distance. This technique can be used
to build multilayer composite fi lms on particles incorporating molecular fragments
such as polymer – polymer, polymer – organic, polymer bimolecular, and polymer –
mineral composition [175 – 177] .
Chemical Vapor Deposition CVD is a crystal growth process whereby a solid
material is deposited from the gas phase onto a controlled substrate using a suitable
mixture of volatile precursor materials which react to produce the desired deposit
on the substrate surface (Table 4 ). Types of fi lms and structures that can be produced
include the following:
Polycrystalline
Amorphous
Epitaxial silicon
Carbon fi ber
Filaments
Carbon nanotubes
Silicon dioxide
Tungsten
Silicon nitride
Titanium nitride
MANUFACTURING TECHNOLOGIES 1301