Biomedical Engineering Reference
In-Depth Information
surface. (ii)
Wet etching
is chemical etching performed with a liquid chemical (etchant) instead of
plasma
[10]
.
11.3
LITHOGRAPHY
Lithography (in Greek “Lithos”—stone; “graphein”—to write) is a planographic printing technique
using a plate or stone with a smooth surface. This technique was invented by Bavarian author Alois
Senefelder in 1976
[12]
. Lithography uses oil or fat and gum arabic to divide the smooth surface
into hydrophobic regions which takes up the ink and hydrophilic regions which does not and thus
become the background. Most topics, indeed all types of high-volume text, are now printed using
offset lithography, the most common form of printing production. Semiconductor industry has bor-
rowed this principle to fabricate ICs and MEMS by photolithography. Biomedical researchers employ
soft-lithography, a technique using elastomeric stamps to fabricate biomaterial micropatterns to study
cell-biomaterial interaction. The following sections will discuss in detail about “soft-photolithogra-
phy,” a combination of photolithography and soft-lithography techniques to fabricate hydrogel micro-
patterns on a silicon substrate.
11.4
HYDROGEL AS A BIOMATERIAL
Hydrogel is a network of polymer chains that swell in aqueous solution. Hydrogel is composed of
long polymer chains connected by cross-links. The cross-links may be degradable or nondegrada-
ble and are ionic interactions between polyelectrolyte chains. Cross-linking of polymer molecules
or polymerization can be achieved by photopolymerization, changes in temperature, radiation, self-
assembly, or by cross-linking enzymes. Hydrogels undergo responsive swelling by absorbing solvent
when placed in an aqueous solution (solvation). Swollen hydrogels can absorb many times their own
weight in water and can switch between swollen and collapsed forms. The key properties of hydrogels
for biomedical applications are better biocompatibility, biodegradability, ability to incorporate biomo-
lecular cues (due to high permeability for oxygen, nutrients, and water-soluble metabolites). These
key characteristics along with the ease of
in situ
fabrication have made hydrogels a biomaterial of
choice in
in vitro
studies for analyzing cell-biomaterial interactions and in biomedical applications
[13]
. Hydrogels are extensively used in cosmetic and reconstructive surgery
[14]
, as a matrix for the
fabrication of artificial organs in tissue engineering
[15]
and as “intelligent” stimuli-sensitive drug
delivery systems
[16]
. Hydrogels have also been extensively used to fabricate contact lenses
[17]
,
breast implants
[18]
, tissue engineering scaffolds
[19]
, delivery vehicles for bioactive drug molecules
[20]
, coatings for biosensors
[21]
, dressings for wound healing and burn injuries
[22]
, and as carrier
scaffolds for guided bone regeneration (GBR) using osteogenic growth factors
[23]
. Although many
polymer hydrogels have been studied, poly(ethylene glycol) (PEG) hydrogel is one of the most widely
investigated systems. PEG hydrogel can be fabricated into three-dimensional microstructures to study
the response of cells for their applications in tissue engineering. PEG hydrogel offers the simplicity
and advantage of incorporating bioactive molecules into the hydrogel matrix passively or by covalent
linking with the PEG monomer. Time-dependent release of these biomolecular cues from the PEG
hydrogel micropatterns serves as an excellent platform for studying cell response in tissue culture.
