Biomedical Engineering Reference
In-Depth Information
8.2.2.3.2 Porous Si Photonic Crystals as Templates for Drug Delivery Materials
As mentioned earlier in this chapter, MCM-41 and siliceous nanocapsules are constructed by apply-
ing templating techniques. The concept of template is in fact applied in many fi elds of science such
as using templates for organic synthesis,
123
preparing porous materials through nanocasting,
124
using
silicone stamps as templates for patterning in soft lithography,
125
and using template-imprinted
nanostructured surfaces for protein recognition.
126
Templates consisting of microporous mem-
branes,
127,128
zeolites,
57
and crystalline colloidal arrays
129-131
have been used to construct elaborate
electronic, mechanical, or optical structures. In particular, the synthesis of materials using micropo-
rous membrane templates has emerged as a useful and versatile technique to generate ordered nano-
structures.
124
The most commonly used microporous membrane templates are track-etched polymer
membranes, porous alumina membranes, and membranes fabricated by lithography and related
techniques, although the choice of templating materials is only limited by imagination. Arnold
and coworkers demonstrated the synthesis of bifunctional polymer-coated silicon/silica core/shell
nanowires from silicon nanowire templates as a way to control the surface properties of nanowires
and nanotubes. The surfaces of the nanowires were fi rst modifi ed with polymer initiators, and then
methacrylate polymer chains were grown from the surface. After the silicon cores were etched
away, the resulting polymer-coated nanotubes with hydrophilic silica cores and hydrophobic poly-
mer shells were obtained.
132
Recent work has employed other membrane templates such as porous silicon
95,118
to fabricate
functional materials. Optical nanostructures consisting of composites of porous Si templates with
various polymers have been demonstrated.
118,133,134
Polymers have been placed in a porous Si tem-
plate by
in situ
polymerization,
134,135
injection molding,
118
or solution casting.
133
The potential of
such hybrid materials in sensing and in controlled release drug delivery has been demonstrated.
118
Porous Si is an attractive candidate for use as a template because the porosity and the average pore
size can be tuned by adjusting the electrochemical preparation conditions, enabling the construction
of photonic crystals, dielectric mirrors, and the more sophisticated optical structures such as
microcavities (the fi rst microcavity made entirely out of porous Si was reported in 1995
89
).
90
Elabo-
rate photonic organic and biological polymers have been constructed by using porous silicon pho-
tonic crystals as the template.
118
Polymer was infused into the porous silicon template by injection
molding
118
of r s of lu t i of n c a s t i n g.
133
Chemical dissolution of the template produces a freestanding porous
polymer fi lm.
118
The polymer replicas inherit an inverse of the optical structure of the template. This
approach was fi rst demonstrated by using a porous silicon rugate dielectric mirror as the template.
118
A rugate dielectric mirror is a structure that contains a sinusoidal refractive index variation, pro-
ducing a sharp spectral feature in the optical spectrum. A porous silicon rugate dielectric mir-
ror is prepared by anodic electrochemical etch of a crystalline Si wafer using a pseudo-sinusoidal
current-time waveform (Figure 8.11).
87,90-93,116,117
The sharp features in its optical refl ectivity spec-
trum of the porous silicon rugate mirror can be controlled by adjusting the frequency and the ampli-
tude of the sinusoidal current-time waveform.
94
For many applications, porous Si is limited by its chemical and mechanical stability. The use
of porous Si as a template eliminates these issues while providing the means for construction of
complex optical structures from fl exible materials that are compatible with biological systems or
harsh environments.
118
A very promising application of such fabricated materials would be in drug
delivery. Such a templating approach enables one to impart the desirable optical features of the
porous Si master to a polymer that possesses the required biocompatibility, resorbability, or drug
solubility parameters. With the optical features inherited, sensing functionality is enabled for the
biopolymer imprints. A self-reporting drug delivery system has been demonstrated using materials
fabricated with this templating approach.
118
A noninvasive, self-reporting drug delivery matrix is desired in many cases. In the case of drug
delivery systems implanted in transparent media such as the vitreous body of the eye, if the systems
change their spectral properties as they degrade or as they release a loaded drug, the drug delivery
status can be monitored using visible light. Similarly, for drug delivery systems implanted in visibly
