Biomedical Engineering Reference
In-Depth Information
exposure (
Figure 8.13d
). As shown in the inset, these microneedles are deep enough to pen-
etrate through the stratum corneum, the top skin layer of keratinized dead cells, and reach the
blood vessels for potential blood extraction (shallower microneedles for difusive drug delivery
are also possible).
For some applications, it may be important to develop very simple, dirt-cheap devices. Mark
Prausnitz's laboratory has recently fabricated planar solid stainless-steel microneedles (700 μm
long, 170 × 55 μm at the base and tapered at the tip with a 5-μm radius of curvature) by laser-
cutting stainless steel sheets. hese solid microneedles, meant only for delivery of transdermal
drugs that can be dried on the tips (see next section), do not sufer from the silicon brittle-
ness and the biocompatibility concerns of older microneedles.
Figure 8.14a
shows an array of
ive microneedles next to a 26-gauge hypodermic needle and a U.S. dime coin for size com-
parison. he microneedles are used to deliver the bacillus Calmette-Guérin (BCG)—the only
licensed vaccine for human use against tuberculosis—in dry form. he BCG-coated micronee-
dles (
Figure 8.14b
) produce minimal tissue damage (
Figure 8.14c
) and eiciently deliver the
(luorescently tagged) BCG vaccine intradermally (
Figure 8.14d
). he microneedle-delivered
vaccine was able to induce an immune response in both the lungs and the spleen of guinea pigs
comparable with intradermal vaccines delivered by traditional hypodermic needles, suggesting
a promising potential for future human use. his group has also been able to develop a process
for fabricating microneedles in biodegradable polymers (polylactic acid, polyglycolic acid, and
their copolymers), which addresses safety concerns of previous microneedle designs made of
metal or silicon.
8.3.2 Microluidic Drug Delivery to the Eye
Drug delivery to the eye is used to treat retinal diseases such as glaucoma, age-related macu-
lar degeneration, diabetic retinopathy, and retinitis pigmentosa, but it can be traumatic to the
patient, especially when administered via surgery or intraocular injection. Ellis Meng's group at
the University of Southern California (Los Angeles) has reported a reillable drug delivery PDMS
microluidic device that is implanted under the conjunctiva (
Figure 8.15a
) and that delivers the
contents of its reservoir into the eye when it is manually pressurized (e.g., with a cotton swab)
beyond the cracking pressure of the check valve (
Figure 8.15b
and
c
). he reservoir features
posts to prevent stiction of the roof against the loor when it is collapsed by pressure application.
he contents of the reservoir are reilled with a sharp needle that punctures the PDMS, leaving
a hole that self-seals.
Rectus muscle
Refillable reservoir
Vitreous gel
a
b
c
Conjunctiva
Iris
Refillable
reservoir
Suture tab
Optic
nerve
Suture
tab
Check
valve
Cornea
Check
valve
Macula
Pupil
Support
posts
Fovea
Support
posts
Lens
Cannula
1 mm
Retina
Iris
FIGURE 8.15
A.microluidic.drug.delivery.device.for.ocular.diseases..(From.Ronalee.Lo,.Po-Ying.
Li,.Saloomeh.Saati,.Rajat.Agrawal,.Mark.S..Humayun,.and.Ellis.Meng,.“A.reillable.microfabricated.
drug.delivery.device.for.treatment.of.ocular.diseases,”.
Lab Chip
,.8,.1027-1030,.2008..Reproduced.
with.permission.from.The.Royal.Society.of.Chemistry.)
