Biomedical Engineering Reference
In-Depth Information
Single tilted
exposure
Two tilted
exposures
Rotated
and tilted
Tilted, reflected
exposure
a
Θ
Θ
Φ
50 µm
FIGURE 1.5
Tilted.microstructures..(From.Han,.M.,.W..Lee,.S.-K..Lee,.and.S.S..Lee,.“3D.micro-
fabrication. with. inclined/rotated. UV. lithography,”.
Sens. Actuators A: Phys.
,. 111,. 14-20,. 2004..
Reprinted.with.permission.from.Elsevier; see.also.the.excellent.review.on.SU8.photolithography.by.
del.Campo,.A..and.C..Greiner,.“SU-8:.A.photoresist.for.high-aspect-ratio.and.3D.submicron.lithog-
raphy,”.
J. Micromech. Microeng.
.17,.R81-R95,.2007.)
Tilted structures can be fabricated simply by tilting the substrate (again, as long as
well-collimated light is used), as shown in
Figure 1.5
.
Overhanging features (e.g., a cantilever) can be made by depositing a sacriicial
shadow mask (e.g., an opaque pad of metal) on unexposed SU-8. hus, the areas shad-
owed by the pad are not cross-linked by subsequent exposures to light (of that layer or
subsequently spun layers).
1.3.5 Biocompatible Photoresists
Most types of photoresists are not biocompatible—cells die when cultured on top of them and
they are not authorized for use in implantable devices. However, there are some notable excep-
tions:
photosensitive poly(dimethyl siloxane)
(PDMS),
Ormocer
, and
water-soluble photo-
resists based on poly (N-isopropylacrylamide) (poly-NIPAM)
.
As we will see in Section 1.6.2, PDMS is an extremely important polymer in BioMEMS,
mostly patterned by molding from a microfabricated master because it cures ther-
mally by a very simple procedure. However, it can also be photosensitized by the
addition of a photo-cross-linker, which circumvents the molding step and allows for
precise alignment of multilayer structures.
Ormocer
(short for “organically modiied ceramics”) was developed by the
Fraunhofer Institute in Germany, originally as a dental implant polymer. It is a nega-
tive resist requiring ~170°C curing temperatures. Two dental illing composites based
on Ormocer (Deinite and Admira) are available.
Recently, Stefan Diez's group at the Max Planck Institute of Molecular Cell Biology
and Genetics in Dresden, Germany, has developed a water-based processing of a pho-
toresist based on a modiied (photocleavable) copolymer of the thermoresponsive
polymer
poly-NIPAM
(
Figure 1.6
). Upon UV illumination, the copolymer's photo-
cleavable groups are deprotected, which results in an increased low critical solution
temperature. he nonirradiated and irradiated copolymer can be dissolved sequen-
tially in water at diferent temperatures, a process that is more biocompatible than
previous solvent-based processing.
