Biomedical Engineering Reference
In-Depth Information
nodes—these cells are located only 40-50
m deep in the epidermis. Vaccine
delivery directly to these cells provides a target of mass vaccination strategies.
6.
Precise control of drug quantities
—The small volumes of drug passing through the
microneedles, either through diffusion or through auxiliary pumping systems,
make the delivery process highly controllable.
Microneedles are presently at various stages of development, although they are yet to
be introduced clinically. They will surely revolutionize drug-delivery systems and provide
more options and possibilities to patients. The day is not far when administering of
polypeptides and even proteins will not be via injection or infusion.
10.8
Designing the Interface Microtools
Microneedle and microelectrode design is the most critical part of designing the bioengi-
neered system. It involves simultaneous consideration of multiple parameters. There is
always present some form of trade-off, for example ease of skin penetration vs. ease of
fluid flow and probe strength vs. biocompatibility. There is no specific optimum design; it
varies with each application. The combinatorial possibilities are vast, and the parametric
interdependency is highly complex. Every application will have a few structural or func-
tional demands that bear higher priority than others. Fortunately, not every application
needs a new microprobe design. Most applications can be grouped into classes having
broadly similar requirements. Thus, generic probes can be designed with specifications
that provide enough latitude for parametric variations within the class.
10.8.1
Functional Requirements or Constraints
Bioengineered interfaces will have to satisfy diverse functional requirements, a large major-
ity of them being either electrical or mechanical in nature. For the sake of discussion, these
requirements are thus broadly classified here as: (1) electrical parameters, (2) mechanical
parameters, and (3) other parameters.
10.8.1.1 Electrical Parameters
Electrical considerations take prominence for sensing applications. The foremost aim for
such applications is optimizing accuracy and reliability of retrieved data. One of the first
factors that affect electrode design is whether it is used in galvanic or nongalvanic contact
with tissue. In galvanic contact, the electrode is the site of shift from electronic to ionic con-
duction. The ionic conduction may be in an applied coating of conduction liquid or in nat-
ural tissue fluids surrounding the electrode. Galvanic contact generally relates to highly
environmentally sensitive probing. It entails cleaning procedures or surface preparation.
The electrode array should thus be designed accordingly (spacing and material).
Nongalvanic contact is often characteristic of a thin isolating layer of Teflon or glass.
Coupling is by capacitive or inductive fields, or by electromagnetic waves. Such electrodes
might employ high voltages and thus have to be designed such. Also, they are prone to
stray capacitances etc. and the array design has to compensate accordingly. Depending on
the application, probes may have to satisfy various electrical specifications, which might
include parameters such as electrical conductivity, resistance, effective electrode area,
