Biomedical Engineering Reference
In-Depth Information
capacitive properties, inductive properties, electromagnetic properties, half-cell potentials,
polarizability, and noise levels, among many others.
10.8.1.2 Mechanical Parameters
Bioengineered interfacing entails insertion of microelectrodes or microneedles into the
upper epidermal layers of the skin, and thus the probes have to be able to withstand the
applied pressure and puncture the skin without fracture. Optimum design demands
characterization of various mechanical features such as mechanical strength, buckling
characteristics, fracture point, elasticity, angular strength and ease of penetration.
Microneedles have to be further characterized for their capability to withstand fluidic
pressure at the sidewalls and the inlet pressure for a particular flow rate.
10.8.1.3 Other Parameters
Microcomponents also have to satisfy certain requirements that are neither electrical nor
mechanical in nature. Most significant of them is biocompatibility of the interface with
the skin. Interaction of nonbiocompatible sensors or electrodes with cells and tissue is
known to trigger “foreign body response” and foul the electrodes and systems.
Biologically, such a system constitutes a breach in the integrity of the organism and
would be responded to as a wound. Thus, it becomes difficult to determine whether one
is sampling “intracellular” fluid from impaled cells or “extracellular” fluid from needles
placed between cells. The most common approach to minimize biofouling is surface pas-
sivation of the functional materials.
Microfluidic properties of microneedles are also important design factors. The design has
to take into account fluid flow characteristics for the passing fluid and also environmental
flow resistance. There should be no clogging of the fluid at orifices or along the needle
lumens. Also capillary forces have to be sufficient to ensure fluid delivery or extraction.
Other common parameters affecting microcomponent design include adhesive interac-
tions (adhesion of analytes to the probe, unwanted adhesion of the probe to the skin, or
other structural parts), life duration, chemical inertness, and biodegradability.
10.8.2
Design Variables
Common variables that control the component characteristics can be broadly classified
into: (1) Material Selection, (2) Geometric Considerations, and (3) Array Layout.
10.8.2.1 Material Selection
There is no one best material available for the fabrication of microelectrodes or needles. The
choice varies according to the application. Metallic microprobes are popular due to strength
and biocompatibility; they have the lowest probability of causing allergic reactions. They
are relatively easy to fabricate, common methods being micromolding or electroforming.
Within metals, there is a wide range of material choices.
Silver is one of the best materials available for current carrying bioapplications. Silver
electrodes are usually coated with AgCl using electrolytic deposition. As body fluids con-
tain Cl
, AgCl forms a nonpolarizable electrode capable of passing currents with less over-
voltage. However, in this case, biocompatibility may be compromised. Platinum is the
metal most preferred for biocompatibility-sensitive applications due to its inertness.
However, it can be highly polarizable by DC currents in a physiological environment.
Nevertheless, it is extremely suited to DC potential reading applications under strict
