NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 297

have been used to overcome these challenges, are described in the next
four sections.
4.1.1 . Sample hydration
Most biological materials are hydrated in their native state, and many
studies have demonstrated the importance of maintaining sample
hydration in biological materials during nanoindentation. 28,41,48-53 The
mechanical properties of mineralized tissues and plants, which have a
low water content relative to soft tissues, have been shown to be strongly
influenced by their degree of hydration. 41,48,49,51-53 The mechanical
properties of soft tissues, which are made up predominantly of water, are
even more heavily influenced by their water content. For example, a
study of fully demineralized dentin showed a 4-fold increase in modulus
after dehydration, with only partial recovery of mechanical properties
after rehydration. 28 Hence, the ability to maintain hydration of soft tissue
samples and other biological materials during both sample preparation
and the nanoindentation process is critical for accurate mechanical
characterization. The fluid used to hydrate the sample has also been
shown to influence the mechanical properties of soft tissues, 19,27,48,54 so
care must also be taken in selecting an appropriate hydration fluid for the
experiment.
The most common method utilized to address this issue of sample
hydration is to indent a submerged sample contained in a fluid
cell. 26,28,48,49,51,53-56 However, submerging the sample complicates data
acquisition and analysis due to the variable meniscus forces acting on the
shaft of the indenter tip. 53,57 In addition, this method requires specialized
tips with long shafts to protect the transducer from contact with the
hydrating fluid and can also make surface detection difficult 57 (see
Section 4.2.1). To avoid these and other challenges, some researchers
have used other approaches to maintain sample hydration during
indentation testing. These methods include applying several drops of
fluid to a surface prior to starting the indent process, 58 removing fluid
from the surface of submerged samples just prior to initiating the indent
process, 20,27 and hydrating the samples from the edges using fluid-soaked
gauze, 18 hydrating foam, 24,25,50 or specialized irrigations systems. 59,60