Civil Engineering Reference
In-Depth Information
containing another material. Nanoparticles included in the solvent can become embed-
ded between the cellulose i brils, or attach to the surface of the cellulose i bers as the
solvent evaporates, essentially creating surface-modii ed cellulose. A variety of materi-
als have been used to modify bacterial cellulose by this impregnation method including
silica [151], and cadmium suli de [152] . h is can result in changes to the mechanical
properties of the cellulose. In addition, bacterial cellulose has been modii ed by sev-
eral materials in order to improve its properties for use in biomedical applications.
Montmorillonite [153], hydroxyapatite [154] and silver nanoparticles [155-157] have
all been used in this way. Montomorillonite impregnated-bacterial cellulose showed
improved water release rate, as well as mechanical and thermal properties [153].
Impregnation of bacterial cellulose by soaking pellicles in hydroxyapatite solution
resulted in an even covering of the cellulose by the hydroxyapatite. It was also found
that impregnation for 14 days, rather than seven days, led to increased hydroxyapatite
covering, and causing much thicker i brils as a result [154]. Modifying bacterial cel-
lulose with silver nanoparticles confered antimicrobial activity [155-157]. Similarly,
bacterial cellulose pellicles have been soaked in solution containing aniline with a vari-
ety of other materials to allow polyaniline to be polymerized directly onto the cellulose
i bers to achieve conducting bacterial cellulose composites [158-162].
Impregnation can be used as a method of directly modifying the surface of the bacte-
rial cellulose, but can also be used as a method of producing bacterial cellulose compos-
ites with other materials.
4.3.3.2
Chemical Modii cations
Bacterial cellulose is typically exposed to an alkaline treatment by being treated with
NaOH at er its removal from the growth media to remove any bacterial cell debris and
to sterilize the pellicle. McKenna
et al.
[163] set out to examine if this chemical treat-
ment was able to alter the cellulose. Despite the identii cation of some minor damage
to the cellulose i bers when visualized by SEM, they found that the low concentra-
tion of NaOH used for this process did not af ect the mechanical properties of the
cellulose. Nishi
et al.
[164], however, found that the treatment of bacterial cellulose
by a higher concentration of NaOH could actually improve its mechanical properties,
likely to be due to the NaOH removing the cell debris and allowing hydrogen bonds to
form within the cellulose due to increased close contact between the i bers. Conversely,
highly concentrated NaOH was found to cause degradation and decrease mechani-
cal properties. Similar results were seen with an oxidating treatment using an NaClO
solution, with even higher mechanical properties observed when both the oxidizing
and alkaline treatments were sequentially applied. h ere are other reports of chemical
modii cation of bacterial cellulose in the literature, using a variety of methods. Many
authors do not attempt to dissolve bacterial cellulose, but rather use it in its native form
and expose the i lm to a solvent exchange process. h is method, commonly completed
as part of an acetylation reaction [125, 165-168], involves the hydrophilic hydroxyl
groups being replaced with less hydrophilic acetyl groups. It consists of a progressive
soaking of the bacterial cellulose pellicle in a series of solvents, such as acetone, fol-
lowed by swelling in acetic acid with toluene and perchloric acid, and then exposure to
acetic anhydride [165]. However there has been a recent report of a solvent-free process
of acetylating bacterial cellulose. h is work involved bacterial cellulose being acetylated
