(Iler 1979). These cyclic species whose formation is most likely promoted during
enzymatic (silicatein-mediated) silica formation (Schr€ oder et al. 2010) hence
become the preferential sites for the addition of further silicic acid molecules.
10.4 Biosilica
Biosilica is a biogenic material, in contrast to bioglass used in bone/tissue
engineering. The inorganic phase of biosilica glass consists of amorphous silica
(SiO 2 ). This inorganic material had already been used about 800 million years ago
(during the Proterozoic) to construct ancient animal skeletal systems (Wang et al.
2010). Today, biosilica can be found in particular in plants, algae, and sponges
(Fig. 10.1b–d) (reviewed in: M€ uller 2003; Schr€ oder et al. 2008). The siliceous
sponges are unique among these organisms in their capability of forming their
siliceous skeleton (consisting of needle-like spicules and biosintered, higher order
structures formed by these elements; Fig. 10.1b, c) through an enzymatic mechanism (see below; reviewed in: Morse 1999; M€ uller et al. 2007b, 2009b; Schr€ oder
et al. 2008). The sponge biosilica is characterized by a quartz-glass-like purity
(M€ uller et al. 2008a). This high purity, in addition to its extreme stability (based on
the fact that this composite material contains, besides inorganic silica, an organic
component) makes this biomaterial of interest for various applications in nanobiotechnology and nano-biomedicine (Schr€ oder et al. 2007a; M€ uller et al. 2009b).
In order to form their biosilica skeletons, which may reach a size of up to 3 m
(example: basal giant spicules of the glass sponge Monorhaphis chuni; a cross section
through a spicule with a diameter of 10 mm is shown in Fig. 10.1d; M€ uller et al.
2008a), the siliceous sponges must accumulate silicon from the environment. In
marine waters, which are silicon poor, accumulation of silicon, in the form of
orthosilicic acid, requires an active transport mechanism. In the marine demosponge
Suberites domuncula, a silicic acid transporter has been identified, which acts as
a co-transporter of Si(OH) 4 and Na
+ ions (Schr€ oder et al. 2004). In diatoms, silicate
uptake is also an energy-consuming process, but uses a different transporter
(Bhattacharyya and Vulcani 1980; Thamatrakoln et al. 2006; Gr€ oger et al. 2007).
Though increasing evidence demonstrates the importance of silicon in controlling
mammalian bone formation (reviewed in: Jugdaohsingh 2007), the mechanism
of uptake of silicon in mammalian cells is not yet known. It is likely that only
monomeric silica (orthosilicic acid or orthosilicate) is taken up by eukaryotic cells.
So far, there are no hints that accumulation of silicon occurs via a passive influx of
silicic acid/silicate into cells. Hence, the existence of an energy-dependent silicic acid
transporter like in sponges has to be assumed. In this context, it should be mentioned that the sponge silicic acid transporter is highly related to the mammalian
Na
+ /HCO 3
À co-transporters (Schr€ oder et al. 2004). In addition, a role of aquaporins
in silicic acid transport might be conceivable (Sasaki 2008; Bhattacharjee et al.
2008). There are, at present, no hints that silicon accumulation into human cells
occurs via uptake of silica nanoparticles. Biosilica nanoparticles formed by sponge
288
H.C. Schr€ oder et al.
enzymatic (silicatein-mediated) silica formation (Schr€ oder et al. 2010) hence
become the preferential sites for the addition of further silicic acid molecules.
10.4 Biosilica
Biosilica is a biogenic material, in contrast to bioglass used in bone/tissue
engineering. The inorganic phase of biosilica glass consists of amorphous silica
(SiO 2 ). This inorganic material had already been used about 800 million years ago
(during the Proterozoic) to construct ancient animal skeletal systems (Wang et al.
2010). Today, biosilica can be found in particular in plants, algae, and sponges
(Fig. 10.1b–d) (reviewed in: M€ uller 2003; Schr€ oder et al. 2008). The siliceous
sponges are unique among these organisms in their capability of forming their
siliceous skeleton (consisting of needle-like spicules and biosintered, higher order
structures formed by these elements; Fig. 10.1b, c) through an enzymatic mechanism (see below; reviewed in: Morse 1999; M€ uller et al. 2007b, 2009b; Schr€ oder
et al. 2008). The sponge biosilica is characterized by a quartz-glass-like purity
(M€ uller et al. 2008a). This high purity, in addition to its extreme stability (based on
the fact that this composite material contains, besides inorganic silica, an organic
component) makes this biomaterial of interest for various applications in nanobiotechnology and nano-biomedicine (Schr€ oder et al. 2007a; M€ uller et al. 2009b).
In order to form their biosilica skeletons, which may reach a size of up to 3 m
(example: basal giant spicules of the glass sponge Monorhaphis chuni; a cross section
through a spicule with a diameter of 10 mm is shown in Fig. 10.1d; M€ uller et al.
2008a), the siliceous sponges must accumulate silicon from the environment. In
marine waters, which are silicon poor, accumulation of silicon, in the form of
orthosilicic acid, requires an active transport mechanism. In the marine demosponge
Suberites domuncula, a silicic acid transporter has been identified, which acts as
a co-transporter of Si(OH) 4 and Na
+ ions (Schr€ oder et al. 2004). In diatoms, silicate
uptake is also an energy-consuming process, but uses a different transporter
(Bhattacharyya and Vulcani 1980; Thamatrakoln et al. 2006; Gr€ oger et al. 2007).
Though increasing evidence demonstrates the importance of silicon in controlling
mammalian bone formation (reviewed in: Jugdaohsingh 2007), the mechanism
of uptake of silicon in mammalian cells is not yet known. It is likely that only
monomeric silica (orthosilicic acid or orthosilicate) is taken up by eukaryotic cells.
So far, there are no hints that accumulation of silicon occurs via a passive influx of
silicic acid/silicate into cells. Hence, the existence of an energy-dependent silicic acid
transporter like in sponges has to be assumed. In this context, it should be mentioned that the sponge silicic acid transporter is highly related to the mammalian
Na
+ /HCO 3
À co-transporters (Schr€ oder et al. 2004). In addition, a role of aquaporins
in silicic acid transport might be conceivable (Sasaki 2008; Bhattacharjee et al.
2008). There are, at present, no hints that silicon accumulation into human cells
occurs via uptake of silica nanoparticles. Biosilica nanoparticles formed by sponge
288
H.C. Schr€ oder et al.