# Nanostructured Mesoporous Silicas for Bone  Tissue Regeneration

Authors: Isabel Izquierdo-Barba; Montserrat Colilla; María Vallet-Regí
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

The research on the development of new biomaterials that promote bone tissue regeneration is receiving great interest by the biomedical scientific community. Recent advances in nanotechnology have allowed the design of materials with nanostructure similar to that of natural bone. These materials can promote new bone formation by inducing the formation of nanocrystalline apatites analogous to the mineral phase of natural bone onto their surfaces, i.e. they are bioactive. They also stimulate osteoblast proliferation and differentiation and, therefore, accelerate the healing processes. Silica-based ordered mesoporous materials are excellent candidates to be used as third generation bioceramics that enable the adsorption and local control release of biological active agents that promote bone regeneration. This local delivery capability together with the bioactive behavior of mesoporous silicas opens up promising expectations in the bioclinical field. In this review, the last advances in nanochemistry aimed at designing and tailoring the chemical and textural properties of mesoporous silicas for biomedical applications are described. The recent developed strategies to synthesize bioactive glasses with ordered mesopore arrangements are also summarized. Finally, a deep discussion about the influence of the textural parameters and organic modification of mesoporous silicas on molecules adsorption and controlled release is performed.


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## Body

## 1. Introduction

Currently,
the biomedical scientific community is demanding new advances in the design of third generation bioceramics for bone tissue regeneration technologies [1]. The
main goal consists in tailoring bone grafts that mimic as much as possible natural
bone tissue biomineralization processes. This involves the growth of
nanocrystalline carbonate hydroxyapatites similar to the biological ones onto
the materials surfaces, that is,
bioactive behavior. Moreover, these biomaterials should stimulate osteoblasts
adhesion, proliferation, and differentiation to reduce the postoperative periods [1–6].Natural hard tissues in vertebrates are natural composite materials,
composed of an organic matrix and an array of inorganic nanoapatites. The
inorganic phase consists of nanocrystalline apatites with average length of 50 nm, width of 25 nm,
and thickness of 2–5 nm [7]. Such
inorganic phase is located into an organic matrix mainly consisting of type I
collagen [8, 9]. The type I collagen molecules are bonded forming linear chains
that are in turn arranged in fibers, giving rise to various macroscopic
structures and leaving small empty interstitial compartments where needle-like apatite
nanocrystals are deposited during a controled biomineralization process. The
integration of both organic and inorganic phases at the nanometric scale
modulates the mechanical properties of each kind of bone. The physiological
formation mechanism of biological nanocrystalline nanoapatites proposed by
Brown [10, 11] involves the growth of biological apatites from two metastables
intermediates. Thus, according to the model proposed by Brown for in vivo conditions, the first compound formed onto the bioactive material surface consists in amorphous calcium phosphate
which is subsequently transformed into octacalcium phosphate and, finally, the
hydrolysis step leads to the formation of a nanocrystalline calcium deficient
carbonate hydroxyapatite
[12, 13]. Bone is a dynamic tissue that is constantly being formed and resorbed
and this remodeling process is mainly governed by osteoblasts and osteoclasts
cells [14, 15]. Osteoblasts govern bone formation processes whereas osteoclasts
modulate bone resorption phases and both processes are controled by mechanical
and hormonal stimuli. Skeletal tissues display different arrangement and
porosity to maintain bone performance, including mechanical, biological, and
chemical functions, such as mechanical support, protection of essential body
organs, and storage of mineral ions, mostly calcium and phosphate. Bone has a
hierarchical organization at many levels of scale which is essential to allow
an appropriate physiological performance [16, 17]. Therefore, pore sizes in
cortical bone range from 1 μm to 100 μm, being 200 μm to 400 μm in trabecular
bone. Moreover, trabecular bone shows 50–95% of porosity,
where 55–70% of such
pores are interconnected.The
efforts committed to develop bioceramics that could promote new bone formation began
in 1971 when Hench reported on the first bioactive composition [18]. Bioactive
materials exhibit highly reactive surfaces that are able to bond to living tissues through
the formation of nanocrystalline calcium deficient carbonate hydroxyapatites
similar to the biological ones. The newly formed apatite, together with the adsorption
of biological moieties, constitutes an excellent layer for the action of
osteoblasts and the subsequent bone-implant fixation. During the last decades,
the development of bioactive materials has allowed important advances in the fields of bone and dental grafting for hard tissue replacement [19–21]. However, the
new trends go further than just ensuring the implant-bone fixation. Therefore,
bone tissue engineering is a new interdisciplinary research field that emerged
because of the excellent potential of certain materials to repair diseased or
damaged tissue via regeneration rather than replacement. Such interdisciplinary
research area is mainly based on the design of 3D-scaffolds made of
biocompatible and bioactive materials that incorporate biologically active
agents (drugs, cell, gens, and proteins) to stimulate bone tissue regeneration [22, 23].
3D-scaffolds will play a significant role in bone tissue regeneration by
preserving tissue volume, providing temporary mechanical function, locally delivering
biologically active agents, and allowing regenerated tissues to assume their function as
scaffold degrades [24]. Thus, one of the main challenges for the scientific
community is the development of scaffolds with 3D interconnected porosity and
with pores similar in size and number to those existing in natural bone. The
size range of these pores is quite wide (1–1000 μm) and at the moment there is a lack of consensus regarding
the ideal porosity necessary for bone in growth [25].Up to date, several porous
bioceramics, such as calcium phosphates, glasses, cements, or different
biphasic combinations exhibiting open texture and high surface area and
porosity, have been reported as excellent candidates to be used as local
carriers for local controled delivery systems [1, 26, 27]. Recently, supramolecular chemistry has allowed
the emergence of a new generation of advanced bioceramics which exhibit fascinating properties for regenerative purposes.
Among such bioceramics,
ordered mesoporous materials show potential features to be used as starting
materials for the subsequent design and fabrication of scaffolds for bone
tissue engineering purposes. These materials exhibit enhanced bioactive capability,
and a very precise control over loading and controled release of biologically
active agents can be achieved (see Figure 1). This review deals with the recent
nanotechnological advances aimed at tailoring the structural, textural, and
chemical properties of nanostructured ordered mesoporous silicas with biomedical
purposes. To reach this goal, two main aspects concerning mesoporous silicas
will be tackled: (i) bioactive behavior
and (ii) capability of loading
and releasing biologically active molecules in a sustained fashion. Such
properties will be described and critically analyzed on the basis of their
textural and structural properties and on their composition and the possibility
of undergoing organic modification.Figure 1
Schematic illustration of functional scaffold based on ordered mesoporous silica for bone regeneration with hierarchically meso- and macroporosity and its corresponding functionality.


## 2. Silica-Based Ordered Mesoporous Materials

Since the discovery of
silica-based ordered mesoporous materials in the early 1990s by scientists of
the Mobil Corporation [28] and Waseda University
[29], these
materials have attracted enormous interest. Such nanostructured mesoporous
materials have been widely employed in catalysis as host matrices of several
guest molecules. Recently, the potential applications of mesoporous silicas as
molecule carriers have been expanded into the biomedical field for bone tissue
regeneration purposes [3].Silica-based mesoporous
materials constitute a new generation of materials that show ordered
arrangements of channels and cavities of different geometry built up fromSiO2 units [30]. These materials exhibit variable pore sizes (2–50 nm), high
surface areas (ca.1000m2/g), high pore
volumes (ca.1cm3/g), and
homogeneous nanostructures, which can be tailored by varying the synthesis
procedure [31]. The synthesis of silica-based
ordered mesoporous materials is based on the formation of liquid crystalline
mesophases of amphiphilic molecules (surfactant) that serve as template for the
in situ polymerization of
orthosilicic acid [32]. Well-known examples are
MCM-41 and MCM-48 materials, which are synthesized by using cationic surfactants
under basic conditions. These mesoporous matrices exhibit pore sizes of ca. 2.5 nm and different pore
arrangements: 2D-hexagonal (p6mm plain
group) for MCM-41 and 3D-bicontinuos cubic (Ia–3d space
group) for MCM-48 [32, 33]. On the contrary, the employment of nonionic
surfactants and acid conditions lead to an increase in the pore wall thickness
and to an enlargement of the pore size (ca. 10 nm) of the resulting mesoporous matrices [34]. SBA-15 [35] and FDU-5 [36]
materials constitute two representative examples of such mesoporous materials.
SBA-15 exhibits 2D-hexagonal structure with micropores interconnecting adjacent
mesopores and FDU-5 presents a 3D-bicontinuos cubic structure similar to that of MCM-48.It was in 2001, owing to their outstanding adsorptive
properties, when mesoporous silicas were proposed for the first time as drug
delivery system and ibuprofen, a common anti-inflammatory, was loaded into the pore
channels of MCM-41 and subsequently released in a sustained manner [37]. Since
then, this research field has experienced a significant growth and much effort
has been devoted to tailor the nanostructure and textural properties (i.e., pore diameter, surface area,
and pore volume) of mesoporous materials that allow to achieve a better control
over molecule loading and controled release. All the parameters that govern the
adsorption and release kinetics of biologically active molecules, with special
emphasis on the organic functionalization of the silica walls, will be tackled
in this review.Moreover, the bioactive behavior of silica-based ordered
mesoporous materials is undoubtedly an added value when considering these materials for bone tissue
regeneration technologies. The combination of both properties, that is, bioactivity and controled delivery
capability, is a remarkable synergy that makes mesoporous silicas excellent
candidates to be used as starting materials for the manufacture of 3D-scaffolds
for bone tissue engineering [38]. The different chemical strategies developed
to accelerate the bioactive response of silica-based ordered mesoporous
materials will be also discussed in this manuscript.


### 2.1. Bioactivity of Silica-Based Ordered  Mesoporous Materials

Silica-based mesoporous materials are characterized by exhibiting high
surface areas and high surface density of silanol groups (Si–OH). It is well known
that silanol groups, which are present in conventional bioactive glasses, are
able to react with physiological fluids to produce nanometer-sized carbonated
apatites [21, 39]. Therefore, nanostructured silica-based ordered mesoporous
materials, which present high amounts of silanol groups covering their
surfaces, are expected to induce bioactive responses. It was in 2006 when several
in vitro bioactivity assays
were performed by soaking three mesoporous materials, SBA-15, MCM-48, and MCM-41,
into simulated body fluid (SBF) [40]. The obtained results revealed that SBA-15
and MCM-48 developed a nanocrystalline carbonate hydroxyapatite layer onto
their surfaces after 30 and 60 days, respectively. On the contrary, no evidence
of formation of an apatite-like layer was observed onto the MCM-41 surface. The
different bioactive behavior of SBA-15, MCM-48, and MCM-41 matrices is related
to the concentration of silanol groups, and also to the textural and structural
properties of the mesoporous matrices. In this case, MCM-41 showed lower concentration
of silanol groups (c.a.2×10−3mmolSiOH⋅m−2) than SBA-15 and MCM-48 (c.a.13×10−3mmolSiOH⋅m−2), which could
explain the absence of bioactivity in MCM-41 after 60 days. In addition, the
textural and structural properties also contribute to the bioactive behavior,
since large and accessible pores systems favor the ionic diffusion into the
mesoporous framework and consequently the apatite formation is accelerated. Hence,
SBA-15, which shows pore size around 9 nm and 3D-pores system with interconnected
microporosity, and MCM-48, with 3D-cubic pores system exhibited bioactive
response. On the contrary, MCM-41, which presents 2D-pores system with pore
sizes close to 2.5 nm did not show bioactive behavior after 60 days of assay. In
fact, traditional pure silica mesoporous materials such as SBA-15 and MCM-48
showed bioactive behavior, but the rate of formation of the hydroxyapatite
layer is too slow (30–60 days) compared
to that of conventional sol-gel bioactive glasses (3 days). Figure 2 displays the bioactivity studies
using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy-energy
dispersive spectroscopy (SEM-EDS) techniques of a conventional sol-gel glass in
the system SiO2-CaO-P2O5 versus SBA-15 mesoporous matrix. The
results indicated that the band at 600cm−1, which corresponds to
amorphous calcium phosphate, began to split into a doublet at 560–600cm−1 after 3 days in the case of conventional sol-gel glass and after 30 days in the
case of SBA-15. Moreover, SEM-EDS studies revealed the formation of spherical
agglomerate of particles consisting of needle-like crystals with a Ca/P ratio
of 1.5 onto the materials surfaces after different periods of time, confirming
the formation of a calcium-deficient nanocrystalline apatite layer.Bioactivity study comparing a conventional sol-gel glass in theSiO2-CaO-P2O5 system and SBA-15 mesoporous material. (a) FTIR study corresponding to the
surface of both materials before and after several times soaked in simulated body
fluid (SBF). (b) SEM and EDS studies corresponding to the surface of both materials
after 3 and 30 days in SBF, respectively.
(a)(b)These findings suggest that high surface areas and porosities are not
enough conditions to achieve satisfactory biomimetic behavior. Consequently,
different chemical strategies have been achieved to promote the bioactive
response of silica-based mesoporous materials and, therefore to accelerate
their bioactive kinetics. Some approaches consisted in the incorporation ofP2O5 units into the silica mesoporous network [41] or the addition of small amount
of bioactive sol-gel glasses [42]. However, the real advance has been to
synthesize SiO2-CaO-P2O5 sol-gel glasses with
the outstanding textural properties (surface area of 195–427 (m2⋅g−1)
and pore volume of 0.46–0.61cm3⋅g)
and ordered porous arrangements of silica-based mesoporous materials [43, 44].
In this sense, the synthesis of ordered mesoporous glasses was carried out
employing the evaporation-induced self-assembly (EISA) method [45]. The
synthesis involved the use of a nonionic triblock copolymer surfactant (EO20PO70EO20)
as structure-directing agent and the employment of tetraethyl orthosilicate (TEOS), triethyl
phosphate (TEP), and calcium nitrate, Ca(NO3)2⋅4H2O
as SiO2, P2O5, and CaO sources, respectively. The textural and structural properties of
this new family of mesoporous glasses can be controled by changing the CaO
content of mesoporous glasses. A progressive evolution from 2D-hexagonal to
3D-bicontinuous cubic structure (see Figure 3) with an increase in the textural
properties was observed when decreasing the CaO content
[46]. The possibility of tailoring both structural and textural features of
mesoporous glasses is undoubtedly an attractive advance towards the development
of biomaterials that is able to fulfill the essential requirements for specific biomedical
applications.TEM images and their corresponding FT diffractograms of mesoporous
glasses in theSiO2-CaO-P2O5 system with
different CaO amounts. (a), (b) TEM images taken with the electron beam parallel
and perpendicular to the pore channels of a 2D hexagonal structure corresponding to a mesoporous glass with 58SiO2-37%CaO-5%P2O5 composition; (c), (d) TEM images taken in
the 100 and 111 directions of 3D bicontinuous cubic structure with Ia–3d space group
corresponding to a mesoporous glass of 85SiO2-10%CaO-5%P2O5 composition [43].
(a)(b)(c)(d)It has been evidenced that this new family of mesoporous glasses exhibits enhanced
bioactive behavior, with even faster apatite phase formation than conventional
bioactive sol-gel glasses. Besides, it has been revealed that the kinetics of
apatite formation in mesoporous glasses is governed by their textural and
structural properties, differing from conventional bioactive glasses, where the
formation kinetics depended on compositional and textural properties. In this
sense, the bicontinuous network in 3D-cubic mesoporous glass  with low CaO
content (10% in mol), which exhibits
high accessibility to the diffusion process in pore systems, has led to the most
accelerated bioactive response reported up to date [46]. Transmission electron microscopy
(TEM) studies revealed that the formation of the crystalline apatite formation
took place after only 4 hours of assay (see Figure 4). Their bioactive response was faster than
mesoporous glasses with higher CaO content and 2D-hexagonal structure.
Moreover, Figure 4 also shows the Ca2+ concentration and pH
evolutions in SBF as a function of soaking time. It should be highlighted that there
was not a significant increase
in calcium and pH, which remained almost constant exhibiting values of 7.45 and
100 ppm, respectively [46]. This is a very important factor to avoid the
cytotoxic effect that high local pH values could produce in living tissues.
On
the other hand, mesoporous glasses with 2D-hexagonal pore arrangements and high CaO
content (37% in mol) provided a biomimetic mechanism where a sequential
transition from amorphous calcium phosphate (ACP) to octacalcium phosphate (OCP) and to calcium deficient carbonate hydroxyapatite (CDHA) maturation, similar to the in vivo bone biomineralization
process, was observed by TEM (see Figure 5) [47]. Such ACP-OCP-CDHA maturation sequence had
never been observed before in any other bioactive ceramic. In this case, the
open channel array and the textural parameters of 2D-hexagonal structure
together with high CaO content allow an intense Ca2+–H3O+ exchange that results in local acid pH values that favor the OCP phase
formation.Bioactive in vitro study corresponding to an ordered mesoporous glass
with 3D bicontinuous cubic structure and85SiO2-10%CaO-5%P2O5 composition. (Inset) schematic representation of 3D cubic pore arrangement. (a)
Variation of calcium content and pH values of SBF after soaking the materials
during different periods. (b) HRTEM study corresponding to the surface of
mesoporous glasses after 4 hours soaked in SBF. Low magnification image
indicates the needle-like crystals formation on the surface of mesoporous
glass. Higher magnification and FT transform showing the (002), (211), and
(112) reflections corresponding to d-spacings of 0.34, 0.28, and 0.27 nm of an
apatite phase.
(a)(b)Bioactive
in vitro study corresponding to
an ordered mesoporous glass with 2D hexagonal structure and58SiO2-37%CaO-5%P2O5 composition. (a) Schematic representation of 2D hexagonal pore arrangement. (b) Variation of pH as a function of time at
the bioceramic surface and SBF, highlighting the pH range where OCP phase is
more stable. (c) HRTEM study after different times of incubation showing a
sequential transition from ACP to OCP and to CDHA. For each time, a low
magnification image, a higher magnification and its corresponding FT transform are shown.
(a)(b)(c)Therefore, the possibility of tailoring the reactive properties of
mesoporous glasses as a function of their textural, structural, and
compositional properties allows to obtain materials with high
biomimetism-derived biocompatibility. In addition, the fast and intense
apatite-phase formation makes mesoporous glasses groundbreaking materials that
will probably point out the future guidelines for the synthesis of the next
bioactive bioceramics.


### 2.2. Silica-Based Ordered
Mesoporous Materials as Controled Delivery Systems

Since 2001, when MCM-41 was purposed for the first time as controled
delivery system [37], much research effort has been
devoted to tailor the chemical properties of mesoporous carriers at the
nanometer scale to achieve a better control over loading and release of
molecules. The mesoporous carrier is selected according to the features of the
guest molecule and the targeted application. Molecule loading is usually
performed by impregnation methods by soaking the mesoporous carrier into a
concentrated molecule solution. Loading solvent is selected attending to the
chemical nature of the guest molecule [48]. Then, the release process is
performed by placing the molecule-loaded mesoporous material into SBF solution
[49], which exhibits ionic concentrations similar to those in the human plasma, or in physiological
serum to make the
detection process easy. Therefore, different guest molecules have been
successfully confined into mesoporous silicas. Some of these molecules are
drugs, such as ibuprofen [37, 50–52], amoxicillin [53],
gentamicin [54, 55], erythromycin [51, 56], vancomycin [57], naproxen [58],
aspirin [59, 60], diflunisal [58], captopril [61], itraconazole [62], and alendronate
[63, 64]. Other guest molecules consisted of biologically active species, such
as proteins (bovine serum albumin (BSA) [65–67] and certain
amino acids (L-tryptophan (L-Trp) [68]).The textural properties (i.e.,
pore diameter, surface area, and pore volume) of mesoporous materials are key
factors that govern molecules adsorption and release [69, 70]. Moreover,
functionalization of silica walls using different organic groups has been
revealed as the main strategy to modulate molecule loading and release.


#### 2.2.1. Influence of Textural Properties

The textural properties of mesoporous materials (pore diameter, surface
area, and pore volume) play an important role in molecule adsorption and
release. The influence of all these parameters on performance of controled
delivery systems will be discussed in the next sections.(a) Pore Diameter
The adsorption of molecules into mesoporous carriers obeys size
selectivity criteria, that is,
pore diameter will determine the size of the guest molecule to be hosted. Most
of the drugs commonly used in biomedicine are in the range of nanometers.
Therefore, they could be easily confined into the pores of nanostructured mesoporous
silica. However, pore diameter is a limiting factor when the adsorption of
large molecules, such as proteins, is aimed. This is the case of serum
albumins, which are one of the major components in plasma proteins. They are
involved in several relevant physiological functions of humans and upper
mammals, such as binding, transport and delivery of biologically active
molecules, including drugs, to specific places in the body [71]. Serum albumins
are usually composed of single chains of 582 amino acids and exhibit average length of 10 nm and width
of 6 nm [72, 73]. The adsorption of globular proteins, such as bovine serum
albumin (BSA) onto MCM-41 matrices, which exhibits pore diameters in the 2–5 nm range, has
been reported [74, 75]. The small diameter of MCM-41 impeded the confinement of
the protein into the mesopores and consequently, the protein was adsorbed on
the outer surface of the mesoporous carrier.
These results evidenced that when the confinement of large size proteins
into mesoporous channels is targeted, large pore matrices are needed. Our
research group recently applied an improved method to increase the pore
diameter of SBA-15 and BSA adsorption and release tests were performed [67].
The increase in the mesopore diameter was achieved by increasing the time of
hydrothermal treatment during the mesoporous synthesis, as described elsewhere
[76, 77]. The pore diameter of the resulting SBA-15 materials ranged from 8.2
to 11.4 nm employing hydrothermal treatments ranging from 1 to 7 days,
respectively. BSA loading tests revealed that there was a direct dependence of
protein adsorption on pore size, that
is, the greater the pore diameter, the higher the amount of protein
loaded. Therefore, conventional SBA-15, which presented pore sizes of 8.2 nm,
loaded 151 mg/g of BSA. The amount of BSA loaded increased to 234 mg/g, 242 mg/g, and 270 mg/g for mesoporous SBA-15 materials with pore sizes of 9.5 nm,
10.5 nm, and 11.4 nm, respectively (see Figure 6.a.1). In vitro delivery tests were carried out by soaking disc-shaped
mesoporous matrices in saline solution (NaCl 0.9%, pH 7.4). Release patterns
exhibited and initial burst profile where more than 90% of the adsorbed protein
was quickly released during the first 24 hours, followed by a sustained release
and the complete delivery was achieved after 192 hours of assay in all tested
samples.
Pore diameter has been revealed as a critical factor that modulates
release rate of molecules to the delivery medium. This fact was firstly
evidenced when MCM-41 materials exhibiting different pore sizes ranging from
2.5 to 3.6 nm were synthesized using cationic surfactants with different length
alkyl chains [78]. Ibuprofen, a common anti-inflammatory,
was selected and release tests were carried out in a simulated body fluid (pH
7.4) [49]. The resulting release profiles, which are displayed in Figure 6.a.2,
revealed that release kinetics strongly depended on mesopore size. Thus, the
amount of ibuprofen released to the delivery medium diminished as the pore size
decreased from 3.6 to 2.5 nm, evidencing that drug dosage can be modulated
depending on the mesopore size.Influence of textural properties. (a) Pore diameter, (b) surface area, and (c) pore volume, on molecule adsorption and release.
(a)
Influence of pore diameter(b)
Influence of surface area(c)
Influence of pore volume(b) Surface Area
The adsorption of molecules into mesoporous matrices depends on the
adsorptive properties of the silica surface. Therefore, the chemical
interactions between the silanol groups covering the silica surface and the
functional groups of the guest molecule will determine the amount of molecule
loaded. Surface area of mesoporous matrix is the textural parameter that points to the silica
surface susceptible to interact with the guest molecule and, consequently, it
is expected that the higher the surface area, the higher the amount of molecule
loaded. This fact was confirmed when several MCM-41 matrices with different
surface areas were employed as ibuprofen delivery systems [78]. MCM-41
mesoporous materials exhibiting surface areas of 768, 936, 1087, and 1157 m2/g
loaded 110, 190, 230, and 340 mg/g of ibuprofen, respectively, evidencing the
increase in the drug loading with the surface area increase.
With the aim of further establishing the effect of surface area on molecule adsorption, two
mesoporous materials, MCM-41 and SBA-15, which exhibit the same structure (2D-hexagonal,p6mm symmetry) but different surface
areas (1157 m2/g for MCM-41 and 719 m2/g for SBA-15) were
tested as alendronate delivery systems [63]. Alendronate is a potent
bisphosphonate commonly employed for osteoporosis treatments that inhibits bone resorption
by osteoclasts and it is a promising drug to be locally applied in bone implant
technologies. As displayed in Figure 6(b), the amount of alendronate loaded into
MCM-41 (139 mg/g) was higher than that into SBA-15 (83 mg/g), confirming the dependence of drug
loading on surface area. Release profiles from both mesoporous carriers into
the delivery medium (NaCl 0.9%, pH 7.4) exhibited an initial burst effect when ca. 20% and ca. 50% of alendronate loaded were quickly released from MCM-41 and SBA-15,
respectively (see Figure 6(b)). Afterwards, alendronate was released to the
delivery medium in a sustained manner, following first-order kinetics for MCM-41
and zero-order or linear kinetics for SBA-15 materials.(c) Pore Volume
As previously mentioned, the adsorption of molecules into mesoporous
matrices is a surface-molecule interaction [79]. For this reason, pore diameter,
as the size-selectivity limiting factor, and surface area are key parameters
that control the adsorption of molecules. However, when the confinement of
really large molecules is targeted, the pore volume available to host the guest
molecule plays also an important role in molecules adsorption.
Recently, mesostructured cellular foams (MCFs) have been tested as host
matrices for the adsorption of several enzymes and proteins [80, 81]. The
synthesis of MCF materials was carried out by using triblock copolymers as
surfactants and introducing a swelling agent, commonly trimethylbenzene, into
the structure directing template solution [82, 83]. TEM image of SBA-15 (see Figure 6(c)) revealed the presence of two-dimensional hexagonal arrays of pores,
typical of hexagonally ordered mesoporous materials with pore sizes of ca. 8 nm. In the case of MCF, spherical
cells of ca. 28 nm with windows of ca. 18 nm were observed (see arrows in Figure 6(c)). These matrices seem suitable to be used as delivery systems of
large-size molecules, such as proteins. The amount of BSA loaded was higher in
MCF (240 mg/g) than in SBA-15 (150 mg/g), following the same trend than pore
volume, being 1.9 cm3/g and 1.1 cm3/g for MCF and SBA-15,
respectively (see Table inset Figure 6(c)). This results evidence that pore
volume is an important parameter to consider when aiming at confining large-size
and large-volume molecules. However, the effect of other parameters, such as
pore diameter into protein loading cannot be overruled. The pore size of SBA-15
is just on the limit of BSA dimensions, which could complicate the protein
adsorption into the mesopores. However, such steric hindrance would not take
place when MCF, with a pore opening of 18 nm, is used as host matrix.


#### 2.2.2. Influence of Organic Functionalization

A
straightforward way to optimize the performance of mesoporous matrices as controled
delivery systems consists in organically modifying the mesoporous silica walls
with appropriate functional groups [69, 70]. On the surface of silica walls,
there is a high surface density of silanol groups that can experience organic
modification by covalently anchoring organic silanes, (RO)3SiR’ [84].
The adsorption and release of molecules can be effectively controled by using
the adequate organic functionalization. To reach this goal, two main strategies
have been widely employed. The first one involves the use of organic groups
that promote attracting host-guest interactions with the functional groups of the
guest molecule. The choice of the functional group would depend on the targeted
molecule. For instance, ibuprofen delivery tests from MCM-41 functionalized
with several organic groups (chloropropyl, phenyl, benzyl, mercaptopropyl,
cyanopropyl, and butyl) revealed that polar groups induced greater ibuprofen
adsorption than nonpolar groups [85]. The second strategy concerns the
functionalization of mesoporous silica walls using hydrophobic species. In the
next sections, some relevant results regarding functionalization using amino
groups and functionalization employing hydrophobic groups will be described.(a) Functionalization Using Amino Groups
Functionalization
using amino groups has been a widely employed strategy to attain a better
control over molecule loading and release. Therefore, amino functionalized MCM-41
and SBA-15 mesoporous matrices have been tested as alendronate delivery systems
[63]. The obtained results indicated that the amount of drug loaded into
amino-modified materials was almost 3 fold that of unmodified matrices. This
fact could be explained by the stronger attracting interactions taking place
between phosphonate groups in alendronate and amino groups of functionalized
matrices compared to the weaker interactions through hydrogen bonds between
phosphonate groups and silanol groups of unmodified materials. Therefore,
amino-modified MCM-41-NH2 loaded more alendronate (366 mg/g) than
unmodified MCM-41 (139 mg/g). Also, amino-modified SBA-15-NH2 matrix
loaded more amount of  drug (220 mg/g) than unmodified SBA-15 (83 mg/g). Moreover, drug release assays revealed
that amino functionalization of mesoporous silica allowed a better control on
the drug release. Thus, the amount of alendronate released after 24 hours was
28% and 58% of the total amount loaded for amino-modified and unmodified
MCM-41, respectively. In the case of SBA-15, the 11% and 56% of the total
alendronate loaded were
released from amino-modified and unmodified materials, respectively, after 24
hours of assay.
Recently,
the amino modification of SBA-15 mesoporous materials using different functionalization
degrees has been revealed as key factor to modulate alendronate dosage [64]. The
amino-modification method was improved by adding a catalyst during the
functionalization process. Alendronate loading as a function of experimental
functionalization degree is displayed in Figure 7. The amount of alendronate
loaded linearly increased as the functionalization degree increased, ranging
from 88 mg/g, for the lowest functionalization degree (0%), to 121 mg/g, for
the highest functionalization degree (47%). Moreover, functionalization degree
also allowed a better control over alendronate release (see Figure 7), that is, the higher the
functionalization degree, the smaller the percentage of alendronate released.
This work evidences the possibility of modulating drug dosage by varying the
functionalization degree of mesoporous matrix, which is particularly
interesting when using bisphosphonates since the high potency of these drugs
implies that only small local doses are needed.
The
organic modification of mesoporous matrices was observed to control release
kinetics of proteins, such as BSA [66, 67]. As previously described, SBA-15
materials exhibiting different pore diameters were obtained by varying the time
of hydrothermal treatment during their synthesis. These materials were then
modified using amino groups to favor attracting interactions with amide groups of protein. It should
be highlighted that organic functionalization always leads to a diminishing of pore
diameter and consequently, as BSA size is on the limit of SBA-15 mesopore
dimensions, after amino functionalization, the amount of BSA loaded decreased
compared to unmodified materials. However, organic functionalization of SBA-15
had a noticeable effect on BSA kinetic release rate. As a consequence of
organic functionalization, the initial burst effect observed in BSA release
from unmodified matrices (ca. 90%) was
drastically reduced after amino modification (ca. 30%). Moreover, the release of protein from the mesopores of
all amino-modified materials was incomplete, whereas the total amount of loaded
BSA was released after 192 hours from unmodified materials.
Moreover,
functionalization of large-pore MCF materials with amino groups has been
demonstrated to increase the affinity for BSA [81]. There was an increase in
BSA adsorption after amino-modification MCF-NH2 (270 mg/g) compared
to unmodified MCF (240 mg/g), despite of the decrease in pore diameter due to
the functionalization. However, amino functionalization has a strong influence
on BSA release rate. Unmodified MCF exhibited an initial burst effect when
almost 60% of the protein loaded was quickly released to the delivery medium. This burst effect
was drastically reduced to ca. 10%
after amino modification. After 24 hours of assay, 62% of the loaded BSA was
released from unmodified MCF material and this percentage decreased to 22%
after functionalization with amino groups.
These
studies evidence that organic functionalization of silica mesoporous matrices
using amino groups allows the control of release kinetics of several molecules
that exhibit functional groups able to undergo attracting interaction with
amino functions.Figure 7
Amount of alendronate loaded and percentage of alendronate released as
a function of functionalization degree.(b) Functionalization Using Hydrophobic Groups
The functionalization of mesoporous silica walls with hydrophobic groups
is an alternative to control the release of certain molecules. Therefore,
SBA-15 mesoporous matrix has been functionalized with different alkyl chains, octyl
(C8) and octadecyl (C18), and erythromycin, a hydrophobic antibiotic was
selected to carry out adsorption and delivery assays [56]. As previously
commented, organic modification led to a decrease of the effective pore
diameter and surface area of SBA-15. The decrease in surface area resulted in diminishing the amount
of erythromycin loaded, 130 mg/g and 180 mg/g for SBA-15-C8 and
SBA-15-C18, respectively, compared to unmodified SBA-15 (340 mg/g). Comparable
results were reported using mesoporous materials modified by silylation as
ibuprofen delivery systems, showing a lower drug loading after silylation [86].
In addition, as a consequence of functionalization with hydrophobic chains,
there was a decrease in the wettability of the surface, which made difficult
the penetration of the aqueous delivery solution inside the mesopores
originating a decrease in the  delivery rate. In fact, the erythromycin release
rate from SBA-15-C18 material was one order of magnitude lower than that from
unmodified SBA-15.
Recently,
functionalized SBA-15 has been proposed as delivery system ofL-tryptophan (L-Trp) [68], a hydrophobic amino acid present in the
three-dimensional structure of many peptides, proteins, and growth factors of
interest in bone tissue regeneration technologies [87–89]. L-Trp presents an aromatic indol ring
that makes necessary
to modify the silanol-rich walls of SBA-15. In fact, unmodified SBA-15 loaded less than
5 mg/g of L-Trp, probably due to the
extremely different chemical nature of hydrophobic amino acid and hydrophilic
SBA-15. The small amount of L-Trp
adsorbed into SBA-15 could be interactions through hydrogen bonds between
deprotonated carboxylic group of amino acid and silanol groups covering the
silica walls (see Figure 8(a)). For this reason, SBA-15 matrix was organically
modified using quaternary amines with different alkyl lengths (∼C3N+Me3 and ∼C3N+Me2C18),
as illustrated in Figures 8(b) and 8(c) [68]. Functionalization with short
alkyl chains (∼C3N+Me3)
allowed coulombic attracting interactions between deprotonated carboxylic
groups of amino acid (–COO−) and protonated quaternary amines (–N+R4)
covering the mesoporous surface. In this case, the amount of L-Trp loaded into SBA-15-C3N+Me3 matrix was higher (43 mg/g) than in unmodified SBA-15 (<5 mg/g). On the
other hand, using long hydrocarbon chains (∼C3N+Me2C18),
two-thirds of the silica surface was functionalized. This high degree of
functionalization with hydrophobic chains promoted interaction of mesoporous
surface with indol group of L-Trp,
and consequently, the amount of amino acid loaded increased to 82 mg/g.
Moreover, release profiles of amino acid from functionalized SBA-15 materials
are displayed in Figure 8(d). In both cases, there is an initial burst effect,
where most of the L-Trp loaded is
quickly released to the delivery medium. After such burst effect, the rest of
amino acid is released in a sustained manner, following first-order and
zero-order or linear kinetics from SBA-15-C3N+Me3 and SBA-15-C3N+Me2C18 matrices,
respectively.Molecular model of interaction betweenL-Trp and (a) unmodified SBA-15, (b) SBA-15 functionalized with
short-chain quaternary amines (∼C3N+Me3),
and (c) SBA-15 functionalized with long-chain quaternary amines (∼C3N+Me2C18).
(d) L-Trp release profiles from
functionalized samples are also displayed.
(a)(b)(c)(d)


## 2.1. Bioactivity of Silica-Based Ordered  Mesoporous Materials

Silica-based mesoporous materials are characterized by exhibiting high
surface areas and high surface density of silanol groups (Si–OH). It is well known
that silanol groups, which are present in conventional bioactive glasses, are
able to react with physiological fluids to produce nanometer-sized carbonated
apatites [21, 39]. Therefore, nanostructured silica-based ordered mesoporous
materials, which present high amounts of silanol groups covering their
surfaces, are expected to induce bioactive responses. It was in 2006 when several
in vitro bioactivity assays
were performed by soaking three mesoporous materials, SBA-15, MCM-48, and MCM-41,
into simulated body fluid (SBF) [40]. The obtained results revealed that SBA-15
and MCM-48 developed a nanocrystalline carbonate hydroxyapatite layer onto
their surfaces after 30 and 60 days, respectively. On the contrary, no evidence
of formation of an apatite-like layer was observed onto the MCM-41 surface. The
different bioactive behavior of SBA-15, MCM-48, and MCM-41 matrices is related
to the concentration of silanol groups, and also to the textural and structural
properties of the mesoporous matrices. In this case, MCM-41 showed lower concentration
of silanol groups (c.a.2×10−3mmolSiOH⋅m−2) than SBA-15 and MCM-48 (c.a.13×10−3mmolSiOH⋅m−2), which could
explain the absence of bioactivity in MCM-41 after 60 days. In addition, the
textural and structural properties also contribute to the bioactive behavior,
since large and accessible pores systems favor the ionic diffusion into the
mesoporous framework and consequently the apatite formation is accelerated. Hence,
SBA-15, which shows pore size around 9 nm and 3D-pores system with interconnected
microporosity, and MCM-48, with 3D-cubic pores system exhibited bioactive
response. On the contrary, MCM-41, which presents 2D-pores system with pore
sizes close to 2.5 nm did not show bioactive behavior after 60 days of assay. In
fact, traditional pure silica mesoporous materials such as SBA-15 and MCM-48
showed bioactive behavior, but the rate of formation of the hydroxyapatite
layer is too slow (30–60 days) compared
to that of conventional sol-gel bioactive glasses (3 days). Figure 2 displays the bioactivity studies
using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy-energy
dispersive spectroscopy (SEM-EDS) techniques of a conventional sol-gel glass in
the system SiO2-CaO-P2O5 versus SBA-15 mesoporous matrix. The
results indicated that the band at 600cm−1, which corresponds to
amorphous calcium phosphate, began to split into a doublet at 560–600cm−1 after 3 days in the case of conventional sol-gel glass and after 30 days in the
case of SBA-15. Moreover, SEM-EDS studies revealed the formation of spherical
agglomerate of particles consisting of needle-like crystals with a Ca/P ratio
of 1.5 onto the materials surfaces after different periods of time, confirming
the formation of a calcium-deficient nanocrystalline apatite layer.Bioactivity study comparing a conventional sol-gel glass in theSiO2-CaO-P2O5 system and SBA-15 mesoporous material. (a) FTIR study corresponding to the
surface of both materials before and after several times soaked in simulated body
fluid (SBF). (b) SEM and EDS studies corresponding to the surface of both materials
after 3 and 30 days in SBF, respectively.
(a)(b)These findings suggest that high surface areas and porosities are not
enough conditions to achieve satisfactory biomimetic behavior. Consequently,
different chemical strategies have been achieved to promote the bioactive
response of silica-based mesoporous materials and, therefore to accelerate
their bioactive kinetics. Some approaches consisted in the incorporation ofP2O5 units into the silica mesoporous network [41] or the addition of small amount
of bioactive sol-gel glasses [42]. However, the real advance has been to
synthesize SiO2-CaO-P2O5 sol-gel glasses with
the outstanding textural properties (surface area of 195–427 (m2⋅g−1)
and pore volume of 0.46–0.61cm3⋅g)
and ordered porous arrangements of silica-based mesoporous materials [43, 44].
In this sense, the synthesis of ordered mesoporous glasses was carried out
employing the evaporation-induced self-assembly (EISA) method [45]. The
synthesis involved the use of a nonionic triblock copolymer surfactant (EO20PO70EO20)
as structure-directing agent and the employment of tetraethyl orthosilicate (TEOS), triethyl
phosphate (TEP), and calcium nitrate, Ca(NO3)2⋅4H2O
as SiO2, P2O5, and CaO sources, respectively. The textural and structural properties of
this new family of mesoporous glasses can be controled by changing the CaO
content of mesoporous glasses. A progressive evolution from 2D-hexagonal to
3D-bicontinuous cubic structure (see Figure 3) with an increase in the textural
properties was observed when decreasing the CaO content
[46]. The possibility of tailoring both structural and textural features of
mesoporous glasses is undoubtedly an attractive advance towards the development
of biomaterials that is able to fulfill the essential requirements for specific biomedical
applications.TEM images and their corresponding FT diffractograms of mesoporous
glasses in theSiO2-CaO-P2O5 system with
different CaO amounts. (a), (b) TEM images taken with the electron beam parallel
and perpendicular to the pore channels of a 2D hexagonal structure corresponding to a mesoporous glass with 58SiO2-37%CaO-5%P2O5 composition; (c), (d) TEM images taken in
the 100 and 111 directions of 3D bicontinuous cubic structure with Ia–3d space group
corresponding to a mesoporous glass of 85SiO2-10%CaO-5%P2O5 composition [43].
(a)(b)(c)(d)It has been evidenced that this new family of mesoporous glasses exhibits enhanced
bioactive behavior, with even faster apatite phase formation than conventional
bioactive sol-gel glasses. Besides, it has been revealed that the kinetics of
apatite formation in mesoporous glasses is governed by their textural and
structural properties, differing from conventional bioactive glasses, where the
formation kinetics depended on compositional and textural properties. In this
sense, the bicontinuous network in 3D-cubic mesoporous glass  with low CaO
content (10% in mol), which exhibits
high accessibility to the diffusion process in pore systems, has led to the most
accelerated bioactive response reported up to date [46]. Transmission electron microscopy
(TEM) studies revealed that the formation of the crystalline apatite formation
took place after only 4 hours of assay (see Figure 4). Their bioactive response was faster than
mesoporous glasses with higher CaO content and 2D-hexagonal structure.
Moreover, Figure 4 also shows the Ca2+ concentration and pH
evolutions in SBF as a function of soaking time. It should be highlighted that there
was not a significant increase
in calcium and pH, which remained almost constant exhibiting values of 7.45 and
100 ppm, respectively [46]. This is a very important factor to avoid the
cytotoxic effect that high local pH values could produce in living tissues.
On
the other hand, mesoporous glasses with 2D-hexagonal pore arrangements and high CaO
content (37% in mol) provided a biomimetic mechanism where a sequential
transition from amorphous calcium phosphate (ACP) to octacalcium phosphate (OCP) and to calcium deficient carbonate hydroxyapatite (CDHA) maturation, similar to the in vivo bone biomineralization
process, was observed by TEM (see Figure 5) [47]. Such ACP-OCP-CDHA maturation sequence had
never been observed before in any other bioactive ceramic. In this case, the
open channel array and the textural parameters of 2D-hexagonal structure
together with high CaO content allow an intense Ca2+–H3O+ exchange that results in local acid pH values that favor the OCP phase
formation.Bioactive in vitro study corresponding to an ordered mesoporous glass
with 3D bicontinuous cubic structure and85SiO2-10%CaO-5%P2O5 composition. (Inset) schematic representation of 3D cubic pore arrangement. (a)
Variation of calcium content and pH values of SBF after soaking the materials
during different periods. (b) HRTEM study corresponding to the surface of
mesoporous glasses after 4 hours soaked in SBF. Low magnification image
indicates the needle-like crystals formation on the surface of mesoporous
glass. Higher magnification and FT transform showing the (002), (211), and
(112) reflections corresponding to d-spacings of 0.34, 0.28, and 0.27 nm of an
apatite phase.
(a)(b)Bioactive
in vitro study corresponding to
an ordered mesoporous glass with 2D hexagonal structure and58SiO2-37%CaO-5%P2O5 composition. (a) Schematic representation of 2D hexagonal pore arrangement. (b) Variation of pH as a function of time at
the bioceramic surface and SBF, highlighting the pH range where OCP phase is
more stable. (c) HRTEM study after different times of incubation showing a
sequential transition from ACP to OCP and to CDHA. For each time, a low
magnification image, a higher magnification and its corresponding FT transform are shown.
(a)(b)(c)Therefore, the possibility of tailoring the reactive properties of
mesoporous glasses as a function of their textural, structural, and
compositional properties allows to obtain materials with high
biomimetism-derived biocompatibility. In addition, the fast and intense
apatite-phase formation makes mesoporous glasses groundbreaking materials that
will probably point out the future guidelines for the synthesis of the next
bioactive bioceramics.


## 2.2. Silica-Based Ordered
Mesoporous Materials as Controled Delivery Systems

Since 2001, when MCM-41 was purposed for the first time as controled
delivery system [37], much research effort has been
devoted to tailor the chemical properties of mesoporous carriers at the
nanometer scale to achieve a better control over loading and release of
molecules. The mesoporous carrier is selected according to the features of the
guest molecule and the targeted application. Molecule loading is usually
performed by impregnation methods by soaking the mesoporous carrier into a
concentrated molecule solution. Loading solvent is selected attending to the
chemical nature of the guest molecule [48]. Then, the release process is
performed by placing the molecule-loaded mesoporous material into SBF solution
[49], which exhibits ionic concentrations similar to those in the human plasma, or in physiological
serum to make the
detection process easy. Therefore, different guest molecules have been
successfully confined into mesoporous silicas. Some of these molecules are
drugs, such as ibuprofen [37, 50–52], amoxicillin [53],
gentamicin [54, 55], erythromycin [51, 56], vancomycin [57], naproxen [58],
aspirin [59, 60], diflunisal [58], captopril [61], itraconazole [62], and alendronate
[63, 64]. Other guest molecules consisted of biologically active species, such
as proteins (bovine serum albumin (BSA) [65–67] and certain
amino acids (L-tryptophan (L-Trp) [68]).The textural properties (i.e.,
pore diameter, surface area, and pore volume) of mesoporous materials are key
factors that govern molecules adsorption and release [69, 70]. Moreover,
functionalization of silica walls using different organic groups has been
revealed as the main strategy to modulate molecule loading and release.


### 2.2.1. Influence of Textural Properties

The textural properties of mesoporous materials (pore diameter, surface
area, and pore volume) play an important role in molecule adsorption and
release. The influence of all these parameters on performance of controled
delivery systems will be discussed in the next sections.(a) Pore Diameter
The adsorption of molecules into mesoporous carriers obeys size
selectivity criteria, that is,
pore diameter will determine the size of the guest molecule to be hosted. Most
of the drugs commonly used in biomedicine are in the range of nanometers.
Therefore, they could be easily confined into the pores of nanostructured mesoporous
silica. However, pore diameter is a limiting factor when the adsorption of
large molecules, such as proteins, is aimed. This is the case of serum
albumins, which are one of the major components in plasma proteins. They are
involved in several relevant physiological functions of humans and upper
mammals, such as binding, transport and delivery of biologically active
molecules, including drugs, to specific places in the body [71]. Serum albumins
are usually composed of single chains of 582 amino acids and exhibit average length of 10 nm and width
of 6 nm [72, 73]. The adsorption of globular proteins, such as bovine serum
albumin (BSA) onto MCM-41 matrices, which exhibits pore diameters in the 2–5 nm range, has
been reported [74, 75]. The small diameter of MCM-41 impeded the confinement of
the protein into the mesopores and consequently, the protein was adsorbed on
the outer surface of the mesoporous carrier.
These results evidenced that when the confinement of large size proteins
into mesoporous channels is targeted, large pore matrices are needed. Our
research group recently applied an improved method to increase the pore
diameter of SBA-15 and BSA adsorption and release tests were performed [67].
The increase in the mesopore diameter was achieved by increasing the time of
hydrothermal treatment during the mesoporous synthesis, as described elsewhere
[76, 77]. The pore diameter of the resulting SBA-15 materials ranged from 8.2
to 11.4 nm employing hydrothermal treatments ranging from 1 to 7 days,
respectively. BSA loading tests revealed that there was a direct dependence of
protein adsorption on pore size, that
is, the greater the pore diameter, the higher the amount of protein
loaded. Therefore, conventional SBA-15, which presented pore sizes of 8.2 nm,
loaded 151 mg/g of BSA. The amount of BSA loaded increased to 234 mg/g, 242 mg/g, and 270 mg/g for mesoporous SBA-15 materials with pore sizes of 9.5 nm,
10.5 nm, and 11.4 nm, respectively (see Figure 6.a.1). In vitro delivery tests were carried out by soaking disc-shaped
mesoporous matrices in saline solution (NaCl 0.9%, pH 7.4). Release patterns
exhibited and initial burst profile where more than 90% of the adsorbed protein
was quickly released during the first 24 hours, followed by a sustained release
and the complete delivery was achieved after 192 hours of assay in all tested
samples.
Pore diameter has been revealed as a critical factor that modulates
release rate of molecules to the delivery medium. This fact was firstly
evidenced when MCM-41 materials exhibiting different pore sizes ranging from
2.5 to 3.6 nm were synthesized using cationic surfactants with different length
alkyl chains [78]. Ibuprofen, a common anti-inflammatory,
was selected and release tests were carried out in a simulated body fluid (pH
7.4) [49]. The resulting release profiles, which are displayed in Figure 6.a.2,
revealed that release kinetics strongly depended on mesopore size. Thus, the
amount of ibuprofen released to the delivery medium diminished as the pore size
decreased from 3.6 to 2.5 nm, evidencing that drug dosage can be modulated
depending on the mesopore size.Influence of textural properties. (a) Pore diameter, (b) surface area, and (c) pore volume, on molecule adsorption and release.
(a)
Influence of pore diameter(b)
Influence of surface area(c)
Influence of pore volume(b) Surface Area
The adsorption of molecules into mesoporous matrices depends on the
adsorptive properties of the silica surface. Therefore, the chemical
interactions between the silanol groups covering the silica surface and the
functional groups of the guest molecule will determine the amount of molecule
loaded. Surface area of mesoporous matrix is the textural parameter that points to the silica
surface susceptible to interact with the guest molecule and, consequently, it
is expected that the higher the surface area, the higher the amount of molecule
loaded. This fact was confirmed when several MCM-41 matrices with different
surface areas were employed as ibuprofen delivery systems [78]. MCM-41
mesoporous materials exhibiting surface areas of 768, 936, 1087, and 1157 m2/g
loaded 110, 190, 230, and 340 mg/g of ibuprofen, respectively, evidencing the
increase in the drug loading with the surface area increase.
With the aim of further establishing the effect of surface area on molecule adsorption, two
mesoporous materials, MCM-41 and SBA-15, which exhibit the same structure (2D-hexagonal,p6mm symmetry) but different surface
areas (1157 m2/g for MCM-41 and 719 m2/g for SBA-15) were
tested as alendronate delivery systems [63]. Alendronate is a potent
bisphosphonate commonly employed for osteoporosis treatments that inhibits bone resorption
by osteoclasts and it is a promising drug to be locally applied in bone implant
technologies. As displayed in Figure 6(b), the amount of alendronate loaded into
MCM-41 (139 mg/g) was higher than that into SBA-15 (83 mg/g), confirming the dependence of drug
loading on surface area. Release profiles from both mesoporous carriers into
the delivery medium (NaCl 0.9%, pH 7.4) exhibited an initial burst effect when ca. 20% and ca. 50% of alendronate loaded were quickly released from MCM-41 and SBA-15,
respectively (see Figure 6(b)). Afterwards, alendronate was released to the
delivery medium in a sustained manner, following first-order kinetics for MCM-41
and zero-order or linear kinetics for SBA-15 materials.(c) Pore Volume
As previously mentioned, the adsorption of molecules into mesoporous
matrices is a surface-molecule interaction [79]. For this reason, pore diameter,
as the size-selectivity limiting factor, and surface area are key parameters
that control the adsorption of molecules. However, when the confinement of
really large molecules is targeted, the pore volume available to host the guest
molecule plays also an important role in molecules adsorption.
Recently, mesostructured cellular foams (MCFs) have been tested as host
matrices for the adsorption of several enzymes and proteins [80, 81]. The
synthesis of MCF materials was carried out by using triblock copolymers as
surfactants and introducing a swelling agent, commonly trimethylbenzene, into
the structure directing template solution [82, 83]. TEM image of SBA-15 (see Figure 6(c)) revealed the presence of two-dimensional hexagonal arrays of pores,
typical of hexagonally ordered mesoporous materials with pore sizes of ca. 8 nm. In the case of MCF, spherical
cells of ca. 28 nm with windows of ca. 18 nm were observed (see arrows in Figure 6(c)). These matrices seem suitable to be used as delivery systems of
large-size molecules, such as proteins. The amount of BSA loaded was higher in
MCF (240 mg/g) than in SBA-15 (150 mg/g), following the same trend than pore
volume, being 1.9 cm3/g and 1.1 cm3/g for MCF and SBA-15,
respectively (see Table inset Figure 6(c)). This results evidence that pore
volume is an important parameter to consider when aiming at confining large-size
and large-volume molecules. However, the effect of other parameters, such as
pore diameter into protein loading cannot be overruled. The pore size of SBA-15
is just on the limit of BSA dimensions, which could complicate the protein
adsorption into the mesopores. However, such steric hindrance would not take
place when MCF, with a pore opening of 18 nm, is used as host matrix.


### 2.2.2. Influence of Organic Functionalization

A
straightforward way to optimize the performance of mesoporous matrices as controled
delivery systems consists in organically modifying the mesoporous silica walls
with appropriate functional groups [69, 70]. On the surface of silica walls,
there is a high surface density of silanol groups that can experience organic
modification by covalently anchoring organic silanes, (RO)3SiR’ [84].
The adsorption and release of molecules can be effectively controled by using
the adequate organic functionalization. To reach this goal, two main strategies
have been widely employed. The first one involves the use of organic groups
that promote attracting host-guest interactions with the functional groups of the
guest molecule. The choice of the functional group would depend on the targeted
molecule. For instance, ibuprofen delivery tests from MCM-41 functionalized
with several organic groups (chloropropyl, phenyl, benzyl, mercaptopropyl,
cyanopropyl, and butyl) revealed that polar groups induced greater ibuprofen
adsorption than nonpolar groups [85]. The second strategy concerns the
functionalization of mesoporous silica walls using hydrophobic species. In the
next sections, some relevant results regarding functionalization using amino
groups and functionalization employing hydrophobic groups will be described.(a) Functionalization Using Amino Groups
Functionalization
using amino groups has been a widely employed strategy to attain a better
control over molecule loading and release. Therefore, amino functionalized MCM-41
and SBA-15 mesoporous matrices have been tested as alendronate delivery systems
[63]. The obtained results indicated that the amount of drug loaded into
amino-modified materials was almost 3 fold that of unmodified matrices. This
fact could be explained by the stronger attracting interactions taking place
between phosphonate groups in alendronate and amino groups of functionalized
matrices compared to the weaker interactions through hydrogen bonds between
phosphonate groups and silanol groups of unmodified materials. Therefore,
amino-modified MCM-41-NH2 loaded more alendronate (366 mg/g) than
unmodified MCM-41 (139 mg/g). Also, amino-modified SBA-15-NH2 matrix
loaded more amount of  drug (220 mg/g) than unmodified SBA-15 (83 mg/g). Moreover, drug release assays revealed
that amino functionalization of mesoporous silica allowed a better control on
the drug release. Thus, the amount of alendronate released after 24 hours was
28% and 58% of the total amount loaded for amino-modified and unmodified
MCM-41, respectively. In the case of SBA-15, the 11% and 56% of the total
alendronate loaded were
released from amino-modified and unmodified materials, respectively, after 24
hours of assay.
Recently,
the amino modification of SBA-15 mesoporous materials using different functionalization
degrees has been revealed as key factor to modulate alendronate dosage [64]. The
amino-modification method was improved by adding a catalyst during the
functionalization process. Alendronate loading as a function of experimental
functionalization degree is displayed in Figure 7. The amount of alendronate
loaded linearly increased as the functionalization degree increased, ranging
from 88 mg/g, for the lowest functionalization degree (0%), to 121 mg/g, for
the highest functionalization degree (47%). Moreover, functionalization degree
also allowed a better control over alendronate release (see Figure 7), that is, the higher the
functionalization degree, the smaller the percentage of alendronate released.
This work evidences the possibility of modulating drug dosage by varying the
functionalization degree of mesoporous matrix, which is particularly
interesting when using bisphosphonates since the high potency of these drugs
implies that only small local doses are needed.
The
organic modification of mesoporous matrices was observed to control release
kinetics of proteins, such as BSA [66, 67]. As previously described, SBA-15
materials exhibiting different pore diameters were obtained by varying the time
of hydrothermal treatment during their synthesis. These materials were then
modified using amino groups to favor attracting interactions with amide groups of protein. It should
be highlighted that organic functionalization always leads to a diminishing of pore
diameter and consequently, as BSA size is on the limit of SBA-15 mesopore
dimensions, after amino functionalization, the amount of BSA loaded decreased
compared to unmodified materials. However, organic functionalization of SBA-15
had a noticeable effect on BSA kinetic release rate. As a consequence of
organic functionalization, the initial burst effect observed in BSA release
from unmodified matrices (ca. 90%) was
drastically reduced after amino modification (ca. 30%). Moreover, the release of protein from the mesopores of
all amino-modified materials was incomplete, whereas the total amount of loaded
BSA was released after 192 hours from unmodified materials.
Moreover,
functionalization of large-pore MCF materials with amino groups has been
demonstrated to increase the affinity for BSA [81]. There was an increase in
BSA adsorption after amino-modification MCF-NH2 (270 mg/g) compared
to unmodified MCF (240 mg/g), despite of the decrease in pore diameter due to
the functionalization. However, amino functionalization has a strong influence
on BSA release rate. Unmodified MCF exhibited an initial burst effect when
almost 60% of the protein loaded was quickly released to the delivery medium. This burst effect
was drastically reduced to ca. 10%
after amino modification. After 24 hours of assay, 62% of the loaded BSA was
released from unmodified MCF material and this percentage decreased to 22%
after functionalization with amino groups.
These
studies evidence that organic functionalization of silica mesoporous matrices
using amino groups allows the control of release kinetics of several molecules
that exhibit functional groups able to undergo attracting interaction with
amino functions.Figure 7
Amount of alendronate loaded and percentage of alendronate released as
a function of functionalization degree.(b) Functionalization Using Hydrophobic Groups
The functionalization of mesoporous silica walls with hydrophobic groups
is an alternative to control the release of certain molecules. Therefore,
SBA-15 mesoporous matrix has been functionalized with different alkyl chains, octyl
(C8) and octadecyl (C18), and erythromycin, a hydrophobic antibiotic was
selected to carry out adsorption and delivery assays [56]. As previously
commented, organic modification led to a decrease of the effective pore
diameter and surface area of SBA-15. The decrease in surface area resulted in diminishing the amount
of erythromycin loaded, 130 mg/g and 180 mg/g for SBA-15-C8 and
SBA-15-C18, respectively, compared to unmodified SBA-15 (340 mg/g). Comparable
results were reported using mesoporous materials modified by silylation as
ibuprofen delivery systems, showing a lower drug loading after silylation [86].
In addition, as a consequence of functionalization with hydrophobic chains,
there was a decrease in the wettability of the surface, which made difficult
the penetration of the aqueous delivery solution inside the mesopores
originating a decrease in the  delivery rate. In fact, the erythromycin release
rate from SBA-15-C18 material was one order of magnitude lower than that from
unmodified SBA-15.
Recently,
functionalized SBA-15 has been proposed as delivery system ofL-tryptophan (L-Trp) [68], a hydrophobic amino acid present in the
three-dimensional structure of many peptides, proteins, and growth factors of
interest in bone tissue regeneration technologies [87–89]. L-Trp presents an aromatic indol ring
that makes necessary
to modify the silanol-rich walls of SBA-15. In fact, unmodified SBA-15 loaded less than
5 mg/g of L-Trp, probably due to the
extremely different chemical nature of hydrophobic amino acid and hydrophilic
SBA-15. The small amount of L-Trp
adsorbed into SBA-15 could be interactions through hydrogen bonds between
deprotonated carboxylic group of amino acid and silanol groups covering the
silica walls (see Figure 8(a)). For this reason, SBA-15 matrix was organically
modified using quaternary amines with different alkyl lengths (∼C3N+Me3 and ∼C3N+Me2C18),
as illustrated in Figures 8(b) and 8(c) [68]. Functionalization with short
alkyl chains (∼C3N+Me3)
allowed coulombic attracting interactions between deprotonated carboxylic
groups of amino acid (–COO−) and protonated quaternary amines (–N+R4)
covering the mesoporous surface. In this case, the amount of L-Trp loaded into SBA-15-C3N+Me3 matrix was higher (43 mg/g) than in unmodified SBA-15 (<5 mg/g). On the
other hand, using long hydrocarbon chains (∼C3N+Me2C18),
two-thirds of the silica surface was functionalized. This high degree of
functionalization with hydrophobic chains promoted interaction of mesoporous
surface with indol group of L-Trp,
and consequently, the amount of amino acid loaded increased to 82 mg/g.
Moreover, release profiles of amino acid from functionalized SBA-15 materials
are displayed in Figure 8(d). In both cases, there is an initial burst effect,
where most of the L-Trp loaded is
quickly released to the delivery medium. After such burst effect, the rest of
amino acid is released in a sustained manner, following first-order and
zero-order or linear kinetics from SBA-15-C3N+Me3 and SBA-15-C3N+Me2C18 matrices,
respectively.Molecular model of interaction betweenL-Trp and (a) unmodified SBA-15, (b) SBA-15 functionalized with
short-chain quaternary amines (∼C3N+Me3),
and (c) SBA-15 functionalized with long-chain quaternary amines (∼C3N+Me2C18).
(d) L-Trp release profiles from
functionalized samples are also displayed.
(a)(b)(c)(d)


## 2.2.1. Influence of Textural Properties

The textural properties of mesoporous materials (pore diameter, surface
area, and pore volume) play an important role in molecule adsorption and
release. The influence of all these parameters on performance of controled
delivery systems will be discussed in the next sections.(a) Pore Diameter
The adsorption of molecules into mesoporous carriers obeys size
selectivity criteria, that is,
pore diameter will determine the size of the guest molecule to be hosted. Most
of the drugs commonly used in biomedicine are in the range of nanometers.
Therefore, they could be easily confined into the pores of nanostructured mesoporous
silica. However, pore diameter is a limiting factor when the adsorption of
large molecules, such as proteins, is aimed. This is the case of serum
albumins, which are one of the major components in plasma proteins. They are
involved in several relevant physiological functions of humans and upper
mammals, such as binding, transport and delivery of biologically active
molecules, including drugs, to specific places in the body [71]. Serum albumins
are usually composed of single chains of 582 amino acids and exhibit average length of 10 nm and width
of 6 nm [72, 73]. The adsorption of globular proteins, such as bovine serum
albumin (BSA) onto MCM-41 matrices, which exhibits pore diameters in the 2–5 nm range, has
been reported [74, 75]. The small diameter of MCM-41 impeded the confinement of
the protein into the mesopores and consequently, the protein was adsorbed on
the outer surface of the mesoporous carrier.
These results evidenced that when the confinement of large size proteins
into mesoporous channels is targeted, large pore matrices are needed. Our
research group recently applied an improved method to increase the pore
diameter of SBA-15 and BSA adsorption and release tests were performed [67].
The increase in the mesopore diameter was achieved by increasing the time of
hydrothermal treatment during the mesoporous synthesis, as described elsewhere
[76, 77]. The pore diameter of the resulting SBA-15 materials ranged from 8.2
to 11.4 nm employing hydrothermal treatments ranging from 1 to 7 days,
respectively. BSA loading tests revealed that there was a direct dependence of
protein adsorption on pore size, that
is, the greater the pore diameter, the higher the amount of protein
loaded. Therefore, conventional SBA-15, which presented pore sizes of 8.2 nm,
loaded 151 mg/g of BSA. The amount of BSA loaded increased to 234 mg/g, 242 mg/g, and 270 mg/g for mesoporous SBA-15 materials with pore sizes of 9.5 nm,
10.5 nm, and 11.4 nm, respectively (see Figure 6.a.1). In vitro delivery tests were carried out by soaking disc-shaped
mesoporous matrices in saline solution (NaCl 0.9%, pH 7.4). Release patterns
exhibited and initial burst profile where more than 90% of the adsorbed protein
was quickly released during the first 24 hours, followed by a sustained release
and the complete delivery was achieved after 192 hours of assay in all tested
samples.
Pore diameter has been revealed as a critical factor that modulates
release rate of molecules to the delivery medium. This fact was firstly
evidenced when MCM-41 materials exhibiting different pore sizes ranging from
2.5 to 3.6 nm were synthesized using cationic surfactants with different length
alkyl chains [78]. Ibuprofen, a common anti-inflammatory,
was selected and release tests were carried out in a simulated body fluid (pH
7.4) [49]. The resulting release profiles, which are displayed in Figure 6.a.2,
revealed that release kinetics strongly depended on mesopore size. Thus, the
amount of ibuprofen released to the delivery medium diminished as the pore size
decreased from 3.6 to 2.5 nm, evidencing that drug dosage can be modulated
depending on the mesopore size.Influence of textural properties. (a) Pore diameter, (b) surface area, and (c) pore volume, on molecule adsorption and release.
(a)
Influence of pore diameter(b)
Influence of surface area(c)
Influence of pore volume(b) Surface Area
The adsorption of molecules into mesoporous matrices depends on the
adsorptive properties of the silica surface. Therefore, the chemical
interactions between the silanol groups covering the silica surface and the
functional groups of the guest molecule will determine the amount of molecule
loaded. Surface area of mesoporous matrix is the textural parameter that points to the silica
surface susceptible to interact with the guest molecule and, consequently, it
is expected that the higher the surface area, the higher the amount of molecule
loaded. This fact was confirmed when several MCM-41 matrices with different
surface areas were employed as ibuprofen delivery systems [78]. MCM-41
mesoporous materials exhibiting surface areas of 768, 936, 1087, and 1157 m2/g
loaded 110, 190, 230, and 340 mg/g of ibuprofen, respectively, evidencing the
increase in the drug loading with the surface area increase.
With the aim of further establishing the effect of surface area on molecule adsorption, two
mesoporous materials, MCM-41 and SBA-15, which exhibit the same structure (2D-hexagonal,p6mm symmetry) but different surface
areas (1157 m2/g for MCM-41 and 719 m2/g for SBA-15) were
tested as alendronate delivery systems [63]. Alendronate is a potent
bisphosphonate commonly employed for osteoporosis treatments that inhibits bone resorption
by osteoclasts and it is a promising drug to be locally applied in bone implant
technologies. As displayed in Figure 6(b), the amount of alendronate loaded into
MCM-41 (139 mg/g) was higher than that into SBA-15 (83 mg/g), confirming the dependence of drug
loading on surface area. Release profiles from both mesoporous carriers into
the delivery medium (NaCl 0.9%, pH 7.4) exhibited an initial burst effect when ca. 20% and ca. 50% of alendronate loaded were quickly released from MCM-41 and SBA-15,
respectively (see Figure 6(b)). Afterwards, alendronate was released to the
delivery medium in a sustained manner, following first-order kinetics for MCM-41
and zero-order or linear kinetics for SBA-15 materials.(c) Pore Volume
As previously mentioned, the adsorption of molecules into mesoporous
matrices is a surface-molecule interaction [79]. For this reason, pore diameter,
as the size-selectivity limiting factor, and surface area are key parameters
that control the adsorption of molecules. However, when the confinement of
really large molecules is targeted, the pore volume available to host the guest
molecule plays also an important role in molecules adsorption.
Recently, mesostructured cellular foams (MCFs) have been tested as host
matrices for the adsorption of several enzymes and proteins [80, 81]. The
synthesis of MCF materials was carried out by using triblock copolymers as
surfactants and introducing a swelling agent, commonly trimethylbenzene, into
the structure directing template solution [82, 83]. TEM image of SBA-15 (see Figure 6(c)) revealed the presence of two-dimensional hexagonal arrays of pores,
typical of hexagonally ordered mesoporous materials with pore sizes of ca. 8 nm. In the case of MCF, spherical
cells of ca. 28 nm with windows of ca. 18 nm were observed (see arrows in Figure 6(c)). These matrices seem suitable to be used as delivery systems of
large-size molecules, such as proteins. The amount of BSA loaded was higher in
MCF (240 mg/g) than in SBA-15 (150 mg/g), following the same trend than pore
volume, being 1.9 cm3/g and 1.1 cm3/g for MCF and SBA-15,
respectively (see Table inset Figure 6(c)). This results evidence that pore
volume is an important parameter to consider when aiming at confining large-size
and large-volume molecules. However, the effect of other parameters, such as
pore diameter into protein loading cannot be overruled. The pore size of SBA-15
is just on the limit of BSA dimensions, which could complicate the protein
adsorption into the mesopores. However, such steric hindrance would not take
place when MCF, with a pore opening of 18 nm, is used as host matrix.


## 2.2.2. Influence of Organic Functionalization

A
straightforward way to optimize the performance of mesoporous matrices as controled
delivery systems consists in organically modifying the mesoporous silica walls
with appropriate functional groups [69, 70]. On the surface of silica walls,
there is a high surface density of silanol groups that can experience organic
modification by covalently anchoring organic silanes, (RO)3SiR’ [84].
The adsorption and release of molecules can be effectively controled by using
the adequate organic functionalization. To reach this goal, two main strategies
have been widely employed. The first one involves the use of organic groups
that promote attracting host-guest interactions with the functional groups of the
guest molecule. The choice of the functional group would depend on the targeted
molecule. For instance, ibuprofen delivery tests from MCM-41 functionalized
with several organic groups (chloropropyl, phenyl, benzyl, mercaptopropyl,
cyanopropyl, and butyl) revealed that polar groups induced greater ibuprofen
adsorption than nonpolar groups [85]. The second strategy concerns the
functionalization of mesoporous silica walls using hydrophobic species. In the
next sections, some relevant results regarding functionalization using amino
groups and functionalization employing hydrophobic groups will be described.(a) Functionalization Using Amino Groups
Functionalization
using amino groups has been a widely employed strategy to attain a better
control over molecule loading and release. Therefore, amino functionalized MCM-41
and SBA-15 mesoporous matrices have been tested as alendronate delivery systems
[63]. The obtained results indicated that the amount of drug loaded into
amino-modified materials was almost 3 fold that of unmodified matrices. This
fact could be explained by the stronger attracting interactions taking place
between phosphonate groups in alendronate and amino groups of functionalized
matrices compared to the weaker interactions through hydrogen bonds between
phosphonate groups and silanol groups of unmodified materials. Therefore,
amino-modified MCM-41-NH2 loaded more alendronate (366 mg/g) than
unmodified MCM-41 (139 mg/g). Also, amino-modified SBA-15-NH2 matrix
loaded more amount of  drug (220 mg/g) than unmodified SBA-15 (83 mg/g). Moreover, drug release assays revealed
that amino functionalization of mesoporous silica allowed a better control on
the drug release. Thus, the amount of alendronate released after 24 hours was
28% and 58% of the total amount loaded for amino-modified and unmodified
MCM-41, respectively. In the case of SBA-15, the 11% and 56% of the total
alendronate loaded were
released from amino-modified and unmodified materials, respectively, after 24
hours of assay.
Recently,
the amino modification of SBA-15 mesoporous materials using different functionalization
degrees has been revealed as key factor to modulate alendronate dosage [64]. The
amino-modification method was improved by adding a catalyst during the
functionalization process. Alendronate loading as a function of experimental
functionalization degree is displayed in Figure 7. The amount of alendronate
loaded linearly increased as the functionalization degree increased, ranging
from 88 mg/g, for the lowest functionalization degree (0%), to 121 mg/g, for
the highest functionalization degree (47%). Moreover, functionalization degree
also allowed a better control over alendronate release (see Figure 7), that is, the higher the
functionalization degree, the smaller the percentage of alendronate released.
This work evidences the possibility of modulating drug dosage by varying the
functionalization degree of mesoporous matrix, which is particularly
interesting when using bisphosphonates since the high potency of these drugs
implies that only small local doses are needed.
The
organic modification of mesoporous matrices was observed to control release
kinetics of proteins, such as BSA [66, 67]. As previously described, SBA-15
materials exhibiting different pore diameters were obtained by varying the time
of hydrothermal treatment during their synthesis. These materials were then
modified using amino groups to favor attracting interactions with amide groups of protein. It should
be highlighted that organic functionalization always leads to a diminishing of pore
diameter and consequently, as BSA size is on the limit of SBA-15 mesopore
dimensions, after amino functionalization, the amount of BSA loaded decreased
compared to unmodified materials. However, organic functionalization of SBA-15
had a noticeable effect on BSA kinetic release rate. As a consequence of
organic functionalization, the initial burst effect observed in BSA release
from unmodified matrices (ca. 90%) was
drastically reduced after amino modification (ca. 30%). Moreover, the release of protein from the mesopores of
all amino-modified materials was incomplete, whereas the total amount of loaded
BSA was released after 192 hours from unmodified materials.
Moreover,
functionalization of large-pore MCF materials with amino groups has been
demonstrated to increase the affinity for BSA [81]. There was an increase in
BSA adsorption after amino-modification MCF-NH2 (270 mg/g) compared
to unmodified MCF (240 mg/g), despite of the decrease in pore diameter due to
the functionalization. However, amino functionalization has a strong influence
on BSA release rate. Unmodified MCF exhibited an initial burst effect when
almost 60% of the protein loaded was quickly released to the delivery medium. This burst effect
was drastically reduced to ca. 10%
after amino modification. After 24 hours of assay, 62% of the loaded BSA was
released from unmodified MCF material and this percentage decreased to 22%
after functionalization with amino groups.
These
studies evidence that organic functionalization of silica mesoporous matrices
using amino groups allows the control of release kinetics of several molecules
that exhibit functional groups able to undergo attracting interaction with
amino functions.Figure 7
Amount of alendronate loaded and percentage of alendronate released as
a function of functionalization degree.(b) Functionalization Using Hydrophobic Groups
The functionalization of mesoporous silica walls with hydrophobic groups
is an alternative to control the release of certain molecules. Therefore,
SBA-15 mesoporous matrix has been functionalized with different alkyl chains, octyl
(C8) and octadecyl (C18), and erythromycin, a hydrophobic antibiotic was
selected to carry out adsorption and delivery assays [56]. As previously
commented, organic modification led to a decrease of the effective pore
diameter and surface area of SBA-15. The decrease in surface area resulted in diminishing the amount
of erythromycin loaded, 130 mg/g and 180 mg/g for SBA-15-C8 and
SBA-15-C18, respectively, compared to unmodified SBA-15 (340 mg/g). Comparable
results were reported using mesoporous materials modified by silylation as
ibuprofen delivery systems, showing a lower drug loading after silylation [86].
In addition, as a consequence of functionalization with hydrophobic chains,
there was a decrease in the wettability of the surface, which made difficult
the penetration of the aqueous delivery solution inside the mesopores
originating a decrease in the  delivery rate. In fact, the erythromycin release
rate from SBA-15-C18 material was one order of magnitude lower than that from
unmodified SBA-15.
Recently,
functionalized SBA-15 has been proposed as delivery system ofL-tryptophan (L-Trp) [68], a hydrophobic amino acid present in the
three-dimensional structure of many peptides, proteins, and growth factors of
interest in bone tissue regeneration technologies [87–89]. L-Trp presents an aromatic indol ring
that makes necessary
to modify the silanol-rich walls of SBA-15. In fact, unmodified SBA-15 loaded less than
5 mg/g of L-Trp, probably due to the
extremely different chemical nature of hydrophobic amino acid and hydrophilic
SBA-15. The small amount of L-Trp
adsorbed into SBA-15 could be interactions through hydrogen bonds between
deprotonated carboxylic group of amino acid and silanol groups covering the
silica walls (see Figure 8(a)). For this reason, SBA-15 matrix was organically
modified using quaternary amines with different alkyl lengths (∼C3N+Me3 and ∼C3N+Me2C18),
as illustrated in Figures 8(b) and 8(c) [68]. Functionalization with short
alkyl chains (∼C3N+Me3)
allowed coulombic attracting interactions between deprotonated carboxylic
groups of amino acid (–COO−) and protonated quaternary amines (–N+R4)
covering the mesoporous surface. In this case, the amount of L-Trp loaded into SBA-15-C3N+Me3 matrix was higher (43 mg/g) than in unmodified SBA-15 (<5 mg/g). On the
other hand, using long hydrocarbon chains (∼C3N+Me2C18),
two-thirds of the silica surface was functionalized. This high degree of
functionalization with hydrophobic chains promoted interaction of mesoporous
surface with indol group of L-Trp,
and consequently, the amount of amino acid loaded increased to 82 mg/g.
Moreover, release profiles of amino acid from functionalized SBA-15 materials
are displayed in Figure 8(d). In both cases, there is an initial burst effect,
where most of the L-Trp loaded is
quickly released to the delivery medium. After such burst effect, the rest of
amino acid is released in a sustained manner, following first-order and
zero-order or linear kinetics from SBA-15-C3N+Me3 and SBA-15-C3N+Me2C18 matrices,
respectively.Molecular model of interaction betweenL-Trp and (a) unmodified SBA-15, (b) SBA-15 functionalized with
short-chain quaternary amines (∼C3N+Me3),
and (c) SBA-15 functionalized with long-chain quaternary amines (∼C3N+Me2C18).
(d) L-Trp release profiles from
functionalized samples are also displayed.
(a)(b)(c)(d)


## 3. Conclusions

Nanostructured mesoporous silicas
are promising candidates to be used as controled delivery systems of a wide
range of biologically active molecules of interest in bone tissue regeneration applications.
Recently, much research effort is being dedicated to enhance the bioactive
capability of ordered mesoporous silicas. Supramolecular chemistry has allowed
the design and synthesis of ordered mesoporous glasses with fascinating
textural and structural features that open many paths for the research on
high-bioactive nanostructured materials for bone tissue regeneration purposes.
These materials have shown high bioactive responses and biomimetic behavior not
reported for any bioceramic up to date, which open promising expectations in
the biomedical field. Furthermore, much attention has been devoted to tailor
the textural properties of nanostructured mesoporous carriers, pore diameter,
surface area, and pore volume, with the aim of optimizing their loading
capacity depending on the guest molecule. Moreover, organic modification of
mesoporous silica walls brings up many possibilities in molecule adsorption and
release by promoting host-guest interactions. The outstanding properties of
mesoporous silicas make them suitable to be used as starting materials for the
design of scaffolds for bone tissue engineering technologies.



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*Source: 106970-2008-11-04.xml*