Nanostructured bioceramics

There are only a few ways of producing nanostructured ceramics. These methods relate to either low-temperature sintering or chemical low-temperature bonding.

Preparation of test samples with the FIB technique; TEM samples ready for lift- out

Figure 1.1 Preparation of test samples with the FIB technique; TEM samples ready for lift- out (left), followed by mounting on the TEM grid, and final polishing to electron transparency (FIB electron mode) (right).

In the preparation of all these nanostructured materials, the size of the precursor (the raw material) is crucial. In the case of sintering, the original ceramic particles of course have to be less than 100 nm, and this reduces the applications of sintered nanostructures. For the chemically bonded ceramics the situation is different in that the precursor particles dissolve. However, the size should be less than approximately 10 pm, since the penetration of water into hydrated bioceramics is reduced to almost zero after 5-10 pm. This is treated in more detail in Ref. [8].

Low-temperature chemical bonding

Most ceramics are formed at high temperatures through a sintering process. By using chemical reactions, ceramic biomaterials can be produced at low temperatures (body temperature), which is attractive from several perspectives: cost, avoidance of temperature gradients (thermal stress), dimensional stability, and minimal negative effect on the system with which the material interacts. Notably the hard tissue of bone and teeth (apatite, a Ca phosphate-based material) also is formed via a biological chemical reaction, and is close in composition to some of the chemically bonded bioceramics. The chemistry of the chemically bonded bioceramic systems is similar to that of the hard tissue found in living organisms. One of the first ceramics to be proposed as a biomaterial was gypsum, Ca(SO4) x ^H2O. The first cement to be proposed and used was a Zn phosphate, which is still used as a dental cement. Examples of typical phases formed in the chemically bonded ceramic systems are presented in Table 1.3.

Chemically bonded ceramics constitute ceramics that are being formed due to chemical reactions, in many cases an acid-base reaction, where the powder is the base and the water the weak acid. The precursor material is a ceramic powder (e.g., Ca silicate or Ca aluminate), which is “activated” in the water-based liquid. A chemical reaction takes place in which the initial powder is partly or completely dissolved and new phases precipitate. The precipitated phases are composed of species from both the liquid and the precursor powder. The degree of reaction of the powder depends on the size of the precursor particles and/or the amount of water/liquid presence, as well as the powder (p) to water (w) ratio (the p/w ratio). This is treated in more detail in Ref [9]. The precipitates can be formed in situ in vivo, often in the nanosized scale due to low solubility of the phases formed (details presented later). The nanostructural chemically bonded bioceramics are especially found within the Ca phosphate, Ca aluminate, and Ca silicate systems. The large pores between the original dissolving precursor powders are increasingly filled with nanocrystals, and the material hardens. The dissolution speed and the solubility products of the formed hydrate phases determine the nanosize, the setting time, and the final curing (hardening) of the material. The setting time can be controlled by selection of the precursor grain size and/or by addition of accelerating or retarding substances. Table 1.4 shows a typical time sequence of the different stages in the formation of chemically bonded bioceramics.

Since the material can be formed from a precursor powder mixed with a liquid, the material can be made moldable simply by controlling the amount of liquid (in relation to the powder) and by the possible addition of small amounts of polymers in the liquid. This makes the chemically bonded ceramics useful as injectable biomaterials, where the final biomaterial is formed in situ in vivo.

Also worth mentioning is a relatively new group of ceramics, called geopolymers [10,11]. These are also produced by chemical reactions but do not involve a hydration,

Table 1.3 Chemically bonded ceramic systems

Group/name

Basic system

Typical phases formed

OPCa

CaO-SiO2-H2O (CSH)

Amorhous CSH, Tobermorite

CAb

CaO-Al2O3-H2O (CAH)

Katoite, and Gibbsite

Gypsum plaster

Ca sulfates

CaSO42H2O

Sorel

MgO-H2O (Cl)

MgOCl

Bioglasses

CaO-Na2O-SiO2-P2O5

Carbohydroxyapatite

Phosphates

CaO-P2O5-H2O (CPH)

Apaties, Brushite, Monetite

Carbonates

CaO-CO2-H2O

Calcite, Aragonite

Geopolymersc

Aluminosilicates, Metakaolin

Amorphous phases

aOPC = Ordinary Portland cements.

bCA = High alumina cements, C3A, C12A7, CA, etc.

cGeopolymers = Metakaolin or synthetic aluminosilicates.

Table 1.4 Time sequence of the formation of nanostructured chemically bonded bioceramics

Dissolution and repeated crystallization

Working and setting time

Initial hardening

Ready to use

Starts immediately

5-15 min

5-20 min

5-60 min

i.e., new uptake of water. The geopolymerization is thus not a hydration process in which the water is consumed. Instead the water resides in the pores but plays an active role as a dissolution medium during the reaction, an inorganic polymerization. The most well-known geopolymer is zeolite [11].

The chemically bonded nanostructured ceramics can further be divided into two main groups: resorbable, including partly resorbable (Ca phosphates, bioglasses, Ca sulfates and Ca carbonates), and stable biomaterials (Ca aluminates and Ca silicates).

 
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