3D Plotting of Bioceramic Scaffolds under Physiological Conditions for Bone Tissue Engineering - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 84

both have their inherent shortages, such as the limited availability and donor
site morbidity for autograft (Paul et al. 2009) or pathogen transmission and
immune responses for allograft (Nemzek et al. 1994; Keating and McQueen
2001). Therefore, alternative strategies for bone repair and regeneration
involving tissue engineering (TE) approaches are required.
TE aims to generate living and functional tissues or organs in vitro by
combining cells (such as autologous stem cells isolated from the patient),
synthetic scaffolds resembling the natural extracellular matrix, growth-stim-
ulating signals, and bioreactor techniques (Gelinsky et al. 2011; O'Brien 2011;
Schroeder and Mosheiff 2011). According to this concept, biomaterials and
3D scaffolds made of them have to provide mechanical support and an envi-
ronment that facilitates cell attachment, migration, growth, and response to
signals (Leong et al. 2008; Srouji et al. 2008; Gelinsky 2009). A broad range
of materials including natural and synthetic polymers, bioceramics, and,
less important, some biodegradable metal alloys have been developed and
widely studied in this field. Compared to metals and polymers, bioceram-
ics have the excellent advantages of osteoconductivity and sometimes even
osteoinductivity when used for tissue engineering of bone. Particularly, cal-
cium phosphate (CaP)-based bioceramics and cements (Yuan et al. 1998) con-
sisting of the inorganic component of natural bone have been widely used as
bone replacement material as well as for preparation of scaffolds for tissue
engineering (Xu et al. 2006; Guo et al. 2009; Dorozhkin 2010).
The porosity of a scaffold is a crucial factor for its success in bone regen-
eration: complete interconnectivity of the pores as well as pore sizes in an
appropriate range are necessary for cell migration, new bone ingrowth, and
vascularization (Karageorgiou and Kaplan 2005; Jones et al. 2009). On the other
hand, the porous structure should still be able to mediate mechanical stability
adequate for the host tissue. Several methods have been developed and are
used for the preparation of porous scaffolds. The most commonly used tech-
nology for fabrication of CaP and other bioceramic scaffolds is particle leach-
ing. In this method, either water-soluble materials (such as NaCl) or those
dissolvable in hydrophobic organic solvents (like paraffin) are incorporated
as porogens into CaP pastes. After shaping, the CaP scaffolds are incubated
in water for setting and dissolving the porogens that lead to mechanical load-
able porous structures, or are treated with an organic solvent after setting to
remove the hydrophobic porogens (Guo et al. 2009). An alternative way is the
utilization of a porous polymer template (such as polyurethane foams), which
is coated with a CaP slurry. By sintering that structure at high temperatures,
a stable, porous CaP scaffold is achieved while the polymer component is
burned out (e.g., Zhang and Zhang 2002). However, these methods are limited
by poor control of pore size and morphology as well as insufficient pore inter-
connectivity. Therefore, in the past decade, advanced techniques of rapid pro-
totyping (RP) based on computer-aided design (CAD) and computer-aided
manufacturing (CAM) were introduced in the field of tissue engineering and
regenerative therapies (Landers et al. 2002; Yang et al. 2002).