SCAFFOLD CONSIDERATIONS FOR OSTEOCHONDRAL TISSUE ENGINEERING - Intelligent Scaffolds for Tissue Engineering and Regenerative Medicine - page 790

implantation in the subchondral bone. Although neither mimics the
complex anatomy of native osteochondral tissue, scaffolds such as
these are undoubtedly a step in the right direction.
Collagen is a favored material in scaffold design for cartilage
due to its biological properties. 49 Furthermore, as a normal con-
stituent of the tissue, collagen has been shown to be a much more
appropriate substrate for bone formation than biomaterials such
as polystyrene, titanium, or PLA and PLGA, which are commonly
used as implant materials. 50 , 51 The major disadvantage of colla-
gen as a scaffold for bone regeneration is that it has relatively
poor mechanical properties. However, the mechanical properties
of collagen-based scaffolds can be improved through cross-linking
mechanisms 52 - 54 orbyallowingbonecellstoproduceosteoidonthe
scaffolds, followed by subsequent mineralization in vitro , 55 which
also leads to improved mechanical properties. Although collagen-
based scaffolds have also inferior properties compared with native
cartilage, they have been implemented in the matrix-induced autol-
ogous chondrocytes implantation (MACI) technique, which has dis-
played promising results for the repair of cartilage. 56 - 58 However,
type 1 collagen is the usual component of these scaffolds and,
although type 2 collagen is the main constituent of articular car-
tilage, it is a component in few scaffolds, either on the market or
currently in development. This is somewhat surprising. Studies by
Nehrer et al . 59 haveshownthepreferentialbehaviorofchondrocytes
in type 2 collagen over type 1 collagen. A product currently under-
going commercial investigation, from the laboratory of one of the
authors, is ChondroColl. 60 This is a highly porous, lyophilized
collagen-based scaffold. In this product, three layers are combined
into one three-dimensional construct, which aims to facilitate the
healing of osteochondral tissue in vivo . This scaffold is produced
using an “iterative layering” technique, resulting in interfacial adhe-
sion between the scaffold layers and allowing the properties of the
individual scaffold layers to be optimized for their respective roles
inosteochondraltissuehealing(Fig.38.2).Thescaffoldthusoffersa
gradient composition and structure consisting of four bioactive and
biocompatible constituents all found in the osteochondral tissue,
namely, collagen type 1 and 2, glycosaminoglycans (GAG), and HA.
The pore size and mechanical properties of the material are varied