Biomedical Engineering Reference
In-Depth Information
Fig. 1
The drastic difference in scales. Both trees and grass are made of cellulose, but they have
different sizes. The trees shown on the
left
are 20-30 cm in diameter and the distances between the
trees are in tens of meters. Animals cannot walk through the trees but between them. Some animals
can climb on the trees (
left panel
). In analogy to cells which are ~5-20 mm in size, and can only
attach to the microfi bers. On the other hand, grass is about 0.5 cm in diameter. When animals walk
in the grass fi eld, they are fully surrounded by the grass, but can walk “through.” In this case, it
appears as animals are embedded in 3-D environment (
right panel
). In analogy to cellular growth
in a nanofi ber-based scaffold, cells are fully embedded within the scaffold that yet allows cells to
migrate/move without much hindrance
fostered a new fi eld of tissue engineering. Attempts have been made to culture cells in
3-D using synthetic polymers or copolymers. However, these synthetic (co)polymers
are often processed into microfi bers ~10-50 mm in diameter that are similar in size
to most cells (~5-20 mm in diameter). Thus, cells attached on microfi bers are more
or less in a 2-D environment eventhough this is somewhat deviating from 2-D by
some curvature imposed by the diameter of the microfi bers. Furthermore, the micro-
pores (~10-200 mm cross section) between the microfi bers are often ~1,000-10,000-
fold greater than the size of biomolecules including vitamins, amino acids, nutrients,
proteins or drugs, which as a consequence can quickly diffuse away. In order to
culture tissue cells in a truly 3-D microenvironment, the scaffold's fi bers and pores
should be signifi cantly smaller than cells so that the cells are surrounded by the
scaffold, similar to the extracellular environment and native extracellular matrices
(Ayad et al.
1998
; Kreis et al.
1999
; Timpl et al.
1979
; Kleinman et al.
1986
; Lee
et al.
1985
; Oliver et al.
1987
) .
Animal-derived biomaterials (e.g., collagen gels, poly-glycosaminoglycans and
Matrigel™) have been used as an alternative to synthetic scaffolds (Kubota et al.
1988
; Kleinman et al.
1986
; Lee et al.
1985
; Oliver et al.
1987
; Bissell et al.
2002
;
Schmeichel and Bissell
2003
; Weaver et al.
1995
; Zhau et al.
1997
; Cukierman et al.
2001
; Cukierman et al.
2002
). However, while they do have an appropriate scale,
they frequently contain residual growth factors, undefi ned constituents or non-quantifi ed
impurities. Due to lack of quality control resulting in lot to lot variations of these
materials, it is thus very diffi cult to conduct a completely controlled study using
these biomaterials. Additionally, impurities pose problems if such scaffolds would be
