Scaffolds for Liver Tissue Engineering

In general, either mixed cultures of primary isolated cells [68] or freshly isolated purified primary hepatocytes are applied to the scaffold [69]. In order to generate a functional liver construct, the liver cells, once seeded onto the scaffolds, must migrate into the scaffold interior, expand, and populate the new tissue constructs. To facilitate these processes the scaffolds must provide a biocompatible surface suitable for cell recognition and adherence. Following attachment, the cells must proliferate and finally differentiate in order to perform their physiological function.

Some of the critical issues in designing a liver scaffold are listed in Table 1.1.

The most frequently used scaffolds for liver tissue engineering are made of synthetic biodegradable polymers such as PGA, PLA and PLGA, natural polymers such as collagen, alginate and chitosan, as well as several combinations of synthetic and/or natural polymers [70, 71]. Based on a more complete understanding of the importance of cell-selective adhesion motifs [71], some of the above polymers have been modified chemically to increase their biocompatibility

[72]. In this context, biocompatibility is defined as the ability of a given material to perform with an appropriate host response in a specific biological application

[73]. While natural proteins, such as alginate and chitosan possess relative good biocompatibility, the biocompatibility of PLGA is transient and biphasic: over a short time period, PLGA is well tolerated, but it may produce inflammatory responses in the long term. Depending on its biological provenance and type or way of preparation, collagen may also induce an inflammatory response. In the context of liver tissue engineering it is also important that the scaffolds will be biodegradable - that is, they will be degraded and eliminated from the body, and also be bioerodible - that is, their degradation products will lack toxicity or immunogenicity (Table 1.1).

Scaffold porosity (see Table 1.1) is crucial for nutrient and gas exchange, which are essential for hepatocyte survival. Pores of100 to 500 ^m diameter are optimal for hepatocyte growth, as they increase the internal surface area ofthe liver scaffold for optimized cell attachment and increased penetration of blood vessels [74]. Other important factors to be considered in the design of liver scaffolds are mechanical properties, such as elasticity and stability (Table 1.1).

Recently, the requirements for the design of liver scaffolds have become more sophisticated. It is increasingly recognized that in aspiring to fully mimic and recreate in vitro the tissue environment, scaffolds, besides providing structural support for cell growth, are an integral part of bioactive, complex systems. Such "high-fidelity" biomimetic systems for liver tissue engineering include blends of different natural and/or synthetic polymers [75], heterotypic cell populations [68], and a variety of growth factors required both for growth and differentiation of the hepatocytes and promotion of blood vessels ingrowth from the host into the scaffold to ensure long-term survival and function of the constructs [76]. This approach is exemplified by the results presented in Figure 1.4.

Table 1.1 Characterization of scaffolds commonly used for liver engineering.

Scaffold

Physical properties

Chemical and mechanical

Bio-

Pharmacokinetics

Advantages and disadvantages

Ref.

(polymer)

(pores shape and size)

properties

compatibility

(bioerodible/

biodegradability)

Polyglycolic

High porosity;

Synthetic; hydrophobic poly

Synthetic,

Yes

Reproducible; manufactured;

[68]

acid (PGA);

pore size can be

mers; insoluble in water;

different from

Scaffold fabrication

easy scale-up; can be used for

[83]

poly-L-lactic

controlled

shape and porosity can be

natural ECM

involves toxic reagents;

printing scaffolds; cell seeding

[221]

acid (PLLA);

easily adjusted; suitable for

FDA approved

degraded upon hydro-

is limited to scaffold periphery;

[222]

polylactide-

chemical modifications

lytic attack of the ester

change physical structure in

[223]

co-glycolide

bond, resulting in

medium or after implantation.

[224]

(PLC A)

random degradation

Tendency to crumble upon

[225]

products

degradation; induced prolonged

inflammatory response

Alginate

Highly inter-connect-

Hydrophilic, high water

Contain

Yes

Able to hold microspheres for

[44]

hydrogel

ing porous structure

retention; convenient

ECM-like

controlled release of growth

[45]

(> 90%); pore size:

preparation method

materials

factors (VEGF, FGF, etc.)

[71]

100-200 |.im; pore

(The gelatin-freeze-dry

enhancing scaffold angiogenesis

[77]

can be modulated by

method); easily isolated from

[79]

freezing temperature

brown algae; can be modified

[226]

to increase cell attachment

[227]

Collagen

High porosity (80%);

Isolation is complicated

Naturally

Yes

Collagen layers play an important

[228]

can shape as fibers

and expensive

derived ECM,

role in preventing dedifferen-

[229]

with diameter of

but immuno

tiation of hepatocytes in long-

[230]

-0.5-2.0 mm;

genic

term culture. Provide good cell

[231]

pore size range:

adhesion; contracted during cell

[232]

200-1650 |.im

culture

Chitosan

Highly porous

Hydrophilic - positively

Contains

Yes

Can be modified to achieve

[233]

structure (> 90%);

charged; high strength

ECM-like

Slowly biodegraded

a good attachment. Preparation

[234]

pore size:

and flexibility; isolation by

materials

by lysozyme. Chitosan

requires acetic acid extraction;

50-200 |.im

deacetylation from chitin

and its biodégradation

therefore, is difficult to neutralize

(arthropod cuticle).

product in vivo, glucose-

and remove completely the

amine, are not toxic.

protons from the scaffold

ECM = extracellular matrix.

ECM = extracellular matrix.

Fig. 1.4 The technology and applicability of alginate composite scaffolds for liver tissue engineering. (A) Macroscopic image of freeze-dried scaffold before population with hepatocytes. (B) Microspheres embedded into the scaffold with minimal reduction of scaffold porosity. (C) The microspheres become an integral part of the freeze-dried

Fig. 1.4 The technology and applicability of alginate composite scaffolds for liver tissue engineering. (A) Macroscopic image of freeze-dried scaffold before population with hepatocytes. (B) Microspheres embedded into the scaffold with minimal reduction of scaffold porosity. (C) The microspheres become an integral part of the freeze-dried

0 0

Post a comment