Tissue Engineering.pdf

Vol. 12

TISSUE ENGINEERING 261

TISSUE ENGINEERING

Introduction

Accidents and diseases lead to devastating tissue losses and organ failures, which

result in more than 8 million surgical operations each year in the United States

alone (1). These problems convert to a national annual healthcare cost of approx-

imately half trillion U.S. dollars. The state-of-the-art clinical therapies to tissue

losses and organ failures can be categorized into three approaches, ie, transplanta-

tion, surgical reconstruction, and the use of prostheses. Each of these approaches

has contributed to solving or alleviating the severity of these clinical problems,

while all of them have serious limitations.

Organ transplantation became successful in the early 1960s because of the

success of immunologic suppression in the clinical setting (2). Transplantation

has saved, and is continuing to save, countless lives. This approach, however, is

severely limited by the dearth of donor organs. For example, fewer than 3000

available donors are way shorter than the needs of approximately 30,000 Amer-

icans for liver transplants each year (3). Surgical reconstruction utilizes tissues

harvested from the patient to rebuild a critically needed body part. The use of the

patient’s own tissue is advantageous in that it has a higher success rate resulting

from the avoidance of immune rejection than using tissues from other sources.

However, the need for second site of surgery, limited supply, inadequate size and

shape, and the morbidity associated with donor site are all major concerns (4,5).

In response to the shortages of needed tissues and organs, prostheses are devel-

oped to replace certain body parts for their structural and mechanical functions.

There are approximately 100,000 people in the United States with transplants,

while there are more than 10,000,000 with biomedical implants (6). However, the

prostheses are made from artiﬁcial materials, are not biologically functional, and

are therefore subject to long-term complications and rejections.

Tissue engineering is a new approach to resolve the missing tissue and organ

problems. Tissue engineering has been deﬁned as an interdisciplinary ﬁeld that

applies the principles of engineering and the life sciences toward the development

of biological substitutes that restore, maintain, or improve tissue function (3).

There are three strategies in tissue engineering (3,7):

(1) The use of isolated cells or cell substitutes to replace those cells that supply

the needed function, including genetic or other manipulations before the

cell infusion (8);

(2) The delivery of tissue-inducing substances, such as growth and differentia-

tion factors, to targeted locations (9,10);

(3) Growing cells in three-dimensional (3-D) matrices (scaffolds) or devices,

where cells can be either recruited from the host tissues in vivo or seeded

(encapsulated) in vitro (3).

The advantage of the use of isolated cells is the simplicity. Cells are often

directly injected into the targeted locations to avoid complex procedures and as-

sociated complications. The disadvantages include cell death and loss of function

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because cells need the right nutritional and substrate environment to grow and

function. The success of using tissue-inducing substances is dependent on the eco-

nomical large-scale production and puriﬁcation of the bioactive molecules, and on

the development of delivery systems that can deliver these molecules with the

desired proﬁles. Certain gene therapy techniques can also be used for this ap-

proach. The above two approaches, ie, the use of isolated cells or tissue-inducing

substances, are considered when the defects are very small and well contained.

For engineering tissues of practical size scale and with predetermined 3-D struc-

tures, these two approaches are seriously limited. Therefore, the third approach,

ie, growing cells in 3-D matrices (scaffolds) or devices, has become increasingly

active.

Some of these matrices or devices are used to develop immobilized cell sys-

tems, either as implants or extracorporeal devices (11,12). The core of this technol-

ogy is semipermeable membranes or matrices that have well-deﬁned molecular

weight or size cutoff (13,14). They serve as immunoprotective barriers to support

cell growth and function, which allow nutrients, metabolic products, and wastes

to diffuse through, but not immune cells or antibodies. These devices offer certain

biological functions but are not living tissue/organ replacements (see M EMBRANE

T ECHNOLOGY ). Some other 3-D matrices work as templates (called scaffolds) to

guide cells to grow, synthesize biological molecules and extracellular matrix com-

ponents, and facilitate the organization and formation of functional tissues and

organs (15–17). After fulﬁlling the templating function, the scaffolds degrade and

disappear, leaving nothing foreign to the biological system (Fig. 1). There are

many additional advantages in this approach. Patient-derived cells (stem cells or

differentiated cells) or future universal cell sources (nonimmunogenic) can be used

so that there will be minimum complication associated with immune rejections.

These cells can be expanded in vitro to solve the donor shortage limitations. Any

tissue/organ structure can be potentially mimicked by the scaffolding design. The

engineered tissues will have the capacity of growing, modeling, and remodeling in

concert with dynamic changes in physiological environment of the body. In this ap-

proach, biodegradable polymers (natural or synthetic) are the materials of choice.

Polymers (or macromolecules) are currently used as scaffolds for nearly every tis-

sue type including bone and other mineralized tissues. Besides polymers, only

limited inorganic materials are used for certain mineralized tissue engineering

research.

Although growing cells on two-dimensional (2-D) substrates (such as petri

dishes and culture plates) dates back centuries, designing 3-D scaffolds for tissue

engineering is a new ﬁeld, where polymer science and engineering play pivotal

roles.

There are a few basic requirements that have been widely accepted for de-

signing polymer scaffolds. First, the scaffold has to have high porosity and proper

pore size. These pores allow cell seeding and migration to achieve the needed

relative uniform distribution. The pores also provide the space for cell prolifera-

tion and neo tissue deposition. The pores also satisfy the needed mass transport

requirements for nutrients, signaling molecules, metabolic products, and wastes.

The porous structure should also allow for vascularization and innervation for

sustained function of the regenerated tissues when implanted in vivo . High poros-

ity is also beneﬁcial because of the reduced polymer amount and its degradation

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TISSUE ENGINEERING 263

Fig. 1. Schematic diagram showing the tissue engineering concept. Scaffolding materials

(temporary synthetic extracellular matrices) are designed, on which mammalian cells can

grow to regenerate the needed tissue or organ in three dimensions (3-D). Because the

scaffolds are biodegradable, they will resorb after fulﬁlling the template function and leave

nothing foreign in the body.

products in the neo tissues, which can provoke unwanted inﬂammatory response

from the host. Second, a high surface area (a.k.a. surface-to-volume ratio) is

needed, which is the 3-D surface area of the porous scaffolds (not just the out-

side surface area). Many cell types are anchorage-dependent, ie, they can survive,

grow, and function only when they are attached to an appropriate substrate. The

high surface area provides cells with the sufﬁcient area to attach, grow, and de-

posit neo tissue components. Third, biodegradability is generally required, and a

proper degradation rate is needed to match the neo tissue formation. If the scaf-

fold degrades too fast, it can collapse before the new tissue is formed so that it

fails to serve as a 3-D guidance for the neo tissue organization. If the scaffold

degrades too slowly, it remains for a prolonged time period after the neo tissue is

formed and stabilized. It may hinder the new tissue replacement of the scaffold

space, and may cause complications associated with long-term foreign body reac-

tions. Fourth, the scaffold must have the needed mechanical integrity to maintain

the predesigned tissue structure to serve the 3-D guidance. However, scaffolds

are often not as mechanically strong as the tissues to be replaced because of the

required high porosity for scaffolds. They usually serve the scaffolding purpose

well as long as they can maintain the structural integrity under the cultivation

or implantation conditions, while the goals are that the engineered tissues from

the scaffolds are mechanically and biologically functional as their natural coun-

terparts. Fifth, the scaffold should not be toxic to the cells (biocompatible). In

addition to the polymer, the degradation products of the polymer should not be

toxic to the cells, which is usually a more restricting requirement than for the

polymer. Sixth, ideally the scaffold should positively interact with cells, including

enhanced cell adhesion, growth, migration, and differentiated function. To achieve

these positive cell–scaffold interactions, surface or bulk modiﬁcations of the

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polymer structure are often employed (18–21). Even drug, protein, or gene de-

livery techniques are often considered in the scaffold design (22–25).

Polymers in Tissue Engineering

As discussed above, polymers play a pivotal role in tissue engineering. To fulﬁll

the diverse needs in tissue engineering, various polymers have been exploited in

tissue engineering research, including natural polymers (macromolecules), natu-

ral polymer-derived materials, synthetic polymers, and synthetic polymers made

of natural monomers or modiﬁed with natural moieties. Various copolymers, poly-

mer blends, or polymeric composite materials are also used. This section is not

intended to be a complete and exhaustive review of all the polymers used in tissue

engineering. Instead, some of the most frequently used polymers (macromolecules)

in tissue engineering are brieﬂy reviewed.

Polymers for Porous Scaffolds. The polymers to be discussed in this cat-

egory can form stable porous structures in the solid form to serve as pre-designed

3-D scaffolds. They generally do not dissolve or melt under in vitro tissue culture

conditions (in an aqueous tissue culture medium) or when implanted in vivo .

Linear Aliphatic Polyesters. Linear aliphatic polyesters are the most fre-

quently used synthetic biodegradable polymers in tissue engineering and many

other biomedical applications (26–28). These polymers degrade through hydroly-

sis of the ester bonds in the polymer backbone. The degradation rates and proﬁles

differ between these polymers owing to their compositional, structural, and molec-

ular weight differences.

Polyglycolide, also called poly(glycolic acid) (PGA), polylactide, also called

poly(lactic acid) (PLA), and their copolymers, poly(lactide- co -glycolide), also called

poly(lactic acid- co -glycolic acid) (PLGA), are a family of linear aliphatic polyesters

called poly(

-hydroxy acids) or poly(

-hydroxy esters) (Fig. 2). These polymers can

be synthesized by direct condensation of the hydroxy acid monomers (resulting in

low molecular weight polymers, such as lower than 10,000) or more commonly by

a ring-opening polymerization of the cyclic dimers (to achieve a higher molecular

weight), from where the names of polyglycolide and polylactide stem.

Among the family of glycolic acid and lactic acid homopolymers and copoly-

mers, PGA is the simplest in chemical structure, has many advantageous prop-

erties, and is therefore one of the most widely used scaffolding polymers (17).

Because of the chain structural regularity, PGA is highly crystalline and has a

high melting point of around 220 ◦ C (17). It does not dissolve in most common or-

ganic solvents. Because of its hydrophilic nature, PGA degrades rapidly in aqueous

solutions or in vivo , and loses mechanical integrity between 2 and 4 weeks depend-

ing on the molecular weight and physical structure of the scaffolds or implants,

and in vitro or in vivo conditions (17,29). It was the material used to develop the

ﬁrst synthetic absorbable suture, and has been processed into nonwoven ﬁbrous

fabrics as one of the most widely used scaffolds in tissue engineering today.

PLA is also widely used for scaffold fabrication because of its biodegrad-

ability. Because of the extra methyl group in PLA repeating unit in comparison

to PGA, PLA is more hydrophobic. The hydrophobic methyl group reduces

the molecular afﬁnity to water and leads to a slower hydrolysis rate of PLA.

Fig. 2. Polymers frequently used as scaffolds for tissue engineering.

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