B Y M o h a m e d M a h m o u d A b d u l - m o n e m

A S S I S TA N T L E C T U R E R D E N TA L B I O M AT E R I A L S D E PA R T M E N T

FA C U LT Y O F D E N T I S T R YA L E X A N D R I A U N I V E R S I T Y

E G Y P T

M o h a m e d _ m a h m o u d . b i o m a t e r i a l s @ y a h o o . c o m

Scaffolds for tissue engineering

Contents

IntroductionWhat are scaffolds?Biommetic scaffoldsRequirements of scaffoldsProperties of scaffoldsRole of scaffoldsFabrication techniquesArchitectureTypes of scaffoldsReferences

Introduction

Tissue engineering (TE) is a rapidly growing scientific area that aims to create, regenerate, and/or replace tissues and organs by using combinations of cells, biomaterials, and/or biologically active molecules.

TE intends to help the body to produce a material that resembles as much as possible the body’s own native tissue.

The classical TE strategy consists of:

1.Isolating specific cells through a biopsy from

a patient, growing them on a biomimetic scaffold under controlled culture conditions.

2.Delivering the resulting construct to the desired site in the patient’s body.

3. Directing the new tissue formation into the scaffold that can be degraded over time.

Tissue engineering Triad

What are scaffolds?

Scaffolds: Serve as temporary or permanent artifical Extracellular Matrices (ECM) to accommodate cells and support 3D tissue regenerations .

What is ECM? blend of macromolecules (proteins and

carbohydrates) around cells—as space fillers.

Biomemtic Scaffolds

Biomimetics is defined as the application of methods and systems, found in nature, to technology and engineering.

Mimicking the naturally occurring extracellular matrix (ECM) and how this is a promising approach to effectively tailor cell response and to successfully engineer replacement tissues.

This biomemtic approach is used in developing scaffolds for tissue engineering of several tissue types.

These include :Hard tissue, such as :1. Bone (trabecular scaffolds, nanofibrous

scaffolds)2. Bone/ligament junctions(triphasic scaffolds), Soft tissue, including :3. Eye (limbal-corneal junction scaffolds),4. Nervous (neural regeneration through the

use of neural progenitor cells [NPC]), 5. Vascular tissues (hydrogels for angiogenesis).

Trabecular scaffolds

Triphasic scaffolds

Requirements of scaffolds

(i) Three-dimensional and highly porous with an interconnected pore network for cell growth and flow transport of nutrients and metabolic waste.

(ii) Biocompatible and bioresorbable with a controllable degradation and resorption rate to match cell/tissue growth in vitro and/or in vivo.

(iii) Suitable surface chemistry for cell attachment, proliferation, and differentation .

(iv) Mechanical properties to match those of the tissues at the site of implantation.

Properties of scaffolds

Scaffold composition Materials that constitute the scaffold can be

distinguished by the chemical composition. Pure, non-organic materials can be

distinguished from composite materials (also containing organic materials) and sole organic materials.

Materials can also be grouped by whether they are in a solid or gel-like condition .

Macrostructure The macrostructure reflects the external

geometry and gross internal structure of the scaffold.

A three-dimensional scaffold that is congruent to the external geometry of the tissue to be replaced is desired for scaffold placement and fixation in the clinical situation.

Porosity and pore interconnectivityScaffolds are constituted of either bulk materials or

they have a pore or tube geometry. Pores or tubes can be introduced in scaffolds in an

isolated fashion or they can be interconnected.

An advantage of an interconnected porous or tubular systems is the improved nutritional supply (by diffusion or directed fluid flow) in deeper scaffold areas, thereby enabling cells to survive in these regions.

As researchers indicated the need for pore sizes ranging from 200–500 μm for vascular ingrowth.

Scaffolds containing tubular structures of such diameters seem to be beneficial in bone tissue engineering applications.

Surface/volume ratio A high overall material surface area to

volume ratio is beneficial in respect to allowing large numbers of cells to attach and migrate into porous scaffolds.

Mechanical properties Scaffolds should ideally have sufficient

mechanical strength during in vitro culturing to resist the physiological mechanical environment in regenerating load-bearing tissues (cartilage, bone) at the desired implantation site.

Degradation characteristics The ideal scaffold degradation must be

adjusted appropriately such that it parallels the rate of new tissue formation and at the same time retains sufficient structural integrity until the newly grown tissue has replaced the scaffold’s supporting function.

Scaffolds should degrade without release of toxic products.

Role of scaffolds in tissue engineering

Serve as a framework to support cell migration into the defect from surrounding tissues

Serve as a delivery vehicle for exogenous cells,growth factors and genes

Serve as a matrix for cell adhesionStructurally reinforce the defect to maintain the

shape of the defect and prevent distortion of surrounding tissues

Serve as a barrier to prevent the infiltration of surrounding tissues that may impede the regeneration process

Fabrication techniques

Conventional Rapid prototyping

Solvent casting/particulate leaching

3D printing

Fiber meshing 3D plotting

Melt moulding Laser sintering

Gas foaming

Membrane lamination

Freeze drying

Gas foaming and freeze drying

Solvent casting and particulate leaching

Fiber electrospining

Rapid prototyping

Architecture

Fiber –meshSponge-likeFine filament meshInjectable hydrogels3D-printed

Fiber-Mesh Sponge -like

Architecture

3D printed Hydrogels

Architecture

Types of scaffolds

Scaffolds

Biocompatibility

Bioinert Bioactive Bioresorbable

Material

Natural synthetic

Types of scaffolds

Bioinert : The term bioinert refers to any material that once placed in the human body has minimal interaction with its surrounding tissue. e.g Titanium

Bioactive :refers to a material, which upon being placed within the human body interacts with the surrounding bone and in some cases, even soft tissue. e.g HA

Bioresorbable :refers to a material that upon placement within the human body starts to dissolve (resorbed) and slowly replaced by advancing tissue (such as bone). e.g Tricalcium phosphate

Classification of bioceramics according to their bioactivity; (a)bioinert, (Dental implant), (b) bioactive, hydroxyapatite (Ca10(PO4)6(OH)2) coating on a

metallic dental implant, (c) Surface active, bioglass (d) bioresorbable tri-calcium phosphate [Ca3(PO4)2].

Polymeric scaffolds

Polymeric Scaffolds for bone regeneration

Two categories:A) Materials for porous solid-state scaffolds and B) Materials for hydrogel scaffolds

The choice of scaffolding materials depends on the environment of original ECM due to specific application for scaffold.

E.g : CartilageECM=Hydrated, Bone ECM=Dense

Solid porous scaffolds Hydrogels

Hydrogels

Highly hydrated hydrophilic polymer networks contain pores and void space between the polymer this provide many advantages over the common solid scaffold materials, including an enhanced supply of nutrients and oxygen for the cells.

Pores within the network provide room for cells, and after proliferation and expansion, for the newly formed tissue.

All hydrogels contain approximately 90% water

Materials for porous solid-state scaffolds

Application: Bone tissue

engineering

Material properties: Solid and stable porous

structures. Donot dissolve or melt

under in vitro tissue culture condition or when implanted in-vivo

Degrade through hydrolysis of the chemical bonds.

Materials for hydrogelScaffolds

Application: Blood vessels, skin,

cartilage, ligaments, and tendons

Material properties:• Ability to fill

irregularly shaped tissue defects.

• the allowance of minimally invasive procedures such as arthroscopic surgeries

• the ease of incorporation of cells and bioactive agents

Polymeric scaffolds

Solid state or Hydrogels

Natural

Protein origin

CollagenFibrinGelatinAlbumin

silk

Polysaccharide origin

AlginateChitosan

Hyaluranon

synthetic

Aliphatic polyesters

PLAPGA

PLGAPCL

Polymeric scaffolds(Natural origin)

Protein origin polymeric scaffolds1.CollagenCollagen is one of the main components of

the extracellular matrix in many mammalian tissues.

It is composed of triple-helical peptide strands that arrange in several tissue-specific combinations.

Due to the fact that collagen is derived from natural sources that include animals,there are always concerns of immunogenicity and contamination with viruses.

2.Gelatin

Gelatin is a polymer that is directly derived from collagen.

It can be obtained via basic or acidic hydrolysis of collagen from different tissues of various mammalian species or fish.

With regards to biocompatibility,it is similar to collagen.

Both collagen and gelatin have the advantage of already containing a sufficient number of adhesion sites for cells; Because of this, further functionalization is not necessary to promote cellular adhesion.

3.Fibrin

Fibrin formation naturally occurs as part of the blood coagulation process in damaged blood vessels and wounds.

Fibrin is enzymatically obtained by cleavage of fibrinogen in the presence of thrombin.

The liberated fibrin then forms distinct aggregates that lead to coagulation.

Show excellent biocompatibility for many tissue-engineering applications and the materials are commonly used as wound sealant in surgical procedures.

However, their long-term stability is very limited, as fibrin is readily degraded by fibrinolysis in the patient, which is the naturally occurring elimination mechanism during wound healing

Polymeric scaffolds(Natural origin)

Polysaccharide origin polymeric scaffolds1.HyaluranonNaturally occurring polysaccharidesHyaluronic acid is naturally involved in tissue

repair and is also the main component of the ECM of cartilage, making it an ideal material for cartilage tissue engineering.

Because of its hydrophilic nature, it requires further modification with adhesion-mediating peptides to allow sufficient cell attachment.

2.Alginate

Alginate is a hydrophilic and negatively charged polysaccharide.

It is derived from brown algae and is obtained after several extraction and hydrolysis steps.

It is formed of guluronic acid (G-blocks) which is one of the two components of alginate, and works through the formation of egg-carton-like structures that are able to complex the calcium ions between neighboring polymer chains.

The other component of alginate, isomeric mannuronic acid blocks (M-blocks) , does not take part in the cross-linking step.

3.Chitosan

Chitosan is a polysaccharide which is derived from arthropod exoskeletons.

It shares some characteristics with glycosaminoglycans from articular cartilage of mammals, and it is therefore used frequently as a scaffold material for cartilage and bone tissue engineering.

Polymeric scaffolds(Synthetic polymers)

Synthetic polymers are less prone to undesirable issues such as :

Remaining byproducts (allergenic or pathogenic)

Batch-to-batch variations . Risk of immunogenecity

which are common problems associated with natural polymers.

Polylactic acid (PLA)

PLA is semicrystalline and brittle .

PLA polymers are generally considered to be lipophilic polymers that only take up about 5–10% water in aqueous surrounding(hydrophobic) .

Hydrolytic degradation yields lactic acid which is a natural metabolite.

They would not traditionally be classified as hydrogel forming polymers.

However, through copolymerization with more hydrophilic monomers or the incorporation of short poly(ethylene glycol) (PEG) chains, PLA polymers can even be rendered water soluble.

Polyglycolide (PGA)

PGA is hard,tough and crystalline.

Hydrolytic degradation yields glycolic acid which is a natural metabolite.

Currently polyglycolide and its copolymers (poly(lactic-co-glycolic acid) with lactic acid, poly(glycolide-co-caprolactone) with ε-caprolactone, and poly (glycolide-co-trimethylene carbonate) are widely used as a material for the synthesis of absorbable sutures.

Metallic scaffolds

Metallic scaffolds

The main disadvantage of metallic biomaterials is their lack of biological recognition on the material surface.

To overcome this restraint, surface coating or surface modification presents a way to preserve the mechanical properties of established biocompatible metals improving the surface biocompatibility.

Another limitation of the current metallic biomaterials is the possible release of toxic metallic ions and/or particles through corrosion or wear that lead to inflammatory cascades and allergic reactions, which reduce the biocompatibility and cause tissue loss.

Tantalum

Porous tantalum is a biomaterial with a unique set of physical and mechanical properties.

It has a high-volume porosity (>80%) with fully interconnected pores to allow secure and rapid bone ingrowth.

Magnesium and its alloys

These alloys have great potential, and it has been shown that they are fully bioresorbable, have mechanical properties aligned to bone, induce no inflammatory or systemic response, are osteoconductive, encourage bone growth, and have a role in cell attachment.

However, concerns over the toxicity of dissolved Mg have been raised, but it has been shown that the excess of magnesium is efficiently excreted from the body in urine.

In addition, the dissolution rate in physiological conditions is rapid, potentially leading to hyper-magnesia.Symptoms include weakness, confusion, decreased breathing rate, and cardiac arrest.

Titanium and its alloys

Titanium is found to be well tolerated and nearly an inert material in the human body environment.

In an optimal situation titanium is capable of osseointegration with bone .

In addition, titanium forms a very stable passive layer of TiO2 on its surface and provides superior biocompatibility.

The nature of the oxide film that protects the metal substrate from corrosion is of particular importance.

Titanium meshes and porous Ti granules

References

1. Fundamentals of tissue engineering and regenerative medicine,2009

2. Tissue engineering from lab to clinic ,2011