Scaffolds for Tissue Engineering

Topics:

  1. Scaffold Approaches in Tissue Engineering
  2. Scaffold Fabrication Techniques
  3. Physiochemical characterization of scaffolds

1. Scaffold Approaches in Tissue Engineering

There are basically two scaffolding approaches which have been typically used in tissue engineering. These are:

  1. Top-down approach
  2. Bottom-up approach
Tissue Engineering approaches (Nicol et al., 2009)

1.1. Top-down approach

The top-down approach involves the use of a polymeric scaffold or decellularized extracellular matrix (ECM) scaffold. Cells are seeded onto these biomaterial scaffolds and cultured until the cells fill the support structure to create an engineered tissue. Thus, the top-down scaffolding approach can be divided further into two groups.

  1. Pre-made Polymeric Porous Scaffolds
  2. Decellularized ECM

1.1.1.Pre-made Polymeric Porous Scaffolds

Pre-made scaffolds are fabricated from polymers using FDM 3D printing, electrospinning, freeze-drying and other techniques discussed in section 2 below. Cells are seeded onto these polymer scaffolds after fabrication and sterilization. However, this approach is limited by the inhomogeneous distribution of cells. Moreover, mimicking the native architecture of the tissue can also be a problem.

1.1.2. Decellularized ECM scaffolds

Decellularized matrices of xenogenic or allogenic tissues are nature simulating scaffolds in terms of their composition and mechanical properties. However, the difficulty in retaining the ECM, immunogenicity upon incomplete decellularization, and inhomogeneous distribution of cells are major challenges involved.

Decellularization of a tissue or organ is usually done using one of the following 3 approaches.

  1. Enzymatic Treatment: Trypsin, endonucleases and exonucleases can be used to digest the cells intact.
  2. Chemical Treatment: Washing the tissues with non-ionic detergents like Triton X-100 or ionic detergents like Triton X-200 and Sodium Dodecyl Sulfate (SDS).
  3. Physical or mechanical methods: Freeze-thaw, agitation, and sonication

After the decellularization process, these scaffolds are disinfected, dehydrated by lyophilization or vacuum pressing, and then sterilized. Proper care should be taken during each step as the microscopic and ultrastructural features can be damaged and thus, the mechanical property can get altered. These microscopic features play an important role in modulating cell behavior such as nutrient infiltration and cell migration.

Q: Why do we need to dehydrate the ECM scaffolds after decellularization?
A: Presence of water in the scaffold leads to the leaching of growth factors like VEGF and b-FGF during packaging. Dehydration makes the scaffold easier to handle and increases the shelf life without causing significant changes in the strength and bioactivity.

A: Presence of water in the scaffold leads to the leaching of growth factors like VEGF and b-FGF during packaging. Dehydration makes the scaffold easier to handle and increases the shelf life without causing significant changes in the strength and bioactivity.

The collagen fiber architecture and alignment of the decellularized ECM scaffolds are specific to the tissue-type. For example, the collagen fibers are highly aligned along the long axis of the tissue to provide the greatest resistance to strain in a load-bearing application in ligaments and tendons. Collagen fibers in the small intestine submucosa (SIS) have a preferred alignment along the native longitudinal axis of the small intestine.

1.2. Bottom-up approach

In the bottom-up approach, there are multiple methods for creating modular tissues, which are then assembled into engineered tissues with specific microarchitectural features.

A summary of various top-down and bottom-up scaffolding approaches in Tissue engineering. (Source: Chan et al, 2008)

2. Scaffold Fabrication Techniques

2.1. Freeform Printing

2.2. Porogen Leaching

2.3. Freeze Drying

3. Physiochemical Characterization of Scaffolds

In order to ensure the cytocompatibility of scaffolds, they are characterized by the following 5 ways.

  1. Surface properties (e.g., surface energy, chemistry, charge, surface area)
  2. Porosity and pore size
  3. Interconnectivity and tortuosity
  4. Degradation kinetics and its characterization (e.g., biodegradation)
  5. Mechanical competence (e.g., compressive and tensile strength)

3.1. Surface Properties

Cell attachment to surfaces is a complex process and it is affected by surface hydrophobicity, surface roughness, and protein adsorption. Proteins like vitronectin, fibrinogen, fibronectin and immunoglobulins adsorb onto the scaffold surfaces when in contact with physiological fluids.

3.2. Porosity and characterization of pores

Types of pores in scaffolds (Source: Chang et al.)

Optimal pore size is critical to cell survival on the scaffold. For osteoconduction, the optimal pore size should be 200 µm- 350 µm while for hepatocytes it’s nearly 20 µm. If pore sizes are too small, pore occlusion may occur and it may prevent cell penetration to the depths of the scaffold. Pores in scaffolds can be open, closed or blind end pores.

How pore tortuosity is defined. It’s the ratio of L/X in Figure (B). In other words. it’s the ratio of actual path length to shortest linear distance.

Pore interconnectivity and tortuosity are also important parameters. Tortuosity defined as the ratio of the actual path length through connected pores to Euclidean distance (shortest linear distance). Nutrient diffusion to the central regions of the scaffold is difficult for a highly tortuous scaffold. Tortuosity also affects cell migration rates.

3.5. Mechanical Competence

There are different requirements for mechanical properties based on the target tissue. For example, the scaffolds for load-bearing tissues like the bone has to be strong enough to withstand the high physiological stresses. However, they should not be so strong as to cause stress shielding at the implanted site, preventing mechanical stimulation for local bone tissue ingrowth. On the other hand, scaffolds for soft tissue regeneration have to be pliable and elastic.

SurajPanigrahi

Leave a Reply

Your email address will not be published. Required fields are marked *