8.3 Scaffolds for Tissue Engineering

Scaffold Structure and Properties

Scaffolds serve as temporary 3D frameworks that guide cells to regenerate functional tissue. They need to mimic the structural and mechanical environment of native tissue while degrading at just the right pace. Getting scaffold design right means balancing porosity, mechanical strength, degradation rate, and biological signaling all at once.

Porosity and Pore Characteristics

Porosity is the percentage of void (empty) space within a scaffold. Most tissue engineering scaffolds target 60–90% porosity, though the exact value depends on the tissue you're trying to regenerate.

Why does porosity matter so much? Three reasons:

• Cell migration — cells need open space to move through the scaffold and colonize it
• Nutrient diffusion — oxygen and nutrients must reach cells deep inside the scaffold, not just on the surface
• Waste removal — metabolic byproducts need a path out

Pore size is just as important as total porosity, and optimal ranges differ by tissue type:

• Bone: 100–350 μm
• Skin: 20–125 μm
• Hepatocytes (liver cells): ~20 μm

Beyond size and volume, interconnectivity describes how well pores are linked to each other throughout the scaffold. A scaffold could have 80% porosity but still fail if those pores are isolated dead ends. High interconnectivity ensures uniform cell distribution, continuous nutrient transport, and consistent tissue growth across the entire construct.

Mechanical and Degradation Properties

A scaffold's mechanical properties need to approximate those of the tissue it's replacing. If the scaffold is too stiff or too soft relative to the native tissue, cells receive the wrong mechanical cues and the construct can fail under physiological loads.

Target elastic modulus values vary enormously:

• Bone: 10–1,500 MPa
• Cartilage: 0.5–1 MPa
• Soft tissues like brain or fat: well below 0.1 MPa

Biodegradation is the other half of the equation. The scaffold should degrade as new tissue forms, gradually transferring mechanical load to the regenerating tissue. If it degrades too fast, the construct loses structural support before the tissue can bear load. Too slow, and the persistent scaffold material blocks full tissue remodeling.

You can tune degradation rate by:

• Changing material composition (e.g., adjusting the ratio of PLA to PGA in a copolymer)
• Altering crosslinking density (more crosslinks = slower degradation)
• Modifying molecular weight of the polymer

The ideal scenario is that scaffold degradation rate and new tissue formation rate are closely matched, so there's never a gap in structural integrity.

Cell-Scaffold Interactions

Cell Seeding and Attachment

Cell seeding is the process of introducing cells onto or into a scaffold. The method you choose directly affects how uniformly cells distribute throughout the construct, which in turn affects tissue quality.

Three main seeding approaches:

1. Static seeding — cells are pipetted onto the scaffold and allowed to settle by gravity. Simple and inexpensive, but cells tend to concentrate on the scaffold surface with poor penetration into the interior.
2. Dynamic seeding — the scaffold is placed in a spinner flask or centrifuge to agitate the cell suspension. This improves distribution compared to static methods but can subject cells to shear stress.
3. Perfusion seeding — cell suspension is actively pumped through the scaffold's pore network. This produces the most uniform distribution, especially in thick scaffolds, because fluid flow carries cells deep into the interior.

Once cells reach the scaffold, they need to attach. Attachment depends on surface chemistry and topography. Smooth, hydrophobic surfaces tend to resist cell adhesion, so several surface modification strategies are used:

• Plasma treatment increases surface hydrophilicity and introduces reactive functional groups
• Protein coatings (fibronectin, collagen) provide familiar binding sites for cell integrins
• Cell-adhesive peptides, particularly the RGD sequence (Arg-Gly-Asp), can be grafted onto scaffold surfaces to specifically engage integrin receptors and promote attachment and spreading

Vascularization and Growth Factor Delivery

Any engineered tissue thicker than about 100–200 μm faces a diffusion limit: cells in the interior can't get enough oxygen and nutrients by passive diffusion alone. Vascularization solves this by establishing a blood vessel network within the construct.

Strategies to promote vascularization:

• Angiogenic growth factors like VEGF (vascular endothelial growth factor) and bFGF (basic fibroblast growth factor) stimulate new blood vessel sprouting
• Co-culture systems that combine endothelial cells with tissue-specific cells (e.g., osteoblasts for bone) encourage endothelial cells to self-organize into vessel-like structures
• Pre-formed vascular channels created using microchannels or sacrificial templates (materials that dissolve after the scaffold is fabricated, leaving hollow channels behind)

Growth factor delivery extends beyond vascularization. Different factors drive different cell behaviors:

Controlled release is critical because a single burst of growth factor won't sustain tissue development. Two common approaches:

• Encapsulation in degradable microspheres or nanoparticles, which release the factor as the carrier degrades
• Covalent binding to the scaffold material, providing release tied to scaffold degradation

Both methods give you spatial control (where the factor is released) and temporal control (when and how fast it's released).

Scaffold Fabrication Techniques

Additive Manufacturing Methods

3D printing has transformed scaffold fabrication because it offers precise, reproducible control over architecture, pore geometry, and material composition.

Key 3D printing techniques for scaffolds:

• Stereolithography (SLA) — a UV laser selectively photopolymerizes a liquid resin layer by layer. Produces high-resolution structures with complex geometries, but material choices are limited to photocurable polymers.
• Fused deposition modeling (FDM) — a thermoplastic filament is melted and extruded through a nozzle, building the scaffold layer by layer. Straightforward and cost-effective, though resolution is lower than SLA.
• Selective laser sintering (SLS) — a laser fuses powder particles (polymers, ceramics, or composites) into solid layers. Good for mechanically robust scaffolds, particularly for bone applications.
• Bioprinting — a specialized form of 3D printing that deposits "bioinks" containing living cells and hydrogel biomaterials simultaneously. This allows direct fabrication of cell-laden constructs with defined spatial organization.

A major advantage across all these methods: scaffolds can be designed from patient-specific medical imaging data (CT or MRI scans), producing constructs that match the exact geometry of a defect site.

Fiber-Based and Biological Scaffold Fabrication

Electrospinning produces fibers ranging from nanometer to micrometer scale by applying a high voltage to a polymer solution, drawing it into a thin jet that solidifies into fibers as it travels to a collector. The resulting fibrous mats closely mimic the structure of native extracellular matrix (ECM), which is itself a nanofibrous network.

You can control fiber properties by adjusting processing parameters:

• Fiber diameter — tuned by changing polymer concentration, voltage, or flow rate
• Fiber alignment — random fibers result from a stationary collector; aligned fibers (useful for nerve or tendon scaffolds) are produced using a rotating collector

Electrospun scaffolds have been applied to skin, blood vessels, and neural tissue, among others.

Decellularized matrices take a fundamentally different approach: instead of building a scaffold from scratch, you start with a native tissue or organ and strip away all the cells, leaving behind the ECM scaffold intact.

The decellularization process typically involves:

1. Perfusing the tissue with detergents (e.g., SDS or Triton X-100) to lyse and remove cells
2. Washing with enzymatic solutions (DNase, RNase) to clear residual DNA
3. Sterilizing the remaining matrix

What you're left with is a scaffold that preserves the natural architecture, mechanical properties, and biochemical composition of the original tissue. This includes embedded growth factors and structural proteins that synthetic scaffolds can't easily replicate. Decellularized scaffolds have been explored for whole organ engineering, including heart, lung, and liver, where recreating the complex internal architecture from scratch remains extremely challenging.