L15 - Tissue Engineering and Nanobiotechnology

Controlled Release System

Although millimeter-scale and micrometer-scale controlled release systems are well studied, nanometer-scale polymer delivery systems

• Can be easily injected or inhaled
• Can be internalized by many types of cells
• Can inject particles for circulation or used to release drug locally

Can be used to deliver higher drug doses into cell interior $\to$ higher efficacy

Nanoparticle Surface Modifications

Polyethylene Glycol (PEG)

• Suppress nonspecific interaction with the body
• Reduce blood clearance level of the drug carriers
• Achieve a stealth effect
• Increase stability during storage or application

Antibody

Y-shaped protein that neutralize pathogens by recognizing antigen of the pathogen

Targeting Ligands

Composed of ions or neutral molecules that bond to a central metal atom or ion $\to$ controlled interactions

Cellular Uptake and Endocytosis of Nanoparticles

• Passive diffusion of free drug (concentration gradient, high to low)
• Nonspecific phagocytosis of a nanoparticle
• Drug entrapped in fluid and uptake by pinocytosis
• Receptor-mediated endocytosis

Tissue Engineering

• Maintain tissues
• Repair tissues
• Replace tissues
• Enhance tissue functions

Processes

1. Induction of aggregation
   • Addition of polymers that stimulate cell adhesion
2. Provision of scaffolds
   • Control microarchitecture and adhesion for invasion by specific cells
3. Sustained release of bioactive agents from localized sources

Principles

• Cells
• Matrix (Scaffold)
  • Porous, biodegradable materials
  • Regulate cell functions
• Regulators
  • Chemicals
  • Mechanical

Degradable Scaffolds

• For molding the overall tissue size and shape
• Optimize scaffold microgeometry for cell recruitment
• The synthetic polymer can be programmed to dissolve as the tissue from emerges

Artificial Skin

Integra

• Only nonliving components
  • Silicone upper layer
  • Extracellular matrix lower layer

In clinical use, the silicone layer is replaced after 3 weeks with a thin epidermal graft.
Include autografted cells in the lower layer prior to use in the patient

Apligraf

• Newborn human foreskin from circumcision
• Supply in 1 week
• Allograft (from another human)
• High growth potential
• Consists of all the cell types making up the skin tissue

But it lacks the formation of a stable vasculature (so no blood cells), which also affects the immune system

Production

Blood Vessels

1. Human endothelial cells and microspheres are suspended with collagen/fibronectin in 3D gels
2. Cells $\to$ cords $\to$ blood vessels
3. Implement the construct into a mouse
4. New blood vessels lined with human endothelial cells

Cells Source

• Patient
  • Limited supply
  • Cannot be off-the-shelf
• Animal/Other humans
  • Immune acceptance
  • Disease transmission

Scaffold Biomaterials

• Provide a surrounding that mimics that natural matrix environment
• Provide right clues (protein signals)
• Provide a mechanical environment that the normal tissue is exposed to (blood vessels, bone)

Vascular Beds

1. Human endothelial cells and microspheres that slowly release vascular endothelial growth factor are both suspended with collagen/fibronectin in 3D gels.
2. Cells begin to form cords, then eventually become new blood vessels
3. Implantation of the construct into a mouse
4. New blood vessels lined with human endothelial cells are formed

Individual Vessels

1. Tissue engineered arteries formed in bioreactor over 8 weeks
2. Tubes of a synthetic, biodegradable polymer (polyglycolic acid) mesh were seeded with vascular smooth muscle cells and then grown in a special reactor
3. Using pulsatile flow through the tubes, the growing tissue experienced pulsatile radial strain (just like the physiological environment of blood vessels)
4. Degrading polymer mesh replace by cells and secreted extracellular matrix (collagens and glycosaminoglycans)

Offline production and transport $\to$ off-the-shelf, no wait time