Video summary

noc19 bt23 lec28 Tissue Engineering – I

Main summary

Key takeaways

Educational

Main ideas and concepts

1) Recap: protein adsorption on implant surfaces (from previous class)

  • When biomaterials/implants enter the body, proteins do not stay in a single layer; they adsorb as multi-layer protein structures.
  • Over time, this can form:
    • Hard corona: relatively stable proteins that don’t change much.
    • Soft corona: more weakly bound proteins that can exchange in and out.

Quantifying protein adsorption discussed:

  • SDS-PAGE: Denature surface proteins with heated SDS, then run gel electrophoresis to assess proteins and molecular weights.
  • Western blot: Use a specific antibody to estimate/summarize amounts of a target protein (semi-quantification) and identify which proteins are present.
  • SPR (Surface Plasmon Resonance): Measure protein adsorption through changes in surface dielectric properties as proteins bind.

2) Example/lesson from nanoparticles: material design can still trigger inflammation

  • A paper discussed gold nanoparticles coated with poly(acrylic acid) (PAA).

Mechanism highlighted:

  1. After entering the body, proteins such as fibrinogen adsorb.
  2. Adsorption/denaturation can expose hidden binding sites.
  3. Exposed sites interact with a receptor (Mac-1).
  4. This leads to upregulation of NF-κB, a key immune regulator.
  5. Result: increased cytokine release and downstream immune/inflammatory responses.

Key takeaway:

  • Even “seemingly biocompatible” materials (like gold) can cause inflammation depending on:
    • Surface coating
    • Surface charge
    • Particle size

Examples of how outcomes change:

  • Replacing PAA with a different polymer reduces fibrinogen-based inflammation (implied comparison).
  • Smaller nanoparticles (e.g., ~20 nm) caused less inflammation than larger ones (exact size comparisons were mentioned as a trend).

Design implication:

  • Drug delivery/tissue engineering materials must be designed considering how the protein corona forms, because that often drives immune effects.

Tissue Engineering: what it is and why it matters

Definition

  • Tissue engineering = combining cells + materials to improve, repair, or replace biological tissues.

Why it’s important (context)

  • Described as a “very exciting” and fast-moving field, with strong promise for patients.
  • The course will integrate tissue engineering concepts with drug delivery (because tissue engineering often involves delivering bioactive molecules/cues).

Real-world examples of tissue engineering / bioimplants

The lecture lists common clinical/industrial implant categories:

  • Hip joint replacement (metals: stainless steel, titanium; sometimes cobalt-chromium)
  • Heart pumps (for patients where heart pumping is insufficient)
  • Total knee replacement (often metal-based support for damaged cartilage context)
  • Fracture plates and screws (stabilization of structural load-bearing tissues)
  • Lenses and contact lenses (glasses are an external example; contact lenses can be polymer-based)
  • Artificial kidneys / dialysis (external support for kidney function)
  • Spinal and rib implants for structural stabilization
  • Additional mention:
    • Nasal deformity reconstruction (polymer additions)
    • Silicone breast implants

Controversial/illustrative landmark example

  • A widely discussed 1997 study by Joseph (Josefa) Vacanti’s group:
    • Cow chondrocytes (cartilage cells) were seeded into a scaffold and grown on a mouse.
    • They produced an ear-shaped tissue on the mouse.
    • The concept at the time: tissue-engineered grafts could be used for humans.
  • Follow-up trend:
    • Other groups reported similar results using human cells cultured and grown in animals.
  • Important experimental detail explained:
    • The mice used were nude mice, meaning they have no immune system, so they are less likely to reject introduced human/cow cells.

The “domains” (major approaches) of tissue engineering

The lecture describes tissue engineering as having roughly four to five domains (presented as categories), including both speculative and more translational approaches:

A) Whole tissue generation in vitro, then replacement (least direct clinically)

  • Generate an entire tissue in vitro (e.g., in a culture dish).
  • Implant/replace in the patient once ready.
  • Notes given:
    • The mouse-based demonstration raises ethical/translational issues (e.g., cross-species components like blood vessels).

B) Partial tissue generation for repair (more feasible translation)

  • If a patient’s tissue function is reduced (e.g., ~50% capacity), the goal is restore toward normal.
  • Approach:
    • Grow a part of the tissue in vitro.
    • Replace damaged portions in the patient.
  • Caveat:
    • Still requires more work before widespread human use.

C) Scaffold-based tissue growth by providing cues (promising but still hard)

  • Instead of growing entire tissues in vitro:
    • Provide a scaffold.
    • Seed/add cells and growth factors (as needed).
    • Use the scaffold’s properties to create a niche so cells can populate, grow, and regenerate damaged tissue in vivo.

D) Support/filler material to replace missing function (most clinically used)

  • This is described as highly translational and already used in clinics.
  • Examples:
    • Structural support: bone support using metals like stainless steel or titanium.
    • Vision support: contact lenses (polymer-based) where natural lenses/vision are impaired.
    • Dental support: artificial teeth replacing damaged teeth.

E) Regeneration domain status

  • Regeneration (i.e., creating/repairing tissue) is described as:
    • Translational but less clinic-used compared to support/filler materials.
  • Whole transformation is implied to still be difficult for near-term human application.

Deeper dive: “whole tissue replacement” approaches and barriers

1) Organ transplant (standard clinical approach)

Two main sources discussed

  • Human-to-human organ donation

    • Benefits:
      • Already heavily used in clinics.
    • Major problems:
      • Lack of donors
      • Disease transmission risk (e.g., chronic diseases like HIV)
      • Immune rejection
    • Immune rejection handling:
      • Immunosuppressants are used.
      • But immunosuppression increases susceptibility to infections.
      • Even then, rejection can still occur; typical survival described as only a few years depending on organ/location.

  • HLA typing strategy (to reduce rejection)

    • Humans differ by HLA types.
    • If donor and recipient have closer HLA match, immune rejection is reduced and graft survival improves.

Additional sources considered

  • Blood donation (works well because):
    • Blood cells have a short lifecycle (~3–4 months)
    • Immunogenicity is relatively low
    • Cells can be recycled within that time window
  • Genetically modified pig organs (“xenotransplantation” concept)
    • Pigs are considered relatively close to humans among animals.
    • Ongoing efforts involve genetic modification to reduce immune recognition.
    • Still stated as far from ready for routine clinics due to persistent major immune challenges.

2) Key technical barrier to growing whole tissues: vascularization/oxygen diffusion

  • When tissues get thicker:
    • Oxygen and glucose diffusion to inner cells becomes limiting.
    • Inner regions face cell death.
  • Therefore, tissue engineering struggles to replicate the body’s extensive blood vessel networks.
  • Resulting constraint:
    • Cultured tissues often limited to very thin thickness (up to ~millimeter scale mentioned).

3) Attempted solution: 3D printing of tissue structures

  • 3D printing can create larger tissue constructs with:
    • Channels/space for vessels
    • Intended to allow media flow and support larger tissue areas
  • Caveat:
    • Still “nascent” and unclear if constructs will be reliably functional in animal settings.

Decellularization (dECM/“decellularized” scaffolds)

Method described (research-focused; limited clinical use)

  • Take donor tissue (from cadavers/animals), e.g. heart/lung tissue.
  • Process it to remove cells and reduce immune-recognizable material:
    • Use reagents to remove:
      • Cellular components
      • Free proteins
      • Genetic material
  • Goal:
    • Make the scaffold non-immunogenic (or less immunogenic) by removing elements that would trigger immune recognition.
  • Then:
    • Seed patient-derived cells back onto the scaffold.
    • Existing/scaffold vessel structures can be used/repopulated.
  • Status:
    • Widely used in research; “not a whole lot” used clinically (as stated).

Speakers / sources featured

  • Joseph (Josefa) Vacanti’s group (1997 mouse ear tissue engineering landmark)
  • Mac-1 (receptor mentioned in the nanoparticle inflammation mechanism)
  • NF-κB (immune regulator mentioned in the inflammation pathway)
  • HLA typing / HLA (immunology concept; not a person/source)
  • SPR (Surface Plasmon Resonance) (technique)
  • SDS-PAGE and Western blot (techniques)
  • SDS (reagent used in SDS-PAGE context)
  • Decellularization (method category; not a person/source)

Original video