Hydrogels

• What are hydrogels?

  Hydrophilic polymer networks that are able to swell and retain large amounts of water and maintain 3D swollen structures
• What is rheology?

  The study of flow and deformation in shear - can be used to measure viscosity and stiffness

  
  

  Shear = a motion where two objects go in opposite directions (twist-type)
• What is viscosity?

  A liquid's resistance to flow
• What is stiffness?

  Mechanical properties of a solid, primarily the elasticity
• When is something considered a gel?

  When the components of the liquid phase contribute to a change in viscosity/stiffness
• What are some biomedical applications of hydrogels?

  • Contact lenses - ideal properties like transparent, wettable, permeable to oxygen, biocompatible
  • Drug delivery, e.g. Cervidil - carrier, high retention rate, promotes differentiation upon trigger
  • Wound dressings, e.g. GranuGel - used a filler to maintain a moist environment to promote healing
  • Implant coating - provide cell-friendly "envelope" on device
  • Regenerative medicine - used to promote acceptance from the body to not attack implanted device
• Characteristics of hydrogels to consider for regenerative medicine

  • Non-toxic
  • Mechanically stable/controlled degradation
  • Provide chemical and physical cues to the cells
  • Physical architecture - e.g. how thick/thin a fibre, whether a lot of space to move through or not
  • Support cells/guide cells
  • Promote healing and tissue reconstruction
  • Integrate with native tissue
  • Promote functionality
• Hydrogels from natural materials

  Most hydrogels originate from derivatives of nature

  • Collagen (often rat tail, can be bovine tendon)
  • Laminin (mouse sarcoma)
  • Fibrin (human blood plasma)
  • Alginate (brown seaweed)
  • Carrageenan (red seaweed)
  • Agarose (seaweed)
  • Decellularised tissue
• Hydrogels from synthetic materials

  • Poly(lactic acid)
  • Poly(glycolic acid)
  • Calcium phosphate - often used to recreate bone, resembles cement but carried by hydrogels
  • Poly(urethane)
  • Synthetic peptides
• Natural vs. synthetic biomaterials?

  Natural - already done most of the hard work

  • Fibrous
  • Non-toxic (mostly)
  • Can be functionalised with cell-guiding cues

  Synthetic

  • Can be designed (and therefore controlled)
  • No batch-batch variation
  • No sacrifice of animal
• What causes gelation in hydrogels?

  Connection of polymer chains via. chemical bonds or entanglement causes gelation - gives rise to 3D network
• Describe chemical crosslinking

  • Formation of covalent bonds between polymer chains within the hydrogel network
  • Typically created by the reaction of functional groups on the polymer chains with crosslinking agents or reactive species
  • Advantages = strong/permanent bonds that enhance the mechanical strength, stability, and durability of the hydrogel. Control over crosslinking density and network architecture allows tuning of hydrogel properties for specific applications
  • Disadvantages = excessive crosslinking or cytotoxic crosslinking agents may affect cell viability and biocompatibility

  
• Describe physical crosslinking

  • Non-covalent interactions, such as hydrogen bonding, ionic, and hydrophobic interactions to stabilise the hydrogel network
  • Reversible and dynamic interactions - formation and dissolution of crosslinks in response to environmental stimuli
  • Advantages = mild processing conditions, biocompatible, encapsulate sensitive bioactive molecules/cells within hydrogel (stimuli-responsive)
  • Disadvantages = weaker/less stable hydrogel networks. Reversible nature may lead to structural changes or degradation over time, affecting long-term stability/function of the hydrogel

  
• What are the types of gelation mechanisms

  Broad category

  • Chemical
  • Physical

  Sub category

  • Ionic
  • Crystallisation
  • Enzyme catalysed
  • LCST/UCST (lower critical solution temperature/upper critical solution temperature)
  • UV
  • Dual

  Hydrogels can be bound by more than 1 mechanism
• Describe dual crosslinking

  • Combine both chemical and physical crosslinking mechanisms to create a hybrid polymer network with synergistic properties
  • Benefit from the strength and permanence of chemical bonds as well as the responsiveness and reversibility of physical interactions
  • Common strategies include incorporating physical crosslinking motifs into chemically crosslinked hydrogels or vice versa - forms secondary crosslinks that enhance the stability, mechanical properties, and responsiveness of the hydrogel
  • Complex gelation
• How are hydrogels characterised?

  • Mechanical strength
  • Porosity
  • Degradation
  • Biocompatibility
• Mechanical strength of hydrogels

  Hydrogel needs to account for the varying mechanical strengths within different parts of the body, e.g. brain tissue vs. ligament for joint movement

  
  

  Measured by

  • Tensile - deformation in stretch
  • Compressive - deformation through 'crush' (ability to withstand load)
  • Bend - deformation through flexion (ability to withstand stress at 3 different points)
  • Rheological - deformation in shear

  Test used is dependent on the application of the hydrogel

  
• Porosity of hydrogels

  Porosity enables material to be introduced into the body and allows cells to move

  
  

  Consider

  • How porous is the material? i.e. must account for the movement of cells throughout a structure
  • How big/small are the pores?
  • Are the pores interconnected? i.e. not just sitting in pcokets, but able to respond to each other
• Degradation of hydrogels


  As cells produce proteins for the ECM, they also produce enzymes which chew up the environment to make space for the newly synthesised proteins - essentially regenerating a new tissue matrix/environment

  
  

  Characteristics of degradation

  • Loss of structure/mechanical strength - breaking bonds of polymer chain leaves little support, gel needs to be able to stay in place long enough for cells to take over
  • Change in geometry - swelling ratio, potential gaps with nothing to fill it
  • Byproducts and potential toxicity issues - inflammatory responses?
• Biocompatibility of hydrogels

  • Cell response to material architecture and chemistry - hydrogels should be an accurate mimic
  • How cells affect hydrogel - do they accelerate the rate of degradation?
  • Inflammatory response - hydrogels provide base components that are supportive of cell environment (architecture and chemistry) but also promote certain behaviours, e.g. healing or regeneration
• How are hydrogels fabricated?

  • Original method of molding/casting
  • Limited by the technology we can currently access
  • Extruding method = syringes, electrospinning and 3D bioprinting
  • 3D bioprinting uses bioinks (including cells and GFs)
• Which hydrogel to use?

  Method 1: using what we have - decellularised ECM materials

  • Body already used to it - correct chemistry and physical structure
  • Can it be used as it is?
  • Can it be modified for our purpose?

  Method 2: creating what we want from scratch - peptide-based materials

  • Precise control
  • Where do you start?
  • Can we create something complex enough (based on current technology)?
  • Often inspired by nature (ECM)
• Describe decellularised tissue hydrogels

  • Consists of tissue where the cellular components have been removed, leaving behind the extracellular matrix (ECM) - retains native architecture, mechanical properties, and bioactive molecules
  • Generally involves freezing tissue then enzymatic degradation/mechanical degradation/detergents
  • Suitable as scaffolds for tissues/organs including skin, cartilage, bone, heart, liver, and blood vessels
  • Can be further functionalised to enhance their properties
  • Disadvantages = needs a donor (ethical concerns), expensive, time-consuming, wrong geometry
• Describe peptide-based hydrogels

  • Short peptide sequences that self-assemble into 3D networks via. non-covalent interactions - e.g. hydrogen bonding, hydrophobic interactions, electrostatic interactions, or π-π stacking
  • Peptides designed to mimic structural motifs found in natural ECM proteins - e.g. collagen, elastin, or fibronectin
  • Advantages = tunable mechanical properties, biodegradability, biocompatibility, and the ability to incorporate bioactive motifs for cell signaling and tissue regeneration
• De novo peptides in hydrogels

  • Trying to mimic the design rules that lead to functional protein quaternary structure - it is difficult to recreate whole proteins but secondary structure is more manageable
  • Many researches attempting to create a-helices or B-sheets - bring them together to form a coiled shape
  • Created hSAF (hydrogenating self-assembling fibres) for coiled shape - by themselves hSAF do not mimic ECM, need a secondary complementary structure to wrap around, and form a dimeric unit (and thus fibres) which can entangle and form 3D hydrogels