Question

1. name at least four mechanisms that may hold a hydrogel together without covalent bonds? In other words, physical crosslinking?

2. provide 2 examples on how pH sensitive polymers/hydrogels may be useful in biomedical field?

Please provide detain explanation how pH changed in human body and how the polymer in response to such change?

3. Why do think we need to incorporate cell instructive peptide (such as RGD) into PEG hydrogel

4. What are the two major degradable strategy in hydrogel?

What are the chemical components of normal enzymatic degradable crosslinker? Do you know why?

Answer 1

Answer -1

A hydrogel is simply a hydrophilic polymeric network cross-linked in some fashion to produce an elastic structure.Thus, any technique which can be used to create a cross-linked polymer can be used to produce a hydrogel. Copolymerization/cross-linking free-radical polymerizations are commonly used to produce hydrogels by reacting hydrophilic monomers with multifunctional cross-linkers.Mechanisms that may hold a hydrogel together without covalent bonds are-

(a) chain entanglements; (b) H-bonds; (c) affinity recognition; (d) ionic interactions; (e) hetero-dimer complexation; (f) stereocomplex formation; (g) crystallite formation.Any of the various polymerization techniques can be used to form gels, including bulk, solution, and suspension polymerization.

Polymerization by irradiation is a new technique for hydrogel prepration In this method ionizing high energy radiation, like gamma rays and electron beams, has been used as an initiator to prepare the hydrogels of unsaturated compounds. The irradiation of aqueous polymer solution results in the formation of radicals on the polymer chains. Also, radiolysis of water molecules results in the formation of hydroxyl radicals, which also attack the polymer chains, resulting in the formation of macro-radicals.

Answer 2-

All the pH-sensitive polymers contain pendant acidic (e.g. carboxylic and sulfonic acids) or basic (e.g. ammonium salts) groups that either accept or release protons in response to changes in environment pH.

Drug Delivery-pH-sensitive hydrogels have been most frequently used to develop controlled release formulations for oral administration. The pH in the stomach is quite different from the neutral pH in the intestine, and such a difference is large enough to elicit pH dependent behavior of polyelectrolyte hydrogels.This type of delivery mechanism is ideal for the the release of the antibiotics in case of the infection of Helicobacter pyroli.

In case of cancer one of the leading disease of the world, these environment sensitive polymers find immense usage.When tumor cells proliferate and divide its forms an irregular mass of the cells. The inner layer of this mass has no oxygen and the cancer niche is acidic. By targeting these polymers site specific drug delivery to cancer cells can be acheived.

Design of Biosensor-

pH-sensitive hydrogels have also been used in making biosensors and permeation switches.The pH-dependent hydrogels for these applications are usually loaded with enzymes that change the pH of  the local microenvironment inside the hydrogels.One of the common enzymes used in pH-sensitive hydrogels is glucose oxidase which transforms glucose to gluconic acid. The formation of gluconic acid lowers the local pH, thus affecting the swelling of hydrogels.

Answer 3-

Arginylglycylaspartic acid (RGD) is a sequence of the most common peptide motif responsible for cell adhesion to the extracellular matrix. Cell adhesion proteins, integrins recognize this sequence and helps in binding of the cell.RGD is the principal integrin-binding domain present within ECM proteins such as fibronectin, vitronectin, fibrinogen, osteopontin, and bone sialoprotein.

The RGD sequence can bind to multiple integrin species, and synthetic RGD peptides offer several advantages for biomaterials applications. Because integrin receptors recognize RGD as a primary sequence (although conformation of the peptide can modulate affinity), the functionality of RGD is usually maintained throughout the processing and sterilization steps required for biomaterials synthesis, many of which cause protein denaturation. The use of RGD, as compared with native ECM proteins, also minimizes the risk of immune reactivity or pathogen transfer, particularly when xenograft or cadaveric protein sources are utilized. Another benefit is that the synthesis of RGD peptides is relatively simple and inexpensive, which facilitates translation into the clinic. Finally, RGD peptides can be coupled to material surfaces in controlled densities and orientations. These advantages of straightforward synthesis, minimal cost, and tight control over ligand presentation cannot readily be achieved when using full-length native matrix proteins to functionalize material surfaces.

Answer 4-

Polymeric material properties are typically changed through polymerization/cross-linking (bond forming events) or through controlled degradation and/or release (bond breaking events). Bond forming events typically often use small molecule reagents (initiators, catalysts, monomers, ligands to be conjugated to the material) while bond breaking typically does not rely on exogenous reagents. Small molecules often have more adverse effects in vitro and in vivo than polymeric reagents so many research groups use degradation as a tool for in situ manipulation of polymeric biomaterials.

Hydrolytic Degradation

The mechanism of degradation most commonly utilized in hydrogels is hydrolysis, in which a molecule of water adds to the polymer backbone, causing chain scission. Anhydrides, esters and amides are all susceptible to hydrolysis. Anyhydrides typically hydrolyze too quickly, and the uncatalyzed hydrolysis of amides is too slow, so most hydrogels that degrade hydrolytically utilize ester linkages. In order to obtain hydrolytically degradable hydrogels with physiologically relevant time scales of degradation, researchers typically functionalize PEG with degradable ester linkages using lactide or glycolide segments.

lcohol chain ends on PEG can initiate ring-opening reactions of 3,6-dimethyl-1,4-dioxane-2,5-dione and 1,4-Dioxane-2,5-dione to generate PEG-lactide and PEG-glycolide, respectively

Figure-Synthesis of PEG-lactide and PEG-glycolide.

Enzymatic Degradation

Although ester linkages are enzymatically degradable, most researchers utilize sequence-specific enzymatic degradation of peptides incorporated into hydrogels rather than non-specific enzymatic degradation of esters and amides. Hubbell′s group pioneered this approach by incorporating matrix metalloproteinase (MMP) sensitive linkages into hydrogels via Michael addition of cysteine-functionalized peptides across acrylates, maleimides and vinyl sulfones

Figure-Enzymatically degradable hydrogels via Michael addition of cysteinecontaining peptides to vinyl sulfone groups

In both hydrolysis and enzymolysis, the rate of degradation is predetermined by the chemistry of the macromer. In hydrolysis, the degradation rate of the material is pre-engineered through the identity (e.g., hydrophobicity or hydrophilicity) and number of the hydrolysable groups, and cannot be changed once the material is fabricated. In enzymolysis, the degradation typically occurs in an area local to the cells producing the enzyme. While hydrolysis and enzymolysis are both effective methods for sustained hydrogel degradation and sustained release of therapeutic agents, the rate of release cannot be adjusted or arrested after the hydrogel is fabricated, and release is not spatially controlled.