18.2.3 Hydrogels Based on Zwitterionic Polymers

Zwitterionic polymers can mimic cell membranes and can be an ideal candidate to en­

gineer biomaterials able to avert hostile interactions with biological cells. Sulfobetaine/

carbobetaine/phosphobetaine polyelectrolytes are highly biocompatible, encompassing

some natural analogs such as taurine, glycine betaine, etc. prospects in biomedical devices

and other such applications are being explored. Zwitterionic polymer is a special kind of

polymer that consists of both cationic and anionic groups on each monomer throughout

the polymeric backbone. Due to the presence of an equal number of anionic and cationic

groups in the polymeric backbone, they are highly hydrophilic and antifouling. They

interact with water via strong ion-dipole interaction and form a stable and dense hy­

dration shell. Polyzwitterions can interact with water molecules via electrostatic inter­

actions and the hydration layers around them can “distance” any other biomolecules

including protein. They are an ideal replacement for PEG. Their application as a drug

delivery vehicle has been recently reviewed [8].

Gels with a polyzwitterionic backbone were extensively reported. The potential use of

zwitterionic hydrogels is constrained by their poor mechanical strength. Mechanical

properties of non-fouling zwitterionic hydrogels were improved by substituting the

methacrylate backbone of sulfobetaine methacrylate (SBMA) by a vinyl imidazole back­

bone (SBVI)]; the non-fouling characteristics of the zwitterionic sulfobetaine group were

retained while aiding on mechanical properties diffusion behavior of fluorescence-labeled

model proteins in PEG, polySBMA and mixed PEG-sulfobetaine methacrylate hydrogels

(SBMA:PEG 4:1, SBMA:PEG 1:4) was studied [9]. Four hydrogels showed varied diffusion

characteristics for either a negatively charged protein or positively charged protein de­

pending on protein-polymer interaction and the labile water content available in the

hydrogel matrix.

Protein loading efficiency may increase as stronger interaction between protein-PEG is

observed than protein-polySBMA and controlled release is expected by changing the ratio

of PEG to SBMA in a hydrogel [10]. A narrowly dispersed zwitterionic poly(amido

amine) (PAA) nanogels tethered with N,Ndimethylethylenediamine having a positive

surface charge were developed for drug delivery and imaging. They showed excellent

stability in serum and minimal cytotoxicity. Stretchable tissue adhesive and antibacterial

hydrogels with zwitterionic monomers (strong dipole−dipole interaction, electrostatic

interaction, and hydrogen bonding with the skin surface) seem promising for wound

dressings and implantable devices. A polymer with phosphorylcholine (PC) and poly

(propylene glycol) (PPG) showed a steady release of insulin. On injecting the polymer

aqueous solution subcutaneously, a hydrogel was formed in the injection site and very

mild tissue responses around the injection site were observed. Overall, zwitterionic

polymer-based hydrogels possess remarkable characteristics such as being thermo­

responsive, good cytocompatibility, anti-biofouling nature, controlled protein adsorption,

cell adhesiveness, implantable nature, and many more.

18.2.4 Ion Conductive Hydrogels

Hydrogels consist of free ions from ionic electrolytes usually exhibit higher conductivity

and found good application in bioelectronics applications. Ionic salts like NaCl, LiCl,

CaSO4, Na2SO4, etc. are generally added to hydrogels to make them ionically conductive.

The resulting ionic hydrogels exhibit greater ionic conductivity, higher stretchability, and

Conductive Hydrogels for Bioelectronics

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