Layer-by-Layer Assembly of Biopolyelectrolytes onto Thermo/pH-Responsive Micro/Nano-Gels

Abstract

This review deals with the layer-by-layer (LbL) assembly of polyelectrolyte multilayers of biopolymers, polypeptides (i.e., poly-l-lysine/poly-l-glutamic acid) and polysaccharides (i.e., chitosan/dextran sulphate/sodium alginate), onto thermo- and/or pH-responsive micro- and nano-gels such as those based on synthetic poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAA) or biodegradable hyaluronic acid (HA) and dextran-hydroxyethyl methacrylate (DEX-HEMA). The synthesis of the ensembles and their characterization by way of various techniques is described. The morphology, hydrodynamic size, surface charge density, bilayer thickness, stability over time and mechanical properties of the systems are discussed. Further, the mechanisms of interaction between biopolymers and gels are analysed. Results demonstrate that the structure and properties of biocompatible multilayer films can be finely tuned by confinement onto stimuli-responsive gels, which thus provides new perspectives for biomedical applications, particularly in the controlled release of biomolecules, bio-sensors, gene delivery, tissue engineering and storage.

Keywords: biomedical applications; layer-by-layer; polyelectrolytes; polypeptides; polysaccharides; thermoresponsive gels.

Scheme 1

Chemical structures of poly(N-isopropylacrylamide) (PNIPAM), PNIPAM-co-methacrylic acid (PNIPAM-co-MAA), poly(acrylic acid) (PAA), hyaluronic acid (HA) and dextran-hydroxyethyl methacrylate (DEX-HEMA).

Scheme 2

Schematic representation of the polypeptides and polysaccharides used for the layer-by-layer (LbL) process: poly-l-glutamic acid (PGA), poly-l-lysine (PLL), poly-l-arginine (pARG), poly-l-aspartic acid (pASP), gelatine (G), chitosan (CHIT), dextran (DEX) and alginate (ALG).

Scheme 3

Schematic representation of the LbL assembly of biopolymers onto soft and porous nanogels (NGs). Reproduced with permission from [8], copyright 2010, Elsevier.

Figure 1

Typical environmental scanning electron microscopy (ESEM) micrographs of: (a) bare NG; (b) NG/(PLL/PGA) and (c) NG/(CHIT/dextran sulfate (DEXS)) in the hydrated state. Reproduced with permission from [8], copyright 2010, Elsevier.

Figure 2

Scanning electron microscope (SEM) images of the top surface and cross-sections of HA MGs (left column) and microgels (MGs) coated with (PLL-HA)₄-PLL, and (PLL-HA)₉-PLL films (middle and right columns, respectively). The arrows indicate diffusion of the PEs into the gel. Reprinted with permission from [37], copyright 2011, Elsevier.

Figure 3

Atomic force microscopy (AFM) images of different alkylamino hydrazide-modified HA/PLL systems with 18 layer pairs: (A) linear C6 derivative with DS of 5%; (B) branched C10 derivative with DS of 5%; (C) linear C10 derivative with DS of 5% and (D) linear C10 derivative with DS of 10%. Reproduced with permission from [30], copyright 2009, American Chemical Society.

Figure 4

Attenuated total reflectance FT-IR spectroscopy (ATR-FTIR) spectra of polypeptide-coated P(NIPAM-co-MAA) MGs. Reproduced with permission from [27], copyright 2012, American Chemical Society.

Figure 5

Electrophoretic mobility μ_e at 20 °C vs. number of layers deposited on PNiPAM-co-MAA NGs: (a) NG/(PLL/PGA) and (b) NG/(CHIT/DEXS). Reproduced with permission from [8], copyright 2010, Elsevier.

Figure 6

Electrophoretic mobility μ_e vs. temperature for different biopolymer-terminated PNIPAM-co-MAA: (a) PLL; (b) PGA; (c) CHIT and (d) DEXS. Reproduced with permission from [8], copyright 2010, Elsevier.

Figure 7

(a) Influence of ionic strength on the hydrodynamic diameter D_h and polydispersity index (μ₂/Г²) of PNIPAM-grafted (CHIT/ALG)₄ hollow microspheres (▲ and ♦) and (CHIT/ALG)₄ hollow microspheres (■ and ★); (b) pH dependence of D_h of PNIPAM-grafted (CHIT/ALG)₄ hollow microspheres at 25 °C; (c) Temperature dependence of D_h of PNIPAM-grafted (CHIT/ALG)₄ microspheres with low (★) and high () grafting degree. Reproduced with permission from [44], copyright 2012, American Chemical Society.

Figure 8

Hydrodynamic radius R_h vs. temperature for PNIPAM-co-MAA with different layers of: (a) PLL/PGA and (b) CHIT/DEXS. For clarity, only the heating cycles are shown. Reproduced with permission from [8], copyright 2010, Elsevier.

Figure 9

Hydrodynamic diameter of PNIPAM-co-MAA MGs upon LbL assembly of: (a) PLL/PGA; (b) PAH/PAA and (c) PDACMAC/PSS at 24 °C (circles) and 37 °C (squares). Reproduced with permission from [27], copyright 2012, American Chemical Society.

Figure 10

(A) Swelling ratio of bare HA MGs, (PLL/HA)₄-PLL and (PLL/HA)₉-PLL and (B) Photographs of the uncoated and coated microgels before and after immersion in phosphate buffered saline. Reproduced with permission from [37], copyright 2011, Elsevier.

Figure 11

(a) AFM image of a hollow DEX-HEMA MG coated with four (DEXS/pARG) bilayers and (b) Height profile along the line indicated in (a). Reproduced with permission from [6], copyright 2007, Wiley-VCH.

Figure 12

Bilayer thickness for different polyelectrolyte layers at 20 °C: (a) NG/(PLL/PGA) and (b) NG/(CHIT/DEXS). The solid and empty symbols correspond to values obtained before and after the heating-cooling cycle is applied, respectively. Reproduced with permission from [8], copyright 2010, Elsevier.

Figure 13

Temporal stability of LbL-coated PNIPAM-co-MMA NGs in water over different periods of time: (a) NG/PLL; (b) NG/(PLL/PGA); (c) NG/CHIT and (d) NG/(CHIT/DEXS). Closed and empty symbols correspond to heating and cooling cycles, respectively. Gels coated with polysaccharide layers (c and d) are reversibly thermoresponsive; for clarity, only the heating cycles are shown. Reproduced with permission from [8], copyright 2010, Elsevier.

Figure 14

(a) Confocal microscopy images of uncoated DEX-HEMA MGs and their size distribution; (b and c) DEX-HEMA MGs coated with four biopolyelectrolyte bilayers before (b) and after (c) degradation of the MG. Reproduced with permission from [6], copyright 2007, Wiley-VCH; (d and e) Confocal microscopy images of polypeptide coated P(NIPAM-co-MAA) microgels at 24 °C (d) and 37 °C (e). Reproduced with permission from [27], copyright 2012, American Chemical Society.

Figure 15

2f-FCS curves of uncoated rhodamine labeled NG detected at two different foci (ACF1 and ACF2) and cross-correlated (CCF) at (a) 25 °C and (b) 40 °C. Reproduced with permission from [42], copyright 2009, American Chemical Society.

Figure 16

Mechanical properties of unmodified and PE coated HA MGs: (A) Compressive stress as a function of the strain; (B) Young’s modulus; (C) ultimate stress and (D) ultimate strain. Reproduced with permission from [37], copyright 2011, Elsevier; (E) Comparison of force vs. deformation curves measured by AFM for VB-g-HA/PLL films with grafting degrees of 14% (squares), 18% (tilted squares), 29% (triangles), and 37% (circles); (F) Variation of Young’s modulus extracted from force-indentation profiles of VB-g-HA/PLL vs. the grafting degree. Reproduced with permission from [29], copyright 2009, American Chemical Society.