Recent Advances in Poly(vinyl alcohol)-Based Hydrogels

Abstract

Poly(vinyl alcohol) (PVA) is a versatile synthetic polymer, used for the design of hydrogels, porous membranes and films. Its solubility in water, film- and hydrogel-forming capabilities, non-toxicity, crystallinity and excellent mechanical properties, chemical inertness and stability towards biological fluids, superior oxygen and gas barrier properties, good printability and availability (relatively low production cost) are the main aspects that make PVA suitable for a variety of applications, from biomedical and pharmaceutical uses to sensing devices, packaging materials or wastewater treatment. However, pure PVA materials present low stability in water, limited flexibility and poor biocompatibility and biodegradability, which restrict its use alone in various applications. PVA mixed with other synthetic polymers or biomolecules (polysaccharides, proteins, peptides, amino acids etc.), as well as with inorganic/organic compounds, generates a wide variety of materials in which PVA's shortcomings are considerably improved, and new functionalities are obtained. Also, PVA's chemical transformation brings new features and opens the door for new and unexpected uses. The present review is focused on recent advances in PVA-based hydrogels.

Keywords: hydrogel; poly(vinyl alcohol); strain sensors; tissue engineering; wound dressings.

Figure 1

The frequency of publications identified on the Web of Science database [1] during the period 2000–2024, using the keywords “PVA hydrogel”.

Scheme 1

The chemical structure of poly(vinyl alcohol) and poly(vinyl alcohol-co-vinyl acetate). The commercial PVA samples have n >> m.

Figure 2

Schematical presentation of (a) temperature–time curve during FT process and (b) structure of PVA networks designed for different applications. Adapted with permission from [71], copyright 2024, BioMed Central Ltd., Part of Springer Nature.

Figure 3

Schematic illustration of PVA-based hybrid hydrogel formation (adapted from [56,58]).

Figure 4

The self-healing behavior of PVA/HPC/BSA hydrogels illustrated through consecutive step strain measurements at 37 °C: (a) G′, G″ and tanδ for the hydrogel with 50% BSA in composition during successive runs of low (1%) and high (100%) strains; (b) G′ for different polymer/protein compositions (w_BSA is the weight percent of BSA in the polymer/BSA mixture) during the first run of strain (1%—↑ 100%—↓ 1%) [58].

Figure 5

(a) A phase diagram of the PVA/CaCl₂/H₂O solution, and the design principle of conventional cryogels obtained by the FT method and suppressed cryogels formed by the SFT method based on the relationship between freezing temperature T_f and cryogenic temperature T_c (ΔT = T_c − T_f). (b–g) The cryogenic state and the corresponding multiscale structures for conventional cryogels (b–d) and suppressed cryogels (e–g). Adapted with permission from [6], copyright 2023, Springer Nature.

Figure 6

(a) The transmittance of cryogels and suppressed cryogels with a thickness of 2 mm (inset: photographs of opaque cryogels and transparent suppressed cryogels (3 M)). (b) The twisted cryogels and suppressed cryogels are placed into liquid nitrogen (−196 °C) and then converted back into ambient temperature (24 °C). (c) Stress–strain curves and the image of flexible suppressed cryogels (ultralow Young’s modulus E) closely contacted with skin. (d) Variations in Young’s modulus with CaCl₂ concentration (CCaCl₂) (C_PVA = 14 wt.%, T_c = −20 °C), PVA concentration (C_PVA) (CCaCl₂ = 3 M, T_c = −20 °C) and cryogenic temperature (T_c) (C_PVA = 14 wt.%, CCaCl₂ = 3 M). (e,f) A schematic presentation of the effect of C_PVA, CCaCl₂ and T_c on the microstructure of cryogels during the FT process (e), and the variation in free/H-bonding –OH groups with the distance (d) between adjacent chains (f). (g) A comparison of Young’s modulus versus fracture strain among different PVA hydrogels, including those of the chemically crosslinked hydrogel (chem-hydrogel), the cryogel treated using directional ice template or mechanical training (oriented cryogel), the cryogel treated using annealing (annealed cryogel), the hydrogel fabricated using solvent exchange (exogel) and the cryogel created using the FT and SFT strategies. (h,i) Variations in adhesion strength with C_PVA and CCaCl₂ under T_c = −20 °C (the adhesion strength of <50 kPa was defined as non-adhesive nature). Error bars = standard deviation (n = 6) in (d,h). Scale bars: 5 mm in (a–c). Adapted with permission from [6], copyright 2023, Springer Nature.

Figure 7

Schematic illustration of PVA/gelatin gel electrolyte DN formation, involving first network of PVA via FT cycles and second network of gelatin via Hofmeister effect. Adapted with permission from [137], copyright 2023, Elsevier Ltd.

Figure 8

(a) Compressive curves of DN hydrogel with various gelatin contents; (b) effect of gelatin content on strength; (c) water content and swelling ratio of DN with various gelatin contents; (d) hydrogels were connected into circuits with a light-emitting diode (LED). Adapted with permission from [137], copyright 2023, Elsevier Ltd.

Figure 9

The electrochemical performance of the PVA/gelatin DN: (a) a schematic diagram of the assembled supercapacitor; (b) CV curves at variable scan rates from 10 to 100 mV/s and (c) galvanostatic charge/discharge profiles of the device at various current densities from 1 to 10 A/g. Adapted with permission from [137], copyright 2023, Elsevier Ltd.

Figure 10

Schematic presentation of PVA/gelatin hydrogels. Adapted with permission from [2], copyright 2024, Elsevier B.V.

Figure 11

Recovery rates at different strain tensile cycles of PGE-SO_4–1.5 hydrogel without interval time (a) and tensile stress–strain curves at different intervals (b). Recovery efficiency at different intervals for hysteresis energy (c), elastic modulus (gray) and toughness (red) (d). Adapted with permission from [2], copyright 2024, Elsevier B.V.

Figure 12

Mechanical behavior of hydrogels at −20 °C. (a) Tensile stress–strain curves of PGE-SO_4–X hydrogels prepared by FT and immersed for 24 h into Na₂SO₄ solutions of X mol/L concentration (X = 0.5, 1, 1.5 and 2 mol/L). Ten stretch loading–unloading cycles under 100% strain (b) and tensile loading–unloading curves at different stresses (c) of PGE-SO_4–1.5. (d) DSC curves of PGE-SO_4–1.5 and PG-SO_4–1.5 hydrogels. Adapted with permission from [2], copyright 2024, Elsevier B.V.

Figure 13

The tunable conformability and flexibility of PVA microfiber composite hydrogels (PVA/MF-CH): (a) The surface roughness of the artificial skins of PVA/MF-CH and polyethylene terephthalate (PET) glue tape. (b) A digital image displaying wrinkles generated from the PVA/MF-CH and PET glue tape induced by squeezing the skin. (c) A schematic wrinkle-generating mechanism of the skin covered by PVA/MF-CH and PET glue tape when squeezing. (d) A schematic diagram of the softness evaluation with a bending diameter (D). (e) Digital images of bending diameters generated from different materials. Specimen size: 1 cm (L) × 0.5 cm (W). (f) The diameter of the bending circle generated in different materials. P-P and V-V mean the distance between two peaks and two valleys, respectively. (g) The Young’s modulus and thickness of different materials. (h) The modulus matching a range of PVA/MF-CH with biological tissues and organs. (i) A digital image of a porcine heart with an attached PVA/MF-CH-based bioelectrode. The inserted picture is the PVA/MF-CH-based bioelectrode. PAN = polyacrylonitrile; PE = polyethylene; PU = polyurethane; PDMS = polydimethylsiloxane; PI = polyimide. Adapted with permission from [5], copyright 2023, Springer Nature.

Figure 14

EMG biosignal monitoring: (a) A schematic of the equivalent circuit model used for monitoring the EMG biosignals. At the electrode level (top three elements), Rd is the charge transfer resistance, C_d is the double-layer capacitance and R_cg is the resistance of the composite gel. At the skin level (bottom three elements), R_e and C_e are the epidermal resistance and capacitance, respectively, and R_sub is the resistance of the dermis and deep tissues. (b) A performance comparison of EMG biosignals collected by the electrode composed of PVA/MF/Gly-CH and a commercial gel. (c) The background noises of the electrode composed of PVA/MF/Gly-CH and a commercial gel. (d) A performance comparison of the electrode composed of PVA/MF/Gly-CH and a commercial gel for the monitoring of EMG biosignals after 48 h. (e) The performance of the electrode composed of PVA/MF/Gly-CH for the monitoring of EMG biosignals after 7 d. (f) EMG biosignals of the forearm are generated from different gestures. (g) EMG biosignals of the forearm are generated from different gripping forces. (h) EMG biosignals of the bicipital muscle of the arm lifting the different masses of the object. (i) A tri-electrode system comprising PVA/MF/Gly-CH for the monitoring of EMG biosignals. (Electrode in red rectangle, GND in yellow rectangle and R_ef electrode in green rectangle). (j) A digital image of the hand with an attached tri-electrode system comprising PVA/MF/Gly-CH. (k) The EMG biosignals of the forearm collected by the PVA/MF/Gly-CH-based bioelectrode. MF = microfiber; CH = composite hydrogel; Gly = glycerol; EMG = electromyography. Adapted with permission from [5], copyright 2023, Springer Nature.

Figure 15

Influence of LG/HG weight ratio (m_(LG:HG)) on (a) the compressive stress at 90% strain, (b) the compressive toughness, (c) the tensile strength and breakage elongation, (d) tensile toughness (10 wt.% solid content, 20% GG in PVA/GG mixture, 5 wt.% TA, 1 M Na₂SO₄). Adapted with permission from [202], copyright 2024, Elsevier Ltd.

Figure 16

(a) The influence of Na₂SO₄ on the conductivity of the composite hydrogels (10 wt.% solid content, the weight ratios of 8:2 for m_(PVA/GG) and m_(LG:HG), 6 wt.% TA); (b) Comparison of GF for the sample without cyclic testing or after 10 consecutive tensile loading–unloading; (c) GG/PVA hydrogel (2 M Na₂SO₄) used for detecting the movement of different parts of the human body. Adapted with permission from [202], copyright 2024, Elsevier Ltd.

Figure 17

(a) Adhesion and resistance of graphene on hydrogel surface under different conditions. (b) Construction diagram of graphene conductive layer on water gel surface under stirring condition and ultrasonic assisted condition. With permission from [203], copyright 2024, Springer Nature.

Figure 18

Relative resistance changes in strain sensor fabricated by PVA/TA/graphene hydrogel when used to monitor various human motions including (a) smiling, (b) frowning, (c) blinking, (d) saying “hello”, (e) saying “goodbye”, (f) finger bending, (g) head-down, (h) knee bending and (i) elbow bending. With permission from [203], copyright 2024, Springer Nature.

Figure 19

Schematic illustration of humidity-responsive shape memory mechanism of PVA/MC composites and its application in hair styling. H-bonds among PVA, cellulose and water molecules are also shown. Adapted with permission from [208], copyright 2023, Wiley-VCH GmbH.

Figure 20

(a) Photographs of humidity-responsive shape fixation and shape recovery process of curly hair bundles coated with PVA and PVA/MC composites. Time curves of (b) length and (c) shape recovery rate of temporarily stretched curly hair bundles when exposed to 80% relative humidity. Adapted with permission from [208], copyright 2023, Wiley-VCH GmbH.

Scheme 2

The most important applications of PVA-based hydrogels.