3.2 Mechanically Strong Hydrophobically Modified
Hydrogels
Hydrogels formed in the presence of a small amount of a hydrophobic monomer via
micellar polymerization discussed in the previous section have a high self-healing
efficiency, but they exhibit an insufficient mechanical performance for load-bearing
applications. Although their mechanical strength could be improved by incorporation of hydrophobic acrylates instead of the corresponding methacrylates into the
hydrophilic polymer backbone, or by increasing the length of alkyl side chain of the
hydrophobic monomers from 12 to 22 carbon atoms, the tensile strength only
slightly increases from 20 to 65 kPa after these modifications [101]. Chen et al.
conducted the micellar copolymerization of AAm and C18A by the addition of
1-pentanol as a cosurfactant of SDS solubilizing the hydrophobe in the micellar
solution [111]. After γ-radiation induced polymerization without any initiator, they
produced a hydrogel sustaining kPa level compressive stresses. However, swelling
of this hydrogel in a second aqueous AAm-BAAm solution followed by polymerization leads to a self-recovery double-network PAAm hydrogel exhibiting a compressive strength of 2.8 MPa under 90% compression [111]. Micellar polymerization
reactions conducted using a polymerizable (acrylated) cationic surfactant in the
absence of free surfactants or reducing surfactant content using amphiphilic hydrophobic monomers lead to hydrogels with tensile strength up to ~300 kPa but with a
low self-healing efficiency [112, 113]. Micellar polymerization of AAm and 2 mol%
C17.3M in aqueous mixtures of cationic and anionic surfactants produces PAAm
hydrogels with a high stretchability (1,800–5,000%) and complete self-healing
efficiency but a low mechanical strength [114]. Thomas et al. prepared
hydrophilic-hydrophobic hydrogels based on polyvinyl alcohol and poly(ethyleneco-vinyl alcohol) with 15–25% water content that exhibit good compressive properties with a modulus of around 20 MPa [115]. Moreover, creating hybrid crosslinked hydrophobically modified hydrogels by incorporation of chemical cross-links
also provides some improvement in the compressive mechanical properties without
affecting much their self-healing behavior, but they are brittle in tension [116]. Considering the load-bearing tissues such as cartilages, tendons, and ligaments
containing 60–75% water and exhibiting a modulus and tensile strength in the
range of MPa, one needs to improve the tensile mechanical performances of
hydrophobically modified self-healing hydrogels to the MPa level. In the following
subsections, two attempts will be discussed for fabrication of high-strength
hydrophobically modified hydrogels with a high self-healing efficiency.
3.2.1 Hydrophobically Modified Polyelectrolyte Hydrogels
with Oppositely Charged Surfactants
One attempt in this direction was to use oppositely charged surfactants in the
preparation of hydrophobically modified polyelectrolyte hydrogels by micellar
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Hydrogels
Hydrogels formed in the presence of a small amount of a hydrophobic monomer via
micellar polymerization discussed in the previous section have a high self-healing
efficiency, but they exhibit an insufficient mechanical performance for load-bearing
applications. Although their mechanical strength could be improved by incorporation of hydrophobic acrylates instead of the corresponding methacrylates into the
hydrophilic polymer backbone, or by increasing the length of alkyl side chain of the
hydrophobic monomers from 12 to 22 carbon atoms, the tensile strength only
slightly increases from 20 to 65 kPa after these modifications [101]. Chen et al.
conducted the micellar copolymerization of AAm and C18A by the addition of
1-pentanol as a cosurfactant of SDS solubilizing the hydrophobe in the micellar
solution [111]. After γ-radiation induced polymerization without any initiator, they
produced a hydrogel sustaining kPa level compressive stresses. However, swelling
of this hydrogel in a second aqueous AAm-BAAm solution followed by polymerization leads to a self-recovery double-network PAAm hydrogel exhibiting a compressive strength of 2.8 MPa under 90% compression [111]. Micellar polymerization
reactions conducted using a polymerizable (acrylated) cationic surfactant in the
absence of free surfactants or reducing surfactant content using amphiphilic hydrophobic monomers lead to hydrogels with tensile strength up to ~300 kPa but with a
low self-healing efficiency [112, 113]. Micellar polymerization of AAm and 2 mol%
C17.3M in aqueous mixtures of cationic and anionic surfactants produces PAAm
hydrogels with a high stretchability (1,800–5,000%) and complete self-healing
efficiency but a low mechanical strength [114]. Thomas et al. prepared
hydrophilic-hydrophobic hydrogels based on polyvinyl alcohol and poly(ethyleneco-vinyl alcohol) with 15–25% water content that exhibit good compressive properties with a modulus of around 20 MPa [115]. Moreover, creating hybrid crosslinked hydrophobically modified hydrogels by incorporation of chemical cross-links
also provides some improvement in the compressive mechanical properties without
affecting much their self-healing behavior, but they are brittle in tension [116]. Considering the load-bearing tissues such as cartilages, tendons, and ligaments
containing 60–75% water and exhibiting a modulus and tensile strength in the
range of MPa, one needs to improve the tensile mechanical performances of
hydrophobically modified self-healing hydrogels to the MPa level. In the following
subsections, two attempts will be discussed for fabrication of high-strength
hydrophobically modified hydrogels with a high self-healing efficiency.
3.2.1 Hydrophobically Modified Polyelectrolyte Hydrogels
with Oppositely Charged Surfactants
One attempt in this direction was to use oppositely charged surfactants in the
preparation of hydrophobically modified polyelectrolyte hydrogels by micellar
44
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