promising in vitro and in vivo studies have shown the
potential of PHAs to be used as medical implants (Lukasiewicz et al. 2018; Chen and Wu 2005), various studies are
now dealing about the fabrication methods tailoring the
scaffold structure and properties (Lim et al. 2017). It has
been stated that PHAs can expect to become a family of
bioimplant materials with rich applications (Chen 2009).
Another medical application is their use for drug delivery
systems (Shrivastav et al. 2013; Xiong et al. 2010).
Regarding the versatile properties of the existing PHAs, the
release rate of the encapsulated drug could be tailored via the
use of proper PHAs macromolecules. For example, it has
been shown that a higher P(3HB) molecular mass tends to
increase the drug release rate (Brophy and Deasy 1986).
In order to extend the field of PHAs applications, some
intrinsic problematic of this polyester has to be solved, like
the high production cost, its poor thermal stability and its
brittleness (Lee 2000). The solutions to their poor
thermo-mechanical properties are either their use in polymer
blends or their chemical modifications (Li et al. 2016;
Ramachandran et al. 2014). Regarding the cost issue, several
research focuses have been mentioned in the literature (Chen
2009). Since 40–50% of the PHAs production costs come
from the raw materials costs (Shen et al. 2009), one possible
solution for a cheaper PHAs production is the use of new
and cheap raw materials as growth media (Du et al. 2012;
Elain et al. 2016). Molasses, whey, glycerol, lignocellulosic
derived materials, fats, oils, and wastewater were successfully used as low-cost raw materials to make PHAs (Du et al.
2012). These PHAs exhibited similar properties to those
produced from pure simple sugars. Besides, the high
value-added PHAs applications would not suffer from the
high production cost (Chen 2009) and the multiple medical
applications seem to be an economically practical area.
3 PHAs-Based Membranes
Since the microstructure of the membrane material is one of
the parameters tailoring the membrane performances (Mulder 1996), it is important to focus, in a first part, on the
structures of PHAs materials and which fabrication process
could be used. Then, few examples of concrete applications
on PHAs-based separation membranes will be discussed.
3.1 From Dense to Porous Structures
Before being considered as a membrane, the material must
be processable in either dense or porous structures. In this
case, it may be challenging for several reasons. Firstly,
PHAs have a poor solubility behavior and a poor thermal
stability, which limit the process. Moreover, it has been
reported that the vitrified amorphous parts of PHAs exhibit a
progressive crystallization at room temperature (Koning and
Lemstra 1993). The latter is the main cause of the PHAs
embrittlement. Hence, this slow crystallization must be
avoided if a stable structure is desired. This paragraph will
deal with the fabrication methods, used in the literature, to
get the suitable dense or porous structures.
The blend of plasticizers in the PHA matrix is one easy
solution to improve the mechanical properties of the material. Indeed, the IUPAC defined plasticizer as “a substance or
material incorporated in a material to increase its flexibility,
workability, or distensibility” (Gurgel et al. 2011). The
added substance improves the mobility of the macromolecules within the amorphous phase by decreasing the
intermolecular forces. Typically, a lower glass transition,
lower stiffness, higher elongation at break, and lower elastic
modulus could be expected in plasticized samples. Some
examples of additives used in PHAs materials are mentioned
in Table 5.
Among the additives tested in the literature and reported
in Table 5, most of them, except for stearic acid, seem to act
as plasticizer. It means that additives molecular chains were
uniformly distributed within the polymer. None of them
decreased significantly the crystallinity ratio. The water
vapor permeability (WVP) was influenced by two phenomena: the additive properties and its hydrophilicity/
hydrophobicity (Parra et al. 2006) and the increasing of
the free volume in the material (Audic and Lemi 2013). In
Table 5, some of the reported additives are biobased and/or
biodegradable like soybean oil and broccoli oil (Choi and
Park 2004; Audic and Lemi 2013). Indeed, since the main
material, PHA, is biobased and biodegradable, the development of plasticizers in accordance with the matrix is relevant. Furthermore, bioplasticizers are also strongly
investigated for other common plastics (Gurgel et al. 2011).
By adding a plasticizer, except the mechanical properties
improvement, a side effect of increased biodegradation rate
was sometimes reported (Parra et al. 2006; Yoshie et al.
2000).
Besides plasticizers, other materials were added in the
polyester matrix in order to improve the mechanical properties. Boron nitride has been used as nucleating agent in P
(3HB) or P(3HB3HV) to improve the crystallization at high
temperature and avoid the slow crystallization at room
temperature (Puente et al. 2013a, b). Multiwall carbon
nanotubes (Yu et al. 2013; Vidhate et al. 2012; Shan et al.
2011; Montanheiro et al. 2015), cellulose nanocrystals
(Bhardwaj et al. 2014), cellulose nanofibrils (Jun et al.
2017), clay (Bittmann et al. 2013), and ZnO (Díez-Pascual
and Díez-Vicente 2014) were also successfully used as
nucleating agents.
Another solution to improve the PHAs properties while
reducing the material cost is the blend with other common
Polyhydroxyalkanoates (PHAs) for the Fabrication …
183
potential of PHAs to be used as medical implants (Lukasiewicz et al. 2018; Chen and Wu 2005), various studies are
now dealing about the fabrication methods tailoring the
scaffold structure and properties (Lim et al. 2017). It has
been stated that PHAs can expect to become a family of
bioimplant materials with rich applications (Chen 2009).
Another medical application is their use for drug delivery
systems (Shrivastav et al. 2013; Xiong et al. 2010).
Regarding the versatile properties of the existing PHAs, the
release rate of the encapsulated drug could be tailored via the
use of proper PHAs macromolecules. For example, it has
been shown that a higher P(3HB) molecular mass tends to
increase the drug release rate (Brophy and Deasy 1986).
In order to extend the field of PHAs applications, some
intrinsic problematic of this polyester has to be solved, like
the high production cost, its poor thermal stability and its
brittleness (Lee 2000). The solutions to their poor
thermo-mechanical properties are either their use in polymer
blends or their chemical modifications (Li et al. 2016;
Ramachandran et al. 2014). Regarding the cost issue, several
research focuses have been mentioned in the literature (Chen
2009). Since 40–50% of the PHAs production costs come
from the raw materials costs (Shen et al. 2009), one possible
solution for a cheaper PHAs production is the use of new
and cheap raw materials as growth media (Du et al. 2012;
Elain et al. 2016). Molasses, whey, glycerol, lignocellulosic
derived materials, fats, oils, and wastewater were successfully used as low-cost raw materials to make PHAs (Du et al.
2012). These PHAs exhibited similar properties to those
produced from pure simple sugars. Besides, the high
value-added PHAs applications would not suffer from the
high production cost (Chen 2009) and the multiple medical
applications seem to be an economically practical area.
3 PHAs-Based Membranes
Since the microstructure of the membrane material is one of
the parameters tailoring the membrane performances (Mulder 1996), it is important to focus, in a first part, on the
structures of PHAs materials and which fabrication process
could be used. Then, few examples of concrete applications
on PHAs-based separation membranes will be discussed.
3.1 From Dense to Porous Structures
Before being considered as a membrane, the material must
be processable in either dense or porous structures. In this
case, it may be challenging for several reasons. Firstly,
PHAs have a poor solubility behavior and a poor thermal
stability, which limit the process. Moreover, it has been
reported that the vitrified amorphous parts of PHAs exhibit a
progressive crystallization at room temperature (Koning and
Lemstra 1993). The latter is the main cause of the PHAs
embrittlement. Hence, this slow crystallization must be
avoided if a stable structure is desired. This paragraph will
deal with the fabrication methods, used in the literature, to
get the suitable dense or porous structures.
The blend of plasticizers in the PHA matrix is one easy
solution to improve the mechanical properties of the material. Indeed, the IUPAC defined plasticizer as “a substance or
material incorporated in a material to increase its flexibility,
workability, or distensibility” (Gurgel et al. 2011). The
added substance improves the mobility of the macromolecules within the amorphous phase by decreasing the
intermolecular forces. Typically, a lower glass transition,
lower stiffness, higher elongation at break, and lower elastic
modulus could be expected in plasticized samples. Some
examples of additives used in PHAs materials are mentioned
in Table 5.
Among the additives tested in the literature and reported
in Table 5, most of them, except for stearic acid, seem to act
as plasticizer. It means that additives molecular chains were
uniformly distributed within the polymer. None of them
decreased significantly the crystallinity ratio. The water
vapor permeability (WVP) was influenced by two phenomena: the additive properties and its hydrophilicity/
hydrophobicity (Parra et al. 2006) and the increasing of
the free volume in the material (Audic and Lemi 2013). In
Table 5, some of the reported additives are biobased and/or
biodegradable like soybean oil and broccoli oil (Choi and
Park 2004; Audic and Lemi 2013). Indeed, since the main
material, PHA, is biobased and biodegradable, the development of plasticizers in accordance with the matrix is relevant. Furthermore, bioplasticizers are also strongly
investigated for other common plastics (Gurgel et al. 2011).
By adding a plasticizer, except the mechanical properties
improvement, a side effect of increased biodegradation rate
was sometimes reported (Parra et al. 2006; Yoshie et al.
2000).
Besides plasticizers, other materials were added in the
polyester matrix in order to improve the mechanical properties. Boron nitride has been used as nucleating agent in P
(3HB) or P(3HB3HV) to improve the crystallization at high
temperature and avoid the slow crystallization at room
temperature (Puente et al. 2013a, b). Multiwall carbon
nanotubes (Yu et al. 2013; Vidhate et al. 2012; Shan et al.
2011; Montanheiro et al. 2015), cellulose nanocrystals
(Bhardwaj et al. 2014), cellulose nanofibrils (Jun et al.
2017), clay (Bittmann et al. 2013), and ZnO (Díez-Pascual
and Díez-Vicente 2014) were also successfully used as
nucleating agents.
Another solution to improve the PHAs properties while
reducing the material cost is the blend with other common
Polyhydroxyalkanoates (PHAs) for the Fabrication …
183