5.3 Stimuli-Responsive Membranes
The stimuli-responsive membranes are the most advanced membranes with response
to an external stimulus, such as pH [29], temperature [30], light [31], biological [32],
magnetic [33], or electrical [34] by inducing changes to their structural or functional
properties [1, 35]. Stimuli-responsive membranes can be synthesized by either direct
blending or grafting of compounds containing stimuli-responsive groups [3]. These
groups show responsiveness to a specific external stimulus, thereby making the
membranes stimuli-responsive in nature. Therefore, these membranes get capabilities to these open various domains of fascinating applications for membrane separation processes, such as controlled drug delivery, wastewater treatment, sensor
development, gas separations, antifouling, and artificial organ development.
The development and advancement in this field of stimuli-responsive membranes
come from the better understanding of the fundamentals of stimuli-responsiveness in
terms of concept and mechanisms. Generally, the external stimulus works as a bulk
stimulus that is the stimulus will be present on the whole system and not at a
particular area or point. This is both good and bad in terms of efficiency and
effectiveness, good in terms of area governed and bad as it results in the wastage
of energy as well as resources. For example, a particular drug is to be delivered on
arrival of a particular cell type in the body, but the bulk stimulus, such as pH, will
make the whole process inefficient in a way that it will trigger the delivery of the
drug even when the said cell type is not present. Therefore, there is need of
development of more accurate, efficient, and effective local stimulus for the betterment of stimuli-responsive membrane separation processes. Furthermore, the external stimuli are categories into three categories, viz., direct, indirect, and field
stimulation, and are shown in Fig. 4. In case of direct stimulation, the external
stimuli come in direct contact with the membranes. Consequently, in indirect
stimulation, the membranes respond to the external stimuli, where the stimulus is
pressure or temperature making a thermodynamic environment. On the other hand,
membranes respond to an external stimuli field, like electromagnetic.
Presently, the applications of stimuli-responsive membranes make them fascinating in the field of bioprocess engineering. The major applications are in the field of
food, pharmaceutical, and biotechnology industries. The applications, such as protein separation, sterilization, acidification or deacidification of beverages, fruit juice
clarifications, antibiotic and other valuable products separation and purification, and
whey protein and cheese production [1–3]. The stimuli-responsive membranes show
better flux, antifouling, and separation efficiencies under the required external
stimulus. For example, Sinha and Purkait showed that the dual, pH and temperature,
responsive polysulfone polymeric ultrafiltration membranes show better antifouling
nature, increased pure water flux, and enhanced bovine serum albumin flux (3.37 L/
m
2 h) and rejection (90%) with change in external stimuli (pH and temperature)
[36]. Further, Singh et al. showed that the photoresponsive Cu 2 O-modified
polysulfone mixed matrix membranes successfully removed ibuprofen from the
feed under the influence of visible light at a rate of 32.63 Â 10
À3 min
À1
22
R. Singh et al.
The stimuli-responsive membranes are the most advanced membranes with response
to an external stimulus, such as pH [29], temperature [30], light [31], biological [32],
magnetic [33], or electrical [34] by inducing changes to their structural or functional
properties [1, 35]. Stimuli-responsive membranes can be synthesized by either direct
blending or grafting of compounds containing stimuli-responsive groups [3]. These
groups show responsiveness to a specific external stimulus, thereby making the
membranes stimuli-responsive in nature. Therefore, these membranes get capabilities to these open various domains of fascinating applications for membrane separation processes, such as controlled drug delivery, wastewater treatment, sensor
development, gas separations, antifouling, and artificial organ development.
The development and advancement in this field of stimuli-responsive membranes
come from the better understanding of the fundamentals of stimuli-responsiveness in
terms of concept and mechanisms. Generally, the external stimulus works as a bulk
stimulus that is the stimulus will be present on the whole system and not at a
particular area or point. This is both good and bad in terms of efficiency and
effectiveness, good in terms of area governed and bad as it results in the wastage
of energy as well as resources. For example, a particular drug is to be delivered on
arrival of a particular cell type in the body, but the bulk stimulus, such as pH, will
make the whole process inefficient in a way that it will trigger the delivery of the
drug even when the said cell type is not present. Therefore, there is need of
development of more accurate, efficient, and effective local stimulus for the betterment of stimuli-responsive membrane separation processes. Furthermore, the external stimuli are categories into three categories, viz., direct, indirect, and field
stimulation, and are shown in Fig. 4. In case of direct stimulation, the external
stimuli come in direct contact with the membranes. Consequently, in indirect
stimulation, the membranes respond to the external stimuli, where the stimulus is
pressure or temperature making a thermodynamic environment. On the other hand,
membranes respond to an external stimuli field, like electromagnetic.
Presently, the applications of stimuli-responsive membranes make them fascinating in the field of bioprocess engineering. The major applications are in the field of
food, pharmaceutical, and biotechnology industries. The applications, such as protein separation, sterilization, acidification or deacidification of beverages, fruit juice
clarifications, antibiotic and other valuable products separation and purification, and
whey protein and cheese production [1–3]. The stimuli-responsive membranes show
better flux, antifouling, and separation efficiencies under the required external
stimulus. For example, Sinha and Purkait showed that the dual, pH and temperature,
responsive polysulfone polymeric ultrafiltration membranes show better antifouling
nature, increased pure water flux, and enhanced bovine serum albumin flux (3.37 L/
m
2 h) and rejection (90%) with change in external stimuli (pH and temperature)
[36]. Further, Singh et al. showed that the photoresponsive Cu 2 O-modified
polysulfone mixed matrix membranes successfully removed ibuprofen from the
feed under the influence of visible light at a rate of 32.63 Â 10
À3 min
À1
22
R. Singh et al.