**1. Introduction**

Membranes are used in desalination or water treatment to separate pollutants from water based on characteristics such as size or charge. Since the late 1950s, reverse osmosis, nanofiltration, ultrafiltration, and microfiltration techniques have been used in water and wastewater treatment and in different application areas. Rapid developments in membrane technologies in the last 50 years have made these technologies the preferred technologies in water and wastewater treatment. However, traditional membranes have a number of disadvantages, including fouling both on surfaces and in internal structures, uncontrollable pore size, and membrane features.

 production technology have also increased the use of industries, such as chemistry, petrochemistry, mineral propharmacy, electronics, paper, etc. Although membrane filters

are used as an alternative water recovery process in many areas, clogging is still one of the biggest problems. Clogging in membranes limits the membrane's permeability [1]. In other words, it causes a decrease in the flux passing through the membrane per unit membrane pressure and, therefore, in the production of treated clean water per unit membrane area. In membranes designed for particulate matter or microbial removal, clogging occurs as a result of the accumulation of materials on the membrane surface or in the membrane pores.

Smart membranes, also known as stimuli-responsive membranes, have recently attracted attention due to their selectivity, tunable permeability, and tunable and/or reversible attributes [2]. This new generation of smart membranes is created by integrating various stimuli-responsive materials into membrane substrates. These multi-functional smart membranes can self-adjust their physical and chemical features in response to environmental signals such as temperature, pH, light, and other stimuli [3].

Because of their smart structures, they have the potential to improve performance by providing high selectivity without reducing the permeability, high mechanical stability, and high resistance against fouling, and can meet requirements such as molecular weight cut-off (MWCO), removal efficiencies, and wastewater quality.

This review of smart membranes is briefly summarized.

### **2. Kinds of Smart Membranes**

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Positively and negatively responsive smart membranes can self-adjust their physical and chemical properties in response to environmental signals such as temperature, pH, light, and other stimuli.

The responsive gating function is divided into two models: positively and negatively responsive smart membranes [4].

Figure 1 represents positively responsive smart membranes.

**Figure 1.** Positively responsive smart membranes ( **A**). The permeability of the membrane increases in response to the presence or increase of a stimulus. (**B**) Temperature-responsive, ( **C**) pH-responsive, (**D**) specific ion-responsive, (**E**) molecule-responsive, (**F**) UV light-responsive, ( **G**) glucose-responsive, and ( **H**) magnetic-responsive [4].

Figure 2 represents negatively responsive smart membranes.

**Figure 2.** Negatively responsive smart membranes. (**A**) The permeability of the membrane decreases in response to the presence or increase of a stimulus. (**B**) Thermo-responsive, (**C**) pH-responsive, (**D**) ion-responsive, (**E**) molecule-responsive, (**F**) UV light-responsive, (**G**) ion strength-responsive, and (**H**) redox-responsive [4].
