Smart Polymers: A Comprehensive Literature Review of Recent Developments and Advancements

الموضوعات :

Ramin Khajavi ¹

1 - ) Department of Polymer and Textile Engineering, South Tehran Branch, Islamic Azad University,Tehran, Iran.

تاريخ الإرسال : 06 الإثنين , رجب, 1446 تاريخ التأكيد : 30 الخميس , رجب, 1446 تاريخ الإصدار : 24 الأحد , شعبان, 1446

الکلمات المفتاحية: Stimuli-Responsive Materials, Thermoresponsive Polymers, pH-Responsive Polymers, Light-Responsive Polmers, Self-Healing Polymers, Shape Memory Polymers (SMPs), Drug Delivery Systems,

ملخص المقالة :

Smart polymers, also known as stimuli-responsive materials, represent a frontier in materials science, distinguished by their ability to undergo significant, often reversible, changes in their physicochemical properties in response to small external triggers. Drawing inspiration from adaptive biological systems, these polymers are at the heart of innovations across numerous scientific and technological domains. This comprehensive review synthesizes recent advancements in the field, systematically classifying smart polymers based on their primary stimuli, including temperature, pH, light, and mechanical forces. For each class, we delve into the fundamental response mechanisms, from the molecular-level hydrophobic-hydrophilic balance and ionization dynamics to macroscopic phenomena like phase transitions and swelling/deswelling. Key synthesis methodologies, advanced characterization techniques, and the structure-property relationships that govern their behavior are discussed in detail. Furthermore, the review highlights the expanding applications of these intelligent materials in high-impact areas such as targeted drug delivery, regenerative medicine, tissue engineering, biosensing, and soft robotics. Finally, we address the current challenges, including the need for enhanced biocompatibility, precise control over response kinetics, and multifunctionality, while outlining future research directions poised to unlock the full potential of smart polymers in creating the next generation of advanced materials.

المصادر:


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نص كامل:



Applied Nanomaterials and smart Polymers,

Online ISSN: 2981-0434

Vol. 2, No. 5

 

 

 

Smart Polymers: A Comprehensive Literature Review of Recent Developments and Advancements

 

Ramin Khajavi *

 

Department of Chemical & Polymer Engineering, ST.C, Islamic Azad University, Tehran, Iran 

 

* Corresponding Author: Ramin Khajavi: [e-mail]; [e-mail]

 

https://doi.org/10.61882/ansp.1213849.2.1.1

 

 

Abstract

Smart polymers, also known as stimuli-responsive materials, represent a frontier in materials science, distinguished by their ability to undergo significant, often reversible, changes in their physicochemical properties in response to small external triggers. Drawing inspiration from adaptive biological systems, these polymers are at the heart of innovations across numerous scientific and technological domains. This comprehensive review synthesizes recent advancements in the field, systematically classifying smart polymers based on their primary stimuli, including temperature, pH, light, and mechanical forces. For each class, we delve into the fundamental response mechanisms, from the molecular-level hydrophobic-hydrophilic balance and ionization dynamics to macroscopic phenomena like phase transitions and swelling/deswelling. Key synthesis methodologies, advanced characterization techniques, and the structure-property relationships that govern their behavior are discussed in detail. Furthermore, the review highlights the expanding applications of these intelligent materials in high-impact areas such as targeted drug delivery, regenerative medicine, tissue engineering, biosensing, and soft robotics. Finally, we address the current challenges, including the need for enhanced biocompatibility, precise control over response kinetics, and multifunctionality, while outlining future research directions poised to unlock the full potential of smart polymers in creating the next generation of advanced materials.

 

Keywords: Stimuli-Responsive Materials, Thermoresponsive Polymers, pH-Responsive Polymers, Light-Responsive Polymers, Self-Healing Polymers, Shape Memory Polymers (SMPs), Drug Delivery Systems, Tissue Engineering

 

 

 

 

 

 

 

 

 

 

 

1.Introduction

Smart polymers have emerged as an important class of materials that can undergo reversible changes in response to environmental stimuli. The concept of intelligent materials draws inspiration from natural biological systems that follow a mechanism of sensing, reacting, and learning (Goy et al., 2024). These materials possess the remarkable ability to adapt their molecular

 

structure and function to different external stimuli or environmental changes, mimicking living systems that adapt to their surroundings. This adaptability opens up a wide range of potential applications and piques the interest of researchers and professionals in materials science and related fields. [1, 2].

Over the past few decades, significant research efforts have been directed toward the development of smart polymeric materials for applications in diverse fields such as drug delivery, tissue engineering, sensors, actuators, and environmental remediation. The versatility of these materials stems from their ability to respond to a wide range of stimuli including temperature, pH, light, electrical fields, magnetic fields, biological molecules, and mechanical forces such as compression, tension, or shear [3, 4].

This review presents an extensive analysis of the latest developments in smart polymers, focusing on their design principles, synthesis methodologies, characterization techniques, and various applications. We systematically categorize these materials according to their response triggers and elaborate on their distinctive properties, advantages, and limitations.

 

2. Classification of Smart Polymers

Smart polymers can be classified based on the type of stimuli they respond to. The major categories include Fig.1:

 

 

Figure 1: Classification of smart polymers due to stimule 

 

2.1 Temperature-Responsive Polymers

Temperature-responsive polymers, also known as thermo-responsive polymers, are a class of smart materials that undergo significant physical or chemical changes in response to temperature variations. These materials have gained considerable attention for their potential in biomedical, agricultural, and technological applications due to their ability to provide controlled, dynamic responses fig. 2  [5, 6].

 

Temperature-responsive polymers exhibit a sharp change in properties, such as solubility, morphology, or shape, at specific temperatures. This is often characterized by a lower critical solution temperature (LCST) or upper critical solution temperature (UCST), where the polymer transitions between soluble and insoluble states or undergoes a sol-gel transformation. The LCST represents the temperature below which the polymer is soluble in a given solvent, while above this temperature, phase separation occurs. 

Conversely, UCST polymers are insoluble below a critical temperature and become soluble above it. The phase transition is driven by the delicate balance between polymer-polymer and polymer-solvent interactions, which are temperature-dependent.

 

 

 

At the molecular level, this behavior is governed by changes in the Gibbs free energy of mixing (Eq. 1):

 

        (1)

 

Where  is the Gibbs free energy of mixing,  is the enthalpy of mixing,  is the absolute temperature, and  is the entropy of mixing.[7-13].

 

 

 

Figure 2. transition between temporary annd permanent shape

 

 

 

2.1.1. Types and Classifications

a) LCST-Type Polymers

Poly(N-isopropylacrylamide) (PNIPAM)

PNIPAM is the most extensively studied temperature-responsive polymer, with an LCST around 32 °C in aqueous solution. Its structure contains both hydrophobic isopropyl groups and hydrophilic amide groups, creating an amphiphilic character essential for its thermoresponsive behavior  (Fig.3)[14].

 

Poly(N,N-diethylacrylamide) (PDEAM)

 

 

 

 

PDEAM exhibits an LCST between 25 °C and 35 °C, depending on molecular weight and solution conditions. The presence of diethyl groups provides different hydrophobic-hydrophilic balance compared to PNIPAM (Fig.3) [15].

Poly(oligo(ethylene glycol) methacrylate)s (POEGMA)

These polymers show tunable LCST behavior (20 °C to 90 °C) depending on the length of the ethylene glycol side chains. They offer excellent biocompatibility and resistance to protein adsorption  (Fig.3) [16].

Poly(2-oxazoline)s

This class includes poly(2-ethyl-2-oxazoline) and poly(2-isopropyl-2-oxazoline), with LCST values ranging from 25 °C to 100 °C depending on the side chain structure  (Fig.3) [17].

 

 

Figure 3: molecular structure of LCST-Type Polymers

 

b) UCST-Type Polymers

Poly(acrylamide-co-acrylonitrile)

These copolymers exhibit UCST behavior in water, with transition temperatures tunable by adjusting the comonomer ratio [6].

Poly(N-acryloylglycinamide) (PNAGA)

 

 

PNAGA shows UCST behavior in water with a transition temperature around 22 °C, driven by strong intermolecular hydrogen bonding [18].

Polyzwitterions

Certain polyzwitterions, such as poly(3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate), exhibit UCST behavior due to electrostatic interaction [19] (Fig.4).

 

 

Figure 4: molecular structure of UCST-Type Polymers

 

c) Dual Responsive Polymers

Some polymers exhibit both LCST and UCST behavior, creating a miscibility window. Examples include:Poly(ethylene glycol) in water at high temperatures and pressures [20], and certain polyzwitterions under specific conditions[21].

Some examples for thermoresponsive polymers are listed and mentioned in Table 1.

 

Table 1: some thermo responsive polymers

Type of Thermo-Responsive Polymer	Key Features	Example Applications
LCST-type	Becomes insoluble above LCST	Drug delivery, hydrogels [7, 9, 10, 22]
UCST-type	Becomes soluble above UCST	Smart coatings, sensors [7-9]
Shape-memory polymers	Change shape with temperature	Biomedical devices [7]
Multi-responsive polymers	Respond to multiple stimuli (e.g., pH, light)	Advanced drug delivery, tissue engineering [8, 12, 23]

 

 

2.1.2. Mechanisms of Temperature Responsiveness

a) Hydrophobic-Hydrophilic Balance

The primary mechanism for LCST behavior involves the temperature-dependent balance between hydrophobic and hydrophilic interactions. Below the LCST, hydrogen bonding between polymer chains and water molecules dominates, maintaining polymer solubility [24]. As temperature increases:

i.       Water molecules gain kinetic energy

ii.       Hydrogen bonds between polymer and water weaken

 

iii.       Hydrophobic interactions between polymer segments strengthen

iv.       Polymer chains collapse and aggregate

This process can be described by the Flory-Huggins interaction parameter () (Eq. 2-6):

                (2)

where  is the enthalpic contribution and  is the entropic contribution.

 

The Enthalpic Contribution () arises from the change in enthalpy when polymer-solvent contacts are formed, replacing polymer-polymer and solvent-solvent contacts. It is directly proportional to the molar enthalpy of mixing, , and inversely proportional to temperature:

                (3)

Here,  is the universal gas constant and  is the absolute temperature. The Entropic Contribution (): This term accounts for the non-combinatorial or "excess" entropy of mixing, . This includes effects not captured by the ideal entropy of mixing chains, such as the specific ordering of solvent molecules around the polymer segments (e.g., hydrophobic hydration). The entropic parameter is defined as:

        (4)

The negative sign is crucial, as a positive excess entropy (disordering) leads to a negative (favorable) , promoting mixing.

By substituting the definitions from Equations enthalpic and entropic into the fundamental relationship in Equation efeq:additive, we obtain the full and correct expression for the Flory-Huggins interaction parameter:

        (5)

This equation is often written in a simplified linear form, which is useful for analyzing experimental data:

                (6)

where the constant  represents the entropic term () and the constant  represents the enthalpic term () [25].

 

 

b) Hydrogen Bonding Dynamics

Temperature affects the strength and dynamics of hydrogen bonds. For LCST polymers like PNIPAM (Poly(N-isopropylacrylamide)), the breaking of polymer-water hydrogen bonds above the transition temperature is crucial [26] . The process involves:

 

 

i.       Below LCST: Strong polymer-water hydrogen bonds maintain extended chain conformation

ii.       At LCST: Critical balance between hydrogen bonding and hydrophobic interactions

iii.       Above LCST: Intramolecular and intermolecular polymer-polymer interactions dominate

 

c) Entropy Effects

The entropy change during phase transition has two main contributions [27]:

i.       Conformational Entropy

a.       Below LCST: Extended, flexible polymer chains (high conformational entropy)

b.       Above LCST: Collapsed, compact structures (low conformational entropy)

ii.       Solvent Entropy

Water molecules form ordered structures around hydrophobic groups (hydrophobic hydration). Phase separation releases these ordered water molecules, increasing system entropy

 

d) Cooperative Effects

The phase transition often exhibits sharp, cooperative behavior due to [28]:

Propagation effects: Initial collapse facilitates further chain collapse

Intermolecular cooperativity: Aggregation of collapsed chains

Critical phenomena: Near-critical fluctuations enhance transition sharpness

 

e) Molecular Architecture Effects

The polymer architecture significantly influences the transition mechanism [29]:

i.       Linear Polymers

a.       Simple coil-to-globule transition

b.       Transition temperature depends on molecular weight (for low MW)

ii.       Branched/Star Polymers

iii.       Core-shell collapse mechanism

iv.       Often sharper transitions than linear analogues

v.       Cross-linked Networks

vi.       Volume phase transition in hydrogels

vii.       Elastic constraints affect transition kinetics

 

f) Electrostatic Interactions

For charged temperature-responsive polymers, electrostatic effects modulate the transition  [30]:

i.       Charged groups increase hydrophilicity, raising LCST

ii.       Salt addition screens charges, lowering LCST

iii.       pH affects ionization state and thus transition temperature

 

 

2.1.3. Factors Affecting Temperature Responsiveness

These factors are generally Structural Factors: The transition temperature and responsiveness can be tuned by modifying molecular weight, copolymer composition, hydrophilic/hydrophobic balance, and cross-linking density. Advanced polymerization techniques allow precise control over

 

 

these parameters, enabling the design of polymers with tailored responses [9-11, 22, 23]. Some important factors influence the transition temperature and behavior can be listed as below:

i.       Molecular Weight: Generally, LCST decreases with increasing molecular weight until reaching a plateau [31] 

ii.       Concentration: Polymer concentration affects transition temperature and phase diagram [32]

iii.       Additives:

-       Salts (Hofmeister series effects)[33] 

-       Surfactants  [34]

-       Cosolvents [35]

iv.       End Groups: Hydrophobic or hydrophilic end groups can significantly affect LCST

 

2.1.4. Applications

Some applications are listed in table 2 and can be catagorized as below:

i. Controlled drug delivery systems - Thermo-responsive polymers enable controlled, site-specific drug release, minimizing side effects and improving efficacy. Nanogels and nanoparticles can encapsulate drugs and release them in response to temperature changes, as demonstrated with doxorubicin and antimicrobial agents.

ii. Tissue Engineering- These polymers serve as scaffolds that support cell growth and can be engineered to mimic natural tissue responses

iii. Other Uses- Applications extend to bioseparation, gene therapy, imaging, and even agrochemical delivery in plants, where temperature triggers the release of protective agents [9-12, 36-41].

 

2.1.5. Design Challenges and Future Directions

Tuning Responsiveness: Achieving precise control over transition temperatures and multi-stimuli responsiveness remains a key challenge. Copolymerization and advanced synthesis methods are being developed to address this  [8, 10, 11, 22, 23].

Biocompatibility and Stability: Ensuring that these polymers are safe and stable in biological environments is critical for clinical and agricultural applications   [9, 10, 22].

Expanding Applications: Research is ongoing to develop polymers with dual or multi-temperature responsiveness and to integrate additional stimuli (e.g., light, pH) for more sophisticated smart materials     [8, 12, 23].

 

Temperature-responsive polymers are versatile smart materials that undergo significant changes in response to temperature, enabling a wide range of applications, especially in drug delivery and tissue engineering. Their properties can be finely tuned through structural modifications, and ongoing research aims to enhance their responsiveness, biocompatibility, and multifunctionality for advanced biomedical and technological uses.

Poly(N-isopropylacrylamide) (PNIPAAm) is one of the most extensively studied thermoresponsive polymers with an LCST around 32°C, which is close to physiological temperature [42]. Below the LCST, PNIPAAm chains are hydrated and exist in an extended conformation, while above the LCST, they undergo a phase transition to a collapsed, hydrophobic state due to the disruption of hydrogen bonds with water molecules [43].

Other important thermoresponsive polymers include poly(N-vinylcaprolactam) (PVCL), pluronics (triblock copolymers of poly(ethylene oxide) and poly(propylene oxide)), and elastin-like polypeptides (ELPs)  [44, 45]. These polymers have been widely employed in controlled drug delivery systems, tissue engineering scaffolds, and smart textiles.

Recent advancements in thermoresponsive polymers have focused on developing materials with tunable LCST/UCST values, improved mechanical properties, and multifunctional capabilities. For instance, copolymerization strategies have been employed to incorporate additional responsive elements or bioactive moieties into thermoresponsive polymers [8, 46].

 

 

 

Table 2: Temperature-Responsive Polymers applivcations 

Polymer Type	Key Features	Applications	Citation
Poly(N-isopropylacrylamide) (PNIPAAm)	LCST around 32°C; reversible hydrophilic-hydrophobic transition; excellent biocompatibility	Drug delivery systems; cell sheet engineering; tissue scaffolds; biosensors	[42, 47-54]
Poly(N-vinylcaprolactam) (PVCL)	LCST between 30-35°C; non-ionic; lower cytotoxicity than PNIPAAm; pH-independent phase transition	Controlled drug release; protein separation; textile finishing; biosensors	[55-62]
Elastin-like polypeptides (ELPs)	Bioderived; precise LCST control through sequence design; biodegradable; stimuli-responsive	Protein purification; targeted drug delivery; tissue engineering; biosensors	[63-72]
Pluronic block copolymers (PEO-PPO-PEO)	Amphiphilic triblock structure; thermoreversible gelation; micelle formation; versatile LCST range	Injectable hydrogels; sustained drug delivery; gene therapy; tissue engineering	[36, 44, 73-79]
Poly(oligoethylene glycol methacrylate) (POEGMA)	Tunable LCST through comonomer composition; excellent biocompatibility; narrow phase transition	Protein conjugation; smart surfaces; biosensors; drug delivery	[80-90]
Poly(2-oxazoline)s	Tunable LCST through side-chain modification; biocompatible; narrow phase transition	Nanomedicine; drug delivery; tissue engineering; anti-fouling coatings	[91-98]
Poly(N-acryloylglycinamide) (PNAGA)	UCST behavior in aqueous solution; stability against salt addition; sharp phase transition	Controlled release; protein separations; thermal actuators; biomedical devices	[99-103]
Temperature-responsive polymer nanocomposites	Enhanced mechanical properties; multi-responsive behavior; improved thermal conductivity	Smart textiles; shape-changing actuators; sensors; 4D printing	[104-114]

 

 

 

 

 

 

2.2. pH-Responsive Polymers

2.2.1. Introduction

pH-responsive polymers, also known as pH-sensitive or pH-triggered polymers, are a class of stimuli-responsive materials that undergo significant physicochemical changes in response to variations in environmental pH [4]. These smart materials have garnered substantial attention due to their potential applications in drug delivery, biosensing, tissue engineering, and various biomedical applications [115-118] (Fig 3).

 

pH-responsive polymers contain ionizable functional groups that can accept or donate protons in response to environmental pH changes. The ionization state of these groups directly affects the polymer’s conformation, solubility, and swelling behavior [119]. The fundamental principle underlying pH-responsiveness is the Henderson-Hasselbalch equation Eq. 7:

 

                (7)

 

where  is the acid dissociation constant,  is the concentration of the conjugate base, and  is the concentration of the weak acid.

The Henderson-Hasselbalch equation is a cornerstone of acid-base chemistry and is fundamental to understanding the behavior of pH-responsive polymers. This equation provides a direct mathematical link between the pH of a buffered solution, the intrinsic acidity of the active functional group (given by its ), and the ratio of its deprotonated to protonated forms. The pH is the independent variable, representing the external environmental condition. It is defined as the negative base-10 logarithm of the hydrogen ion activity, approximated by its concentration :

 

                (8)

 

For a pH-responsive polymer, a change in the solution’s pH is the trigger that initiates a macroscopic change in the material’s properties. The  (the negative logarithm of the acid dissociation constant, ) is an intrinsic, constant value for a given acidic functional group. It defines the pH at which the system is most sensitive to change. At the precise moment when  the concentrations of the protonated and deprotonated forms are equal: . Consequently, the ratio term , and since , the equation simplifies to . This point represents the midpoint of the polymer’s transition from a collapsed to a swollen state (or vice-versa).

 and represent the molar concentrations of the polymer’s functional groups in their two possible states:

-        (Protonated Acid): This is the concentration of the functional groups in their neutral, protonated state (e.g., carboxylic acid, ). In this form, the polymer chains are typically less soluble, compact, and dominated by hydrophobic interactions or intramolecular hydrogen bonding.

 

 

 

-        (Deprotonated Conjugate Base): This is the concentration of the functional groups in their charged, deprotonated state (e.g., carboxylate anion, ). The presence of these charges leads to strong electrostatic repulsion along the polymer backbone, causing the chain to expand and become hydrophilic, leading to swelling and dissolution.

The term  quantifies the state of the polymer system in response to the pH.

-       If , the log term is negative, indicating . The polymer is predominantly in its collapsed, protonated state.

-       If , the log term is positive, indicating . The polymer is predominantly in its swollen, deprotonated (charged) state.

 

Application of Henderson-Hasselbalch equation to pH-Responsive Polymers

The Henderson-Hasselbalch equation (Eq. 7) governs the behavior of both polyacids and polybases:

 

Polyacids (Anionic Polymers)

Examples include Poly (acrylic acid) (PAA) and Poly (methacrylic acid) (PMAA). For these polymers, the HA form is the neutral  group and the A form is the anionic  group.

 

 

As the pH rises above the polymer’s  (around 4.5-5.0), the polymer transitions from a collapsed globule to a swollen, hydrated coil due to electrostatic repulsion.

Polybases (Cationic Polymers)

Examples include Poly (dimethylaminoethyl methacrylate) (PDMAEMA) and Chitosan. For these polymers, the equation is applied to the conjugate acid equilibrium. For polybases, it’s often more intuitive to think in terms of pKb or the pKa of the conjugate acid (BH+). The principle is the same. At low pH (below its pKa≈7.5), the amine groups are protonated (BH+). The polymer is positively charged, expanded, and soluble.

The Henderson-Hasselbalch equation can be written for the conjugate acid, . Here,  (the protonated amine) is the charged "acid" form, and  (the neutral amine) is the "base" form. At low pH (below the  of the conjugate acid,  for PDMAEMA), the polymer is protonated, positively charged (), and soluble. As the pH is raised, it deprotonates to its neutral, hydrophobica form () and collapses. In summary, the Henderson-Hasselbalch equation provides the quantitative framework for predicting and designing the pH at which a smart polymer will undergo its functional transition.

 

2.2.2. Classification of pH-Responsive Polymers

Generally, these polymers can be broadly classified into two categories: polyacids (containing carboxylic or sulfonic acid groups) and polybases (containing amine groups) [120]. Polyacids, such as poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA), remain unionized at low pH values but become increasingly ionized as the pH increases above their pKa values, leading to chain expansion due to electrostatic repulsion between negatively charged groups [121].

 

Conversely, polybases like poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(ethylenimine) (PEI) are ionized at low pH and become deprotonated as the pH increases [122]. Indetail catagorization of pH-Responsive Polymers can be expressed as below:

 

Polyacids (Anionic Polymers)

Polyacids contain acidic groups such as carboxylic acid (-COOH), sulfonic acid (-SOH), or phosphoric acid (-POH) groups. These polymers are protonated at low pH and become deprotonated and negatively charged at high pH [123].

Examples  (Fig. 4  ) :

i.       Poly(acrylic acid) (PAA)

ii.       Poly(methacrylic acid) (PMAA)

iii.       Poly(L-glutamic acid) (PGA)

iv.       Poly(aspartic acid) (PASP)

 

 

 

 

Figure 4 : Polyacids pH sensetive polymers

 

Polybases (Cationic Polymers)

Polybases contain basic groups such as amines, which become protonated and positively charged at low pH and neutral at high pH [124].

Examples (Fig. 5 ):

 

i.       Poly(dimethylaminoethyl methacrylate) (PDMAEMA)

ii.       Poly(diethylaminoethyl methacrylate) (PDEAEMA)

 

iii.       Chitosan

iv.       Poly(L-lysine) (PLL)

v.       Polyethylenimine (PEI)

 

 

Figure 5  :  Polybases examples 

 

 

 

Polyampholytes

Polyampholytes contain both acidic and basic groups within the same polymer chain, exhibiting complex pH-dependent behavior with isoelectric points where the net charge is zero [125].

Examples (Fig. 6 ):

i.       Proteins and polypeptides

ii.       Poly(methacrylic acid-co-dimethylaminoethyl methacrylate)

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6: Polyampholytes examples

 

 

pH-Responsive Hydrogels

These are three-dimensional networks that can absorb large amounts of water and exhibit dramatic volume changes in response to pH variations [126].

Some of pH-Responsive Polymers are presented in table 3.

 

Table3- pH-Responsive Polymers

Polymer Type	Key Features	Applications	Citation
Poly(acrylic acid) (PAA)	Carboxylic acid pendant groups; transitions from collapsed to expanded state above pH 4-5; excellent pH sensitivity	Oral drug delivery; colon-targeted release; sensors; controlled release	[120, 121, 127-134]
Poly(methacrylic acid) (PMAA)	Stronger acid than PAA; pH-dependent swelling; versatile functionalization capacity	Intestinal drug delivery; stimuli-responsive membranes; biosensors; controlled release	[132, 135-143]
Chitosan-based pH-responsive systems	Natural polymer; protonated at acidic pH; biodegradable; biocompatible; mucoadhesive	Oral drug delivery; tissue engineering; wound healing; gene delivery	[116, 144-152]
Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA)	Protonated at acidic pH; hydrophilic-hydrophobic transition; strong buffering capacity	Gene delivery; antimicrobial materials; sensors; controlled release	[153].
Poly(ethylenimine) (PEI)	High cationic charge density; strong buffering capacity; "proton sponge" effect	Gene transfection; antimicrobial coatings; water treatment; carbon capture	[154]
Poly(histidine)	Imidazole groups responsive to physiological pH range (6.0-7.4); endosomal escape capability	Targeted anticancer drug delivery; gene therapy; intracellular delivery	[155]
Block copolymers with pH-responsive segments	Self-assembly into micelles/vesicles; multi-responsive behavior; tunable release properties	Tumor-targeted drug delivery; intracellular delivery; diagnostic imaging	[119]
Poly(acrylamide-co-methacrylic acid)	Tunable pH response through copolymer composition; high swelling capacity; mechanical stability	Controlled drug release; tissue engineering; agricultural applications	[156]

 

 

2.2.3. Mechanisms of pH-Responsiveness polymers

The macroscopic changes observed in pH-responsive polymers, such as swelling, collapse, or sol-gel transitions, are driven by a complex interplay of physicochemical phenomena at the molecular level. The primary trigger for these changes is the protonation or deprotonation of ionizable functional groups along the polymer backbone as the environmental pH crosses the group’s characteristic  value. This ionization event initiates several interconnected mechanisms, which are detailed below [157].

 

Ionization-Induced Conformational Changes

The fundamental mechanism governing pH-responsiveness is the alteration of the polymer’s conformation due to changes in its ionization state. As the pH shifts, the following effects collectively determine the polymer’s structure and solubility:

-       Electrostatic Repulsion: When the functional groups become ionized (e.g., carboxylic acids forming carboxylates, ), the resulting like charges along the polymer chain repel each other. This intramolecular repulsion overcomes cohesive forces, forcing the polymer chains to uncoil and adopt a more expanded, hydrophilic conformation.

-       Osmotic Pressure: The ionization of polymer chains creates a high concentration of fixed charges within the polymer network or hydrogel (FIG 7). To maintain electroneutrality, mobile counter-ions from the surrounding solution migrate into the polymer domain. This influx of ions dramatically increases the internal ion concentration, generating a significant osmotic pressure difference between the interior of the polymer and the bulk solution. The resulting osmotic swelling pressure drives water into the network, causing it to expand.

 

Figure 7 : Osmotic Pressure in inoized polymer network

-       Changes in Hydration: The hydration state of the functional groups is highly dependent on their ionization. Ionized groups (e.g., , ) are significantly more hydrophilic and form stronger hydrogen bonds with water molecules than their neutral counterparts (e.g., , ). This enhanced hydration contributes to chain expansion and increased solubility in aqueous media.

Swelling/Deswelling Mechanisms in Hydrogels

For crosslinked pH-responsive networks (hydrogels), the balance between these expansive forces and the elastic retractive force of the polymer chains determines the equilibrium swelling behavior. The swelling ratio, , can be quantitatively described by the Flory-Rehner theory, which was extended by Peppas and colleagues to account for the ionic contribution. The modified equation is given as Eq.9 [158]:

                (9)

 

where  is the volume swelling ratio,  is the polymer volume fraction in the swollen state,  is the effective crosslinking density,  is the molar volume of the solvent (water),  is the Flory-

 

Huggins polymer-solvent interaction parameter,  is the charge per pendant group,  is the volume of a repeating unit, and  is the ionic strength of the external solution [158]. This equation models how the degree of ionization and external ionic strength directly influence the hydrogel’s equilibrium volume.

Sol-Gel Transitions

pH changes can also induce reversible transitions between a solution state (sol) and a solid-like state (gel). These transitions are typically mediated by the pH-dependent formation or disruption of non-covalent crosslinks, including:

-       Hydrogen Bonding: Shifts in pH can alter the ability of functional groups to act as hydrogen bond donors or acceptors, leading to the formation or breakage of physical crosslinks that define the gel state.

-       Ionic Crosslinking: In systems containing both positive and negative charges (polyampholytes) or in mixtures of oppositely charged polymers, pH changes can modulate electrostatic attractions, leading to the formation of ionic crosslinks and subsequent gelation.

-       Hydrophobic-Hydrophilic Balance: As described by [157] & Gutowska (2002), the protonation/deprotonation of side chains can switch a polymer segment from being hydrophilic to hydrophobic, promoting aggregation and physical crosslinking through hydrophobic interactions, which can trigger gel formation .

Phase Separation Mechanisms

In polymer solutions (non-crosslinked systems), pH changes can induce macroscopic phase separation, where the polymer precipitates out of the solution. This behavior is often linked to the modulation of the polymer’s critical solution temperature:

-       LCST (Lower Critical Solution Temperature) Behavior: For many polymers, such as those containing tertiary amine groups (e.g., PDMAEMA), the ionization state directly influences the LCST. In their protonated (charged) state at low pH, these polymers are highly soluble. As the pH increases, deprotonation renders them more hydrophobic, causing the LCST to decrease. If the LCST drops below the system’s temperature, the polymer will phase separate.

-       UCST (Upper Critical Solution Temperature) Behavior: Conversely, for polymers that become more soluble upon heating, the ionization state can shift the  [159].

 

 

2.2.4. Temperature Responsiveness in pH-Responsive Polymers

Many pH-responsive polymers also exhibit temperature-responsive behavior, creating dual-responsive systems. The mechanisms of temperature responsiveness include:

 

 

 

Hydrophobic-Hydrophilic Balance

Temperature changes affect the hydration of polymer chains and the strength of hydrogen bonds. At the lower critical solution temperature (LCST), polymers undergo a coil-to-globule transition due to:

i.       Disruption of polymer-water hydrogen bonds

ii.       Enhanced hydrophobic interactions

iii.       Entropic effects favoring water molecule release [14]

 

Dual pH/Temperature Response Mechanisms

 

The synergistic interplay between pH and temperature responsiveness in dual-stimuli polymers arises from the direct influence of one stimulus on the polymer’s response to the other. This coupling manifests primarily through two key physicochemical mechanisms:

1)       Ionization-Dependent Lower Critical Solution Temperature (LCST): The phase transition temperature of a thermo-responsive polymer is highly sensitive to its overall hydrophilicity. For polymers containing ionizable groups (e.g., carboxylic acids or amines), the degree of ionization () directly alters the polymer’s charge density and hydration state. As the polymer becomes more charged (ionized), its hydrophilicity increases, which in turn raises its LCST. This relationship can be empirically described by the following linear approximation Eq.10:

                (10)

       where  is the intrinsic LCST of the polymer in its non-ionized state,  is the observed LCST at a given pH,  is an empirical constant that reflects the sensitivity of the LCST to ionization, and  is the degree of ionization, which is a function of pH [160]. This equation illustrates how pH can be used to precisely tune the temperature at which the polymer undergoes its phase transition.

2)       Temperature-Modulated pKa: Conversely, the dissociation constant () of the ionizable groups within the polymer is not fixed but is dependent on temperature. This relationship is governed by the van’t Hoff equation, which describes the change in an equilibrium constant with temperature. For the ionization process, this can be expressed as Eq.11:

                (11)

       where  is the standard enthalpy of ionization for the functional group,  is the universal gas constant, and  is the absolute temperature [161]. This dependency means that a change in temperature can shift the pH range over which the polymer is responsive, effectively altering its pH-triggered behavior.

Examples of Dual-Responsive Polymers

The rational design of polymers incorporating both pH- and temperature-sensitive moieties has led to a variety of sophisticated materials. Prominent examples include:

-       Poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAAm-co-AAc)): This widely studied copolymer integrates the thermo-responsive NIPAAm units (providing LCST behavior) with the pH-responsive AAc units (Fig 7)  . At low pH, the AAc groups are protonated and less hydrophilic, resulting in a lower LCST. At high pH, the deprotonated carboxylate groups increase hydrophilicity, significantly raising the LCST.

 

Figure 7   : P(NIPAAm-co-AAc)

-       Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA): This polymer is intrinsically dual-responsive (Fig. 8  ) . The tertiary amine groups provide pH sensitivity (protonating at acidic pH), while the polymer backbone exhibits an LCST around 50°C in its neutral state.

 

 

Figure 8   : PDMAEMA

-       Chitosan Derivatives: As a natural polysaccharide, chitosan provides a biocompatible and biodegradable backbone. Its primary amine groups confer pH responsiveness. By grafting thermo-responsive polymers like PNIPAAm onto the chitosan backbone, researchers have created dual-responsive systems with significant potential in biomedical applications [123, 162]

2.2.5. Applications Leveraging pH and Temperature Responsiveness

The unique ability of pH and temperature dual-responsive polymers to undergo reversible changes in response to multiple stimuli has opened numerous opportunities across biomedical, environmental, and industrial applications. These smart materials can be precisely engineered to

 

respond to physiological conditions, making them particularly valuable for therapeutic and diagnostic applications [123, 163].

Controlled Drug Delivery: Targeting Specific pH Environments with Temperature-Triggered Release

pH and temperature dual-responsive polymers have revolutionized targeted drug delivery by exploiting the unique microenvironments of diseased tissues. The tumor microenvironment, characterized by acidic pH (6.5-7.2) compared to normal tissues (pH 7.4) and elevated temperatures due to enhanced metabolism, provides ideal conditions for selective drug release [164, 165]. Poly(N-isopropylacrylamide-co-acrylic acid) [P(NIPAAm-co-AA)] copolymers have been extensively studied for cancer therapy, demonstrating enhanced drug accumulation in tumor tissues through pH-triggered swelling and temperature-induced phase transitions [166, 167].

In gastrointestinal drug delivery, pH-responsive polymers protect drugs from the acidic stomach environment (pH 1-3) and enable controlled release in the intestinal tract (pH 6-8). Eudragit® polymers, based on methacrylic acid copolymers, have been successfully commercialized for enteric coating applications [168]. Recent advances include the development of multi-responsive nanocarriers incorporating poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) that respond to both pH and temperature changes throughout the GI tract, improving oral bioavailability of protein drugs  [169, 170],

The integration of pH and temperature responsiveness has enabled sophisticated drug delivery systems with programmable release profiles. Core-shell nanoparticles with pH-sensitive cores and thermoresponsive shells demonstrate sequential drug release triggered by environmental changes [171]. These systems have shown particular promise for combination therapy, where different drugs can be released at specific sites and times based on local pH and temperature conditions [172] [173]

Smart Hydrogels: Self-Regulating Materials for Wound Healing and Tissue Engineering

pH and temperature-responsive hydrogels represent a significant advancement in regenerative medicine, offering dynamic materials that can adapt to physiological changes during healing processes. Wound environments typically exhibit pH variations from 5.5-8.5 depending on the healing stage, while inflammation causes local temperature increases  [174, 175], Smart hydrogels based on chitosan-g-poly(N-isopropylacrylamide) have demonstrated excellent wound healing properties by modulating drug release and moisture retention in response to wound pH and temperature  [176, 177],

In tissue engineering applications, dual-responsive hydrogels serve as dynamic scaffolds that can mimic the natural extracellular matrix. Injectable hydrogels based on poly(ethylene glycol)-b-poly(L-glutamic acid) block copolymers undergo sol-gel transitions at physiological temperature

 

and pH, enabling minimally invasive delivery while providing mechanical support for cell growth  [178, 179], These materials have shown promise in cartilage regeneration, where the pH-responsive components facilitate nutrient transport while temperature sensitivity enables in situ gelation  [180, 181].

Recent developments include self-healing hydrogels that respond to pH and temperature changes to autonomously repair damage. Poly(acrylic acid)-g-poly(N-isopropylacrylamide) hydrogels crosslinked through dynamic covalent bonds demonstrate rapid self-healing at physiological conditions, making them ideal for load-bearing tissue engineering applications  [182, 183], These materials have shown enhanced cell viability and proliferation compared to static hydrogels, attributed to their dynamic mechanical properties [184].

Biosensors: Dual-Responsive Systems for Enhanced Sensitivity and Selectivity

The integration of pH and temperature responsiveness in biosensor design has significantly improved detection sensitivity and selectivity for various biomarkers. Microcantilever sensors functionalized with pH and temperature-responsive polymers demonstrate enhanced mechanical response to target binding, with detection limits in the picomolar range (Bashir et al., 2010; Tamayo et al., 2013). Poly(methacrylic acid-co-N-isopropylacrylamide) brushes grafted on sensor surfaces undergo conformational changes that amplify signal transduction, enabling label-free detection of proteins and nucleic acids   [185, 186].

Optical biosensors incorporating dual-responsive polymers have shown particular promise for continuous monitoring applications. Photonic crystals embedded in pH and temperature-responsive hydrogels exhibit reversible color changes in response to analyte binding, providing visual readouts without external power sources (Hu et al., 2012; Yetisen et al., 2016). These sensors have been successfully applied to glucose monitoring, where enzyme-catalyzed reactions create local pH changes that, combined with temperature variations, produce distinct optical signatures [187, 188].

Electrochemical biosensors utilizing dual-responsive polymer coatings demonstrate improved selectivity through controlled permeability. Poly(N-isopropylacrylamide-co-allylamine) films on electrode surfaces act as molecular gates, regulating analyte access based on environmental conditions [189]. This approach has enabled selective detection of neurotransmitters in complex biological matrices, with applications in neurological disorder diagnosis  [190, 191].

Separation Technologies: pH and Temperature-Controlled Membrane Permeability

Dual-responsive polymers have revolutionized membrane separation technologies by enabling dynamic control over permeability and selectivity. Membranes functionalized with poly(N-isopropylacrylamide-co-acrylic acid) demonstrate reversible pore size changes in response to pH and temperature, allowing for tunable separation of proteins and nanoparticles (Wandera et al.,

 

2010; Zhao et al., 2011). These smart membranes have shown 10-fold changes in permeability between different pH and temperature conditions, significantly improving separation efficiency  [192, 193].

In water treatment applications, pH and temperature-responsive membranes offer energy-efficient solutions for removing contaminants. Poly(vinylidene fluoride) membranes modified with dual-responsive copolymers demonstrate self-cleaning properties through stimuli-triggered surface reorganization, reducing fouling by up to 90% compared to conventional membranes  [194, 195]. These systems have shown particular effectiveness in treating industrial wastewater with varying pH and temperature conditions [196, 197].

Advanced separation systems incorporating magnetic nanoparticles coated with pH and temperature-responsive polymers enable remote-controlled separation processes. These materials combine the advantages of magnetic separation with stimuli-responsive selectivity, achieving high recovery rates for valuable biomolecules and rare earth elements [198, 199]. Recent developments include continuous-flow separation systems that automatically adjust conditions based on feed composition, demonstrating the potential for autonomous operation  [200, 201].

Challenges of pH and Temperature-Responsive Polymers

Despite their tremendous potential, pH and temperature dual-responsive polymers face several significant challenges that must be addressed for widespread clinical and industrial implementation.

Material Design and Synthesis Challenges

The precise control over polymer composition and architecture required for predictable dual-responsive behavior remains a significant challenge. Achieving narrow molecular weight distributions and uniform functionality distribution is crucial but difficult, particularly for complex architectures like star polymers or dendrimers [202, 203]. Batch-to-batch reproducibility issues can lead to inconsistent performance, particularly problematic for biomedical applications requiring regulatory approval [204].

The incorporation of multiple responsive moieties often results in competing effects that complicate the response profile. For instance, increasing the content of pH-responsive groups may interfere with temperature-responsive behavior, requiring careful optimization of copolymer composition [21, 205] . Additionally, the synthesis of biocompatible and biodegradable dual-responsive polymers with appropriate mechanical properties remains challenging, as many responsive polymers are based on non-degradable backbones [206].

 

 

 

Response Kinetics and Reversibility

The response time of dual-responsive polymers to environmental changes can be too slow for certain applications, particularly in drug delivery where rapid release may be required. The diffusion-limited nature of polymer chain reorganization means that bulk materials may take hours to fully respond to stimuli  [207, 208] (Yoshida et al., 2013; Stuart et al., 2010). While nanostructured materials can improve response times, they introduce additional complexity in terms of stability and manufacturing [209].

Reversibility and cycling stability represent another major challenge. Repeated pH and temperature cycling can lead to polymer degradation, irreversible aggregation, or loss of responsive behavior  [210, 211]. This is particularly problematic for long-term implantable devices or reusable separation membranes. The hysteresis observed in many dual-responsive systems further complicates their application in precision-controlled environments  [212].

Biological and Environmental Compatibility

For biomedical applications, ensuring biocompatibility while maintaining responsive behavior is challenging. Many pH-responsive polymers contain charged groups that can interact non-specifically with proteins and cells, potentially causing cytotoxicity or immune responses [213, 214]. The lower critical solution temperature (LCST) of many thermoresponsive polymers is close to body temperature, making it difficult to achieve sharp transitions under physiological conditions without incorporating potentially toxic comonomers [215].

The complex biological environment presents additional challenges, including protein adsorption, enzymatic degradation, and varying ionic strength effects that can alter polymer responsiveness [216, 217]. The presence of salts and proteins in biological fluids can significantly shift transition temperatures and pH responses, requiring careful calibration for each specific application [218].

Scalability and Manufacturing Considerations

Scaling up the synthesis of dual-responsive polymers from laboratory to industrial scale presents significant challenges. Controlled polymerization techniques that produce well-defined polymers often require expensive catalysts, inert atmospheres, and precise temperature control, making large-scale production costly  [219, 220]. Purification processes to remove residual monomers, catalysts, and byproducts can be complex and may alter polymer properties.

The processing of dual-responsive polymers into useful forms (films, particles, fibers) while maintaining their responsive behavior is technically demanding. Conventional processing methods may expose polymers to conditions that trigger unwanted transitions or cause degradation   [221]. Additionally, ensuring long-term storage stability of these materials, particularly in hydrated forms, remains a significant challenge for commercial applications [222].

 

Regulatory and Standardization Challenges

The lack of standardized characterization methods for dual-responsive polymers complicates regulatory approval and quality control. Different research groups often use varying protocols to assess responsive behavior, making it difficult to compare materials or establish performance benchmarks [223]. For medical applications, demonstrating consistent performance across the range of physiological conditions encountered in diverse patient populations is particularly challenging [224].

The regulatory pathway for dual-responsive polymer-based medical devices and drug delivery systems remain unclear in many jurisdictions. The dynamic nature of these materials challenges traditional regulatory frameworks designed for static materials [225]. Establishing appropriate safety margins and failure modes for materials that undergo significant property changes in response to environmental stimuli requires new approaches to risk assessment [226].

Controlled Drug Delivery: Targeting specific pH environments (e.g., tumors, GI tract) with temperature-triggered release

Smart Hydrogels: Self-regulating materials for wound healing and tissue engineering

Biosensors: Dual-responsive systems for enhanced sensitivity and selectivity

Separation Technologies: pH and temperature-controlled membrane permeability

 

Conclusion

Recent advances in pH-responsive polymers have explored their applications in targeted drug delivery, particularly for cancer therapy, where the acidic tumor microenvironment can be exploited for triggered release .The development of pH-responsive nanomaterials has gained significant attention due to their ability to improve the efficiency of drug delivery in vivo, allow targeted drug delivery, and reduce adverse drug reactions [227-230].

pH-responsive polymers represent a versatile class of smart materials with complex mechanisms governing their behavior. The integration of temperature responsiveness adds another dimension of control, enabling sophisticated applications in biotechnology and medicine. Understanding the fundamental mechanisms of both pH and temperature responses is crucial for the rational design of next-generation responsive materials.

 

 

 

2.3 Light-Responsive Polymers

Light-responsive polymers represent a fascinating class of smart materials that undergo reversible or irreversible changes in their physical, chemical, or mechanical properties upon exposure to electromagnetic radiation [231]. These materials have emerged as crucial components in advanced technologies ranging from drug delivery systems to optical data storage, soft robotics, and biomimetic devices [232]. The ability to control polymer behavior with light offers unprecedented advantages including spatial and temporal precision, remote activation, and minimal invasiveness, making these materials particularly attractive for biomedical applications [233].

The development of light-responsive polymers has been inspired by nature's sophisticated photochemical systems, from the rhodopsin proteins enabling vision to the phytochromes 

 

controlling plant growth [234]. By incorporating photoactive chromophores into synthetic polymer chains, scientists have created materials that can mimic these biological processes while offering enhanced stability and tunability [235].

Here are some key examples and concepts from nature that are analogous to synthetic light-responsive polymers:

 

Photoactive Proteins

These are perhaps the most direct natural analogs to light-responsive polymers. Proteins are natural polymers (chains of amino acids), and many of them have evolved to respond to light.

i.       Rhodopsin (and other visual pigments): 

Found in the eyes of animals, rhodopsin is a classic example of a photoactive protein [236]. It consists of a protein (opsin) bound to a light-sensitive chromophore (retinal) [237]. When retinal absorbs light, it undergoes a rapid cis-trans isomerization, which in turn causes a conformational change in the opsin protein [238]. This shape change initiates a signaling cascade that leads to vision, representing a perfect example of light-triggered conformational change in a polymer  [239, 240] (Fig. 9).

 

 

 

Figure 9 : Rhodopsin

 

 

 

 

 

ii.       Phytochromes: Found in plants, bacteria, and fungi, phytochromes are photoreceptors that sense red and far-red light [241, 242]. They exist in two interconvertible forms (Pr and Pfr) [243]. Light absorption triggers a conformational change that shifts the equilibrium between these forms, allowing the organism to sense light quality and quantity, influencing processes like germination, flowering, and shade avoidance   [244-246](Fig.10 ).

 

 

Figure 10  :   Phytochromes

 

 

iii.       Photosystem I and Photosystem II (in photosynthesis): These protein complexes embedded in membranes are central to photosynthesis [247]. They contain pigments (like chlorophyll) that capture light energy [248]. This energy is then used to drive electron transport and ultimately produce ATP and NADPH [249]. While not directly “polymeric” in the same way, the complex interplay of proteins and chromophores to convert light energy into chemical energy is a highly sophisticated light-responsive system [250, 251] (Fig. 11 ).

 

 

Figure 11 :Photosystem I and Photosystem II

 

 

 

iv.       Bacteriorhodopsin and Halorhodopsin: Found in archaea, these are light-driven ion pumps [237, 252]. When they absorb light, they undergo confo rmational changes that result in the vectorial transport of protons or chloride ions across the cell membrane, generating an electrochemical gradient [253, 254].

v.       Light-oxygen-voltage (LOV) domains: These are found in many organisms and are involved in light sensing  [255, 256]. They contain a flavin chromophore that forms a reversible covalent bond with a cysteine residue upon blue light illumination [257]. This covalent bond formation induces conformational changes in the protein, which can regulate various cellular processes   [258, 259].

Melanin and other Pigments

While not typically considered “polymers” in the same context as synthetic ones, melanin (a complex polymer of phenolic and indole units, Fig.12    ) is a well-known light-responsive pigment in animals    [260, 261].

 

 

 

Figure 12   : The esteimated structure of melanin

 

 

i. UV Protection

Melanin absorbs UV radiation, converting it into heat and dissipating it, thereby protecting cells from DNA damage [262, 263]. Its formation (tanning) is a light-induced process   [264, 265].

ii. Camouflage and Display

In some animals (e.g., chameleons, cephalopods), pigments can rapidly change their dispersion or aggregation in response to light signals, allowing for dynamic color changes for camouflage or communication [266-268].

Natural Photocrosslinking/Photopolymerization

While synthetic photopolymers often involve the light-induced crosslinking of monomers to form a solid, nature also has examples where light plays a role in modifying or forming biological structures  [269, 270]:

i. Lignin Formation

Lignin, a complex aromatic polymer in plant cell walls, undergoes free-radical polymerization during its formation  [271, 272]. While not solely light-driven, light (specifically UV radiation) can influence the generation of radicals that initiate or participate in lignin polymerization in certain contexts [273, 274].

ii. Certain enzymatic reactions

Some enzymes are light-activated and can catalyze polymerization or crosslinking reactions in a light-dependent manner. This is more about light activating a catalytic process rather than the direct photopolymerization of the “monomers” themselves  ([275, 276],

Light-secretive polymers, often referred to as light-sensitive or photo-editable polymers, represent a rapidly evolving class of materials capable of storing, revealing, and erasing information at the

 

 

molecular level through exposure to specific wavelengths of light  [277, 278]. Recent research has demonstrated that these polymers can act as molecular-scale “invisible ink,” with their monomer sequences transformed by light to encode, decode, or erase messages  [279, 280]. For example, scientists have shown that information such as chemical symbols can be written and later altered or removed by controlled light exposure, offering a new paradigm for secure information storage and anti-counterfeiting technologies [281, 282] . The ability to manipulate the information content of polymers using light not only mimics biological information systems like DNA but also opens avenues for advanced data storage and dynamic material design  [283, 284]. The broader field of light-responsive polymers encompasses a variety of photochemical processes, including bond formation, degradation, and isomerization, which can be exploited to control polymer structure and properties with high spatial and temporal precision [285, 286]. Light offers unique advantages over traditional stimuli due to its ability to deliver significant energy locally and instantaneously, enabling polymerization, depolymerization, and functionalization reactions under mild conditions ([287, 288]. These properties have been harnessed for applications ranging from smart materials and actuators to biomedical devices and intracellular polymerization, where light-triggered reactions can be used to modulate cellular functions or deliver therapeutics  [289, 290]. As the understanding of photochemical mechanisms and the interplay between light and polymer matrices advances, light-secretive polymers are poised to play a significant role in future adaptive and multifunctional material systems  [291, 292].

Light-responsive polymers undergo structural or property changes upon exposure to light of specific wavelengths. These polymers typically contain photochromic groups such as azobenzene, spiropyran, diarylethene, or coumarin derivatives [293-296].

Azobenzene-containing polymers undergo trans-cis isomerization upon UV irradiation, leading to significant changes in molecular geometry and physical properties [297] (Fig. 13). This photoisomerization is reversible, with thermal or visible light exposure inducing cis-trans back-isomerization. Spiropyran-based polymers exhibit reversible ring-opening/closing reactions upon UV/visible light exposure, accompanied by dramatic changes in polarity and color [124, 298-300].

 

 

Figure 13   : Azobenzene-containing polymers undergo trans-cis isomerization upon UV irradiation

 

Recent developments in light-responsive polymers have focused on shifting the activation wavelength to the near-infrared (NIR) region, which offers deeper tissue penetration and reduced phototoxicity for biomedical applications [301]. Additionally, two-photon activated systems have been developed to achieve higher spatial resolution in applications such as photolithography and 3D printing [302].

The integration of light-responsive elements with other stimuli-responsive components has led to the development of multi-responsive systems with enhanced functionality and versatility [303].

 

 

2.3.1. Scientific Principles

Photochemical Mechanisms

 

The fundamental principle underlying light-responsive polymers involves the absorption of photons by chromophores, leading to electronic excitation and subsequent molecular transformations [295]. These transformations can include:

 

i.       Photoisomerization: The most common mechanism, exemplified by azobenzene's trans-cis isomerization upon UV irradiation. The energy barrier for this process is typically overcome by photon absorption, causing a change in molecular geometry that affects the polymer's macroscopic properties [304].

ii.       Photocleavage: Certain bonds, such as o-nitrobenzyl esters, undergo irreversible cleavage upon UV exposure, enabling controlled degradation or release of encapsulated molecules [305].

iii.       Photocrosslinking: Groups like coumarins and cinnamates undergo [2+2] cycloaddition reactions under UV light, creating crosslinks between polymer chains [306].

iv.       Photoredox reactions: Involving electron transfer processes that can trigger polymerization, degradation, or functionalization reactions [307].

 

Energy Transfer and Quantum Efficiency

The efficiency of photochemical processes in polymers depends on several factors Eq.12:

 

        (12)

 

Where  represents the quantum yield. Factors affecting quantum efficiency include chromophore concentration, polymer matrix effects, and competing deactivation pathways [308].

 

2.3.2. Categorization of Light-Responsive Polymers

Based on Response Type

i. Reversible Systems: Azobenzene-containing polymers [309]   - Spiropyran-based materials [310], & Diarylethene-functionalized polymers (Irie et al., 2014)

ii. Irreversible Systems: o-Nitrobenzyl-containing polymers [311], & Phenacyl ester-based materials [312]

Based on Structural Changes

 i.Shape-changing polymers: Materials that undergo macroscopic deformation upon light, exposure [313]

         ii. Sol-gel transitioning systems**: Polymers that switch between solution and gel states, [314]

iii. Surface-modifying polymers**: Materials with light-controllable surface properties (Luo & Shoichet, 2004)

Based on Activation Wavelength

i. UV-responsive(200-400 nm)

ii. Visible-light responsive(400-700 nm)

iii. NIR-responsive(700-1000 nm) (Rwei et al., 2015)

 

2.3.3. Mechanisms

 

Azobenzene Photoisomerization

The trans-cis isomerization of azobenzene represents one of the most studied photochemical processes Fig 5. This process involves a change in the N=N bond configuration, resulting in a significant geometric change from a planar trans form (distance between para carbons ~9.0 Å) to a bent cis form (distance ~5.5 Å) [315]  .

 

Spiropyran Ring-Opening

Spiropyrans undergo a ring-opening reaction to form merocyanine Fig 6. This transformation involves breaking of the C-O bond and results in a change from a colorless, hydrophobic SP form to a colored, zwitterionic MC form [316].

 

2.3.4. Photocrosslinking Mechanisms

 

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