Smart Nanomaterials for Biomedical Applications—A Review

Recent advances in nanotechnology have forced the obtaining of new materials with multiple functionalities. Due to their reduced dimensions, nanomaterials exhibit outstanding physio-chemical functionalities: increased absorption and reactivity, higher surface area, molar extinction coefficients, tunable plasmonic properties, quantum effects, and magnetic and photo properties. However, in the biomedical field, it is still difficult to use tools made of nanomaterials for better therapeutics due to their limitations (including non-biocompatible, poor photostabilities, low targeting capacity, rapid renal clearance, side effects on other organs, insufficient cellular uptake, and small blood retention), so other types with controlled abilities must be developed, called “smart” nanomaterials. In this context, the modern scientific community developed a kind of nanomaterial which undergoes large reversible changes in its physical, chemical, or biological properties as a consequence of small environmental variations. This systematic mini-review is intended to provide an overview of the newest research on nanosized materials responding to various stimuli, including their up-to-date application in the biomedical field.

Smart nanomaterials are categorized in different groups by means of the applied stimuli. The properties of smart nanomaterials are modified by external triggers in a controlled way [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. By considering their various properties, many kinds of smart nanomaterials are known. These stimuli can typically be classified into three different categories: physical, chemical, and biological, as shown in Figure 1 . The main applications in the biomedical field of these special materials as a function of their ability to respond to one or more stimuli are also highlighted in Figure 1 . 2.1. Physical Responsive Nanomaterials Examples of physico-sensitive nanomaterials and their applications are listed in Table 1 . 2.1.1. Temperature-Responsive Nanomaterials Since 1942, when Huggins and Flory first theoretically described polymer–solvent interaction in solution with varying temperature and the concept of free volume (used to explain the critical temperature lower/upper phenomenon in solution), temperature-responsive polymers have received increasing interest in the biomedical field [ 21 , 22 ]. Thermosensitive polymers are a type of material that go through a sudden change in their solubility as a reply to a small temperature change [ 23 , 24 , 25 ], and they have stimulated researchers’ attention in the biomedical field, taking into consideration that specific infections show temperature changes [ 41 ]. Temperature-responsive polymers have a typical trademark highlight in the presence of a hydrophobic group: propyl, methyl, and ethyl groups. When warmed or cooled over a critical transition point, inside a small temperature range, a break of hydrophobic and intra/intermolecular electrostatic interactions takes place and the impact is a phase transition in the volume. The lower critical solution temperature (LCST) is the temperature above which the polymeric monophasic system endures phase separation and becomes biphasic, hydrophobic, and insoluble. On the contrary, below a lower critical solution temperature (LCST), the polymer is monophasic [ 26 , 42 ]. Above an upper critical solution temperature (UCST), one polymer phase appears, and below this, a phase separation exists [ 43 ]. In polymer solutions, the LCST usually results from a coil to a globule transition, minimizing the contact with the solvent [ 44 ]. The UCST is normally present in soluble polymers [ 45 , 46 ]. A particular class of temperature-responsive materials are the polymers that can display both LCST and UCST properties, but at different temperatures [ 47 , 48 ]. For better results, the LCST value of a stimuli-responsive system must be close to the body temperature and by increasing that value to approximately 42 °C, due to a change in the environmental condition, the initially loaded drug is released. Notwithstanding the LCST, the engineering of nanomaterials might be utilized to adjust drug release kinetics, especially with huge changes in the surface area as a result of the modified porosity and geometries or inclusion of metallic nanoparticles. Some parameters such as polymer concentration and molecular weight or pH can impact the LCST and UCST behaviors of thermoresponsive polymers [ 49 , 50 , 51 ]. In a recent study, Tamaki et al. obtained thermoresponsive polymers by different phenylalanine (Phe)-modified zwitterionic dendrimers [ 50 ]. They obtained carboxy-terminal Phe-modified dendrimers polyamidoamine (PAMAM) and succinic anhydride (Suc) (PAMAM-Suc-Phe and PAMAM-Phe-Suc) utilizing PAMAM dendrimers modified with Phe and Suc. Both these dendrimers indicated UCST-type thermosensitivity. Strangely, PAMAM-Phe-Suc showed LCST-type thermosensitivity at lower pH, but PAMAM-Suc-Phe did not. This shows that, as a function of the pH solution, PAMAM-Phe-Suc can switch LCST/UCST-type thermoresponsivity and the inner tertiary amine and the Phe residue in the dendrimer are indispensable for their stimuli-sensitive behaviors. These dendrimers induced coacervation (liquid–liquid phase separation) during temperature changes. Tissue engineering, drug delivery, or diagnosis applications from thermoresponsive nanomaterials were achieved from different hydrogels with various nanostructures and behaviors [ 27 , 28 , 52 , 53 , 54 ] or smart polymer substrates [ 55 ]. However, for the as-mentioned biomedical applications, responsive behavior must be offset with biocompatibility and degradation kinetics [ 53 ]. Zheng et al. developed an injectable low-fouling zwitterionic thermosensitive hydrogel [ 29 ] ( Figure 2 ). The zwitterionic thermosensitive poly(Nisopropylacrylamide-co-sulfobetaine methacrylate) (PNS) nanogels were synthetized using N-isopropylacrylamide (PNIPAM), sulfobetaine methacrylate (SBMA), and N, N’-methylenebisacrylamide (MBA). The average hydrodynamic diameter of nanogels was ca. 105 nm ( Figure 2 b). The sol–gel phase transition of the nanogel is based on the balance between the hydrophobic interaction betw

Light is regarded as an appealing stimulus because of its controllable and tunable properties. Light-responsive materials are profoundly invaluable for applications since light can be applied immediately and with extremely high spatiotemporal accuracy with an on/off controlling mode. Moreover, light-sensitive intelligent materials are biomarkers which indicate the site of drugs and the targeting capacity and visualize the tumors by fitting a wide variety of parameters. Considering specific applications, an advantage of this stimulus material is that input parameters such as intensity, wavelength, light, beam diameter, and exposure time can be easily tailored. To overcome the issues related to the precise drug release or light-mediated theranostics, smart nanomaterials such as nanopolymers are an alternative way in which the drug delivery on target locations can be controlled and the right concentration of drug delivery in a specific time can be achieved. Several light-sensitive polymers contain chromophores: azobenzene groups [ 64 ], spiropyran groups [ 36 ], or nitrobenzyl groups ( Figure 4 a) [ 65 ]. Various photomaterials were effectively utilized for biomedical applications, but unfortunately the greater part of them is activated only in the UV/VIS region, which restricts tissue penetration and destructive behavior related to ordinary tissues. The development of photoresponsive smart nanomaterials is an approach to overcome these difficulties on the grounds that these materials can successfully create reactive oxygen species (singlet oxygen, peroxide, hydroxyl, etc.) and have targeting ability, good solubility in water, and near-infrared light activation (NIR) [ 37 ]. Photoabsorbing materials are widely used in photothermal therapy due to local hyperthermia which kills cancer/bacteria cells. In biological media, an environment that is overheated produces few risky impacts: cell lysis, protein evaporation of the cytosol, and aggregation or denaturation [ 66 ]. As referenced before, the majority of the recent phototherapeutic systems adequately use NIR-sensitive nanopolymers to defeat the downsides related to photosensitizers, similar to UV/VIS-sensitive fluorophores [ 67 ]. Furthermore, most of the recent diagnosis or therapeutic systems are using NIR fluorescence for observing deep tissue cancer tumors [ 68 ]. Mimicking natural structures, the production of photoresponsive polymers can be achieved by introducing photoactive molecules in the side chains or the polymeric backbone by exposition to UV or near-infrared (NIR) irradiation. Those materials undergo conformational, stereochemical, or structural changes, such as photoisomerization and photocleavage. For azobenzene, this process is visualized by the change in molecular symmetry from a thermally stable trans (E) orientation to a less favorable cis (Z) orientation. In spiropyrans, a ring-opening reaction is induced by the irradiation with the appearance of the isomeric merocyanine form. By UV absorption, the photoactive groups from the nanocarriers suffer a reversible isomerization from cis to trans and the trans isomer can be reconverted into the cis isomer by visible light. In this way, the nanostructures are disrupted and their cargo is released [ 69 ]. Figure 4 ( a ) Examples of photoisomerizable groups for reversible light-responsive nanomaterials. Reproduced from [ 65 ], with permission from © The Royal Society of Chemistry 2017. ( b ) Synthesis of PNc and UV light drug release. ( c ) Synthesis and photoisomerization process of PNSC amphiphilic copolymer. Reproduced from [ 70 ], with permission from © 2020 Mena-Giraldo, Perez-Buitrago, Londono-Berrío, Ortiz-Trujillo, Hoyos-Palacio, and Orozco under CC BY 4.0. Mena-Giraldo et al. synthesized a photoresponsive polymeric nanocarrier by chitosan modification with ultraviolet-photosensitive azobenzene molecules ( Figure 4 b). They considered the impact of UV light on releasing the cargo in a guided manner by means of nanobioconjugates. As cargo models, they chose Nile red and dofetilide and proved the encapsulation/release model. They used photoresponsive polymeric nanocarriers functionalized with a cardiac transmembrane peptide. The viability of the therapeutic regime was improved by the effect of UV light irradiation which increases the concentration of intracellular delivery and decreases the amount and cargo reactions [ 70 ]. In Figure 4 , the notations are: 2-(4-phenylazophenoxy)ethanol (PAPE); 2-(4-(phenylazo)phenoxy)ethanoloxy succinyl ester (PAPESE); N-succinyl chitosan (NSC); N-succinyl-N-4-(2-(4-(phenylazo)phenoxy)ethanoloxy)-succinyl-chitosan (PNSC); and chitosan (CHI). A group of authors anchored uniform zeolitic imidazolate framework (ZIF-8) crystals onto FeOOH nanorods through a simple recombination process to obtain FeOOH@ZIF-8 composites for in vitro direct metabolic analysis of biofluids in diagnostics of gynecological cancers. The obtained FeOOH@ZIF-8 nanorods displayed an enhanced

Examples of chemical-sensitive nanomaterials and their applications are listed in Table 2 . 2.2.1. pH-Responsive Nanomaterials pH stimulus smart nanomaterials react to the pH by displaying new functional properties and are fascinating in the biomedical field because pH changes are present in many specific or pathological systems. The advantage of using those materials is that various segments of the human body have diverse pH levels (for chronic wounds, pH values are 7.4–5.4; for saliva 6.5–7.5; along the gastrointestinal tract the pH changes from the stomach (4–6.5) to the intestine (5–8)) [ 89 ]. Additionally, the pathological state shows strange pH values contrasted with the physiological state pH, for example, tumor micromedia have a lower extracellular pH between 6.5 and 6.9, bacterial infections present acidic pus with pH in the range 6.0–6.6, and an inflamed tissue has a pH value of 6–7. In the light of these extraordinary pH varieties, different pH-responsive materials have been obtained so far. The pH-sensitive polymers are mostly classified as polymers with ionizable moieties and polymers with acid-labile linkages. The critical component for the first category is the existence of ionizable, fragile basic, or acidic moieties (amines and carboxylic acids) that bind to a hydrophobic backbone, for example, polyelectrolytes [ 90 ]. A common pH-responsive material of this class of polymer displays protonation/deprotonation processes by dispersing the charge over the ionizable groups of the molecule. The second class is the polymers with a backbone which involves acid-labile covalent linkages. A dissociation of polymer aggregates or a breaking in polymer chains is determined by the cleavage of these bonds at the reduction in pH. The second category has a slower inner alteration due to the presence of covalent bonding, which encourages their implementation in the drug release field. pH initiates a phase sudden transition in pH-responsive polymers. Ordinarily, the phase switches within a pH range of 0.2–0.3. The most known pH-sensitive polymers are poly(L-lysine), poly(N,N -dimethylaminoethyl methacrylate), poly(methacrylic acid), poly(acrylic acid), poly(N,N -dialkyl aminoethyl methacrylates), poly(ethylenimine), chitosan, aginate, and hyaluronic acid [ 91 ]. Experiments for in vivo wound healing demonstrated that pH-responsive silver nanoparticle clusters formed from the ortho ester inner layer and PEG corona (AgNCs) are effective for the healing of methicillin-resistant Staphylococcus aureus infections [ 77 ]. Transistors are usually utilized as electronic switches since they react pointedly with an amplified current output to an applied voltage threshold. A new hybrid nanoparticle design with a combination of three pH-responsive block copolymers, each of them acting as a sensor at a foreordained pH value, was reported to study endocytic organelles acidification from living cells. This ultra-pH-sensitive (HyUPS) nanotransistor with pH transitions at 6.9, 6.2, and 5.3 opens new insights in biomedical applications [ 78 ]. A novel pH-responsive supramolecular nanomaterial was developed, taking into account the negatively charged octasulfonate-modified zinc (II) phthalocyanine (ZnPcS8) which interacts with positively charged hydroxide double layers (LDH). In neutral conditions, LDH-ZnPcS 8 is not photoactive, being activated in an acidic environment (pH 6.5) [ 79 ]. A melanin-like nanoparticle (MelNP) was obtained to exploit its unique characteristic in developing a highly pH-sensitive tool for in vivo cancer target imaging [ 80 ]. As a confirmation of the idea, a few pH-sensitive nanomaterials are obtained either from polymers with weak acid bonds (vinyl ester, ketal, acetal, orthoester, etc.) whose breaking initiates the surface charge changes or the molecules release, or from polysaccharides (e.g., chitosan) that suffer conformational changes and/or pH-responsive solubility. For instance, chitosan is a polysaccharide with different behavior: under neutral conditions, which are found in a healthy environment, it has a negative or neutral surface charge, while under an acidic condition from a pathological environment, it has a positive surface charge (due to the amino group’s protonation), which easily interacts with the negative surface charge of the cells. This behavior entitles chitosan to be utilized for targeting and giving specific sites for therapeutic viability [ 92 ]. Bonadies et al. developed a pH stimuli-responsive polylactic acid PLA system acting as an implant coating for prolonged period utilization. They described a Resveratrol (RSV)-containing membrane obtained by the electrospinning method, which liberated RSV very fast if pH decreased from a neutral to slightly acidic value around 5.5 (the case of infections). They demonstrated the PLA-RSV membrane’s capability to prevent implant-associated infections [ 81 ]. An illustration of a pH-responsive nanomaterial obtained by Zhou et al. is repre

Examples of biological-sensitive nanomaterials and their applications are listed in Table 3 . 2.3.1. Glucose-Responsive Nanomaterials Sugar-sensitive nanomaterials can mimic normal endogenous insulin production as a response to the presence of glucose by limiting diabetic disorders and by delivering the bioactive agent in a driven way. In particular, polymers have gathered extensive consideration in view of their use in the glucose detection and insulin delivery fields. Despite these points of interest, the significant disadvantages to be overcome are the short reaction time and the low biocompatibility [ 114 ]. Glucose-responsive polymeric systems are precisely engineered to produce insulin and are based on enzymatic oxidation of glucose by glucose oxidase (GOx) and binding of glucose with lectin. A pH change in the environment is caused by glucose oxidation to gluconic acid. The response to the pH change is a volume transition of the polymer, and in this manner, the body’s glucose level is driven by conformational polymer changes [ 115 ]. Some authors synthesized acetalated dextran nanoparticles or acryloyl cross-linked dextran dialdehyde (ACDD) nanocarriers containing enzymes and insulin to facilitate glucose-sensitive release [ 103 , 116 ]. Boronic acid-derived hydrogels were used to obtain glucose-sensitive platforms for drug delivery [ 104 , 117 ]. For production of a glucose-sensitive platform, different systems used the lectin special characteristics of carbohydrate binding. The most used lectin in insulin-driven drug delivery is concanavalin A (Con A) due to its four binding sites. Yin et al. showed that for an ideal insulin treatment, the ConA micro/nanospheres inserted in a polymer matrix can provide good results ( Figure 8 ) [ 105 ]. They obtained a pancreatic system intended to release insulin in a long-term and close-looped manner. The system was engineered by using a chitosan hydrogel whose degradation allowed the leak out of insulin microspheres, enabling an extended insulin delivery. Liu et al. obtained a polyelectronic nanocomplex (PEC) for insulin delivery by synthesis of chitosan-g-polyethylene glycol monomethyl ether (CS-g-mPEG) copolymers with various mPEG graft concentrations [ 106 ]. They studied the hypoglycemic effect by in vivo oral administration. The action of two copolymers, a CS-g-mPEG nanocomplex and fabricated mPEG-CS, was compared and the results demonstrated the best absorption was achieved by CS-g-mPEG at a graft ratio of 10%. Another outcome of this work was the implication of nanoparticles containing insulin in the penetration of the mucus of the intestine. Other authors compared the blood glucose-lowering effect in diabetic rats, in the cases of oral administration versus insulin subcutaneous injection. They used nanocarriers made from calcium carbonate, covered with hyaluronic acid [ 107 ]. The delivery of insulin to the small intestine was effectively conducted by an enteric-coated capsule loaded with chitosan-poly(γ-glutamic acid) nanoparticles [ 108 ]. The release of insulin was evaluated in vitro using carboxymethyl chitosan-phenylboronic acid-Lvaline multifunctional nanoparticles [ 109 ]. The production of biodegradable nanomaterials is favorable for targeted drug release applications due to their bioavailability, biocompatibility, better retention time, lower toxicity, and enhanced permeability. A widely utilized protein polymer is gelatin due to its nontoxic and biodegradable properties [ 118 ].

A step forward for biomedical applications is attained when the smart nanomaterials are simultaneously sensitive to more stimuli. The nanomaterials which are sensitive to a few sorts of stimuli are the key for expanding the efficacy of drug delivery and for supporting the diagnosis by monitoring a few physiological changes at once. The development of nanomaterials with both diagnostic and therapeutic properties is the most powerful technological frontier for moving forward to nanotheranostics. The demand for these technologies is based on the advantage of multiple functions such as multimodal imaging, synergistic therapies, and targeting. The working system of nanotheranostics depends on biological, chemical, and physical triggers considering the activation of the diagnostic and/or the therapeutic properties only at the infected site. In this era of the “war on cancer”, the dual and multi-stimuli-responsive methodology is undeniably appropriate for theranostics as some properties can provide diagnostics, while others could initiate therapy and curing. Consequently, multi-stimuli-sensitive polymers are drawing in expanding consideration for their advantages in the biomedical field. Multi-stimuli-sensitive polymeric nanoparticles were developed as emerging targeting drug delivery systems. In Figure 10 , a scheme of internal and external multi-stimuli action is outlined [ 127 ]. External stimuli such as temperature and pH facilitate the emergence of nanoparticles, while stimuli such as light, the magnetic field, the temperature, and ultrasonic are intended to control drug delivery. Examples of multi-sensitive nanomaterials and their applications are listed in Table 4 . In the actual global fight against cancer, the development of new diagnosis and treatment methods is crucial. The research community developed different systems from multi-responsive stimuli nanomaterials which are proven to be effective in cancer treatment. Yu et al. obtained by a distillation precipitation polymerization a multi-responsive system with controllable drug release and tumor cells destruction. The pH-, redox-, and temperature-responsive drug release system realizes the penetration and the accumulation of the tumor and targeted drug delivery [ 128 ]. A novel system responsive to both pH and ultrasound stimuli was obtained by Chen and Du, based on a poly(ethylene oxide, 2-(diethylamino)ethyl methacrylate, (2-tetrahydrofuranyloxy)ethyl methacrylate PEO- b -P(DEA- stat -TMA) block copolymer [ 129 ]. The anticancer drugs were encapsulated in the system and then released in a controllable way after the action of ultrasound radiation or varying pH stimuli. Figure 10 highlights the obtaining of the pH- and ultrasound-sensitive PEO 43 - b -P(DEA 33 - stat -TMA 47 ) vesicle. Ultrasound radiation and the solution pH influence the system, leading to a smaller vesicle. The system disruption and re-self-assembly are due to the ultrasound radiation. A lab-on-a-chip based on a magnetic- and temperature-responsive nanoparticle system was developed to capture diagnostic targets [ 130 ]. The supermagnetic nanoparticle has γ-Fe 2 O 3 as a core, surrounded by self-assembled carboxylate-terminated PNIPAAm as the temperature-responsive agent (with LCST). Biotin and streptavidin were loaded in this nanoparticle and the final device has walls from PEG-modified polydimethylsiloxane channels. Above MCST, the system formed magnetic-responsive aggregates and a further increase in temperature combined with the magnetic field application induced immobilizations of these aggregates on the microchannel wall. If the temperature is decreased below the LCST and the magnetic field is removed, the aggregates are redispersed and eluted through the channel. Those nanoparticles are useful for capturing diagnostic targets at the right time and at a certain channel position. A recent study describes a novel dual stimuli-sensitive system for a controllable tumor drug delivery. A chemo-photothermal synergetic cancer therapy was achieved by integrating DNA aptamer with dopamine-reduced graphene oxide (rGO-PDA) nanosheets [ 131 ]. The rGO-PDA nanosheets acted at once as an NIR photothermal agent, inducing hyperthermia for photothermal therapy. Some authors obtained a dual stimuli response nanoplatform with superior targeting capacity, good photothermal conversion properties, and smart drug release sensitive to both pH and photothermal heating. The nanosystem was effectively utilized to release DOX to protein tyrosine kinase 7 (PTK7)-overexpressing cancer cells in a controllable manner, by reacting simultaneously to NIR irradiation and to the acidic intracellular environment. The mix of a dual nanocarrier of drug loading and dual stimuli release is effective in chemo-photothermal synergetic therapy. A new approach of a pH- and temperature-sensitive mixed ferrite nanohybrid was recently obtained for theranostic applications [ 132 ]. The system was obtained from polyethyleneimine

This mini-review highlights the tremendous progress of nanotechnology over the last two decades and why it is of vital importance for the further study of smart materials for biomedical applications. The manufacture of stimulus-responsive nanomaterials involves a creative formulation of drugs and polymers so that they respond to biological cues such as differences in pH and temperature between healthy and diseased tissue and the induced response leads to controlled and sustained release of the load. Better understanding of the physiological changes and of the differences between healthy and diseased tissue will improve the possibility of designing materials that uniquely respond to local stimuli. The synthesis of nanoparticles for drug release after encapsulation must be tailored as a function of the therapeutic goals. The efficacy of diagnosis and therapeutics is achieved by designing multifunctional nanoplatforms to provide new methods and strategies for future clinical oncology nanomedicine. Nanotheranostic improvement maybe speaks to the most elevated level of innovative development in the nanomedicine field. Another important field in nanomedicine is lab-on-a-chip innovation, which includes the scaling down of processes that occur on an integrated platform. Stimuli-responsive nanopolymers on the lab-on-a-chip systems have contributed essentially to the decrease in cost, time, efficient reagents, and tests required. Some complex biological in vitro barriers such as improving the biocompatibility, stability, and biodegradability; ensuring nontoxic, timed turns on and off; and precise control of response sites of stimuli-responsive nanoplatforms still remain difficult to realize. In spite of these hurdles, knowledge about plasmonic nanoparticles offers great possibilities for enhanced diagnosis and monitoring for prognostic use towards point-of-care testing. However, even if the macroscopic photothermal effects such as fluid convection, tissue damage, drug release, or chemical reactions were intensively studied, the actual temperature of the plasmonic nanoparticles during these processes was often unknown. In that sense, the recent developments in efficient thermal imaging techniques are expected to contribute to further insight. The synergistic new approach moving from in vivo studies to clinical trial applications in the oncological field holds great promise in nanomedicine but must be studied in more depth. In brief, with the increasing progress and the continuous innovation of science and technology, we have reason to believe that stimuli-responsive nanomaterials will surely lead to effective strategies and bring a substantial benefit in the biomedical field.