In thermal medicine, the procedure of raising the temperature has been used alone to kill cancer cells or as an adjuvant treatment to make surgery or chemotherapy more effective. Raised temperature can provide direct damage, and furthermore, trigger drug or gene release of loaded thermo-sensitive nanocarriers.
Thermo-sensitive polymers have been extensively exploited for the synthesis of thermal responsive nanosystems. In the following, we review some examples of different thermo-sensitive nanocarriers investigated in the last five years for the release of different drugs after stimulation with temperature.
Thermo-sensitive liposomes are composed of lipid membranes that undergo phase transitions (from a gel to a liquid phase) in response to heating, at a characteristic phase transition temperature (Tc). Upon heating, the mobility of the lipid head groups, which were ordered and condensed in the gel phase, increases. At the Tc, the hydrocarbon chains switch configuration and the membrane becomes permeable, presenting solid lipid domains and liquid lipid domains. At temperatures higher than Tc, lipids move freely and the bilayers become fully fluidized. During this transition phase, the load of the liposomes is able to leak out.
Non thermo-sensitive liposomes can be sensitized to temperature by functionalizing them with temperature-responsive polymers that disrupt the membrane in response to heating. Such polymers can also enhance the response to temperature of thermo-sensitive liposomes. Temperature-sensitive polymers experience a sharp coil-to-globule transition and phase separation at a LCST or an UCST [108
] (Figure 3
To date, thermo-sensitive liposomes for the delivery of doxorubicin (ThermoDox, Celsion) are the most advanced and effective temperature-activated nanocarriers available [110
]. Currently, in phase III clinical study, ThermoDox showed a 2.1-year improvement to the overall survival in patients affected with hepatocellular carcinoma. Other liposomal carriers have been recently designed and investigated in vitro
and in vivo
. Mild hyperthermia was used to effectively trigger release of cisplatin from thermo-sensitive liposomes in the vasculature of a human model of triple-negative breast cancer (MDA-MB-231 and MDA-MB-436) resulting in a significant tumor growth delay [111
]. The liposomes used in this study had a lipid phase transition temperature of 41.5 °C and demonstrated their efficiency in a previous work on mice bearing subcutaneously-implanted ME-180 cervical tumor [112
Likewise, in another study by Yoon et al. cisplatin was successfully encapsulated in a thermo-sensitive liposomal formulation in order to selectively treat 4T1 murine triple negative breast cancer via photothermal heating. The thermo-sensitive lipid selected was 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine, which has a phase transition of around 41 °C, in order to minimize thermal damage to the body. The loading efficiency was maximized in the formulation called CL16 and exhibited greater therapeutic outcomes, both in vitro
and in vivo
]. Lv et al. reported a hyaluronic acid-paclitaxel (HA-PTX) prodrug and marimastat (MATT)-loaded thermo-sensitive liposomes for the dual targeting of the tumor microenvironment and breast cancer cells [114
]. In this research, they combined the effect of MATT, which avoids metastases by inhibiting enzymes such as collagenases, gelatinases, and matrix metalloproteinases, with the effect of the antitumor drug docetaxel. Thermo-sensitive liposomes were loaded with MATT and assembled with HA-PTX in a multifunctional nanoplatform. The nanocarriers released their payloads after mild hyperthermia treatment at 42 °C and docetaxel entered the cancer cells via CD44 receptor mediation in 4T1 tumor-bearing BALB/c mice.
Thermo-responsive polymers have been used to synthetize polymeric micelles that can release the cargo in response to temperature. They are formulated through a self-assembly process using amphiphilic block-copolymers that spontaneously assemble into a core-shell structure in an aqueous environment. The polymers, at either a LCST or an UCST, experience a phase transition that induces collapse of the micelle and the consequent cargo release.
Thermo-sensitive polymers, such as PNIPAm, pluronics (PEG-b
-PEG, triblock copolymers of polypropylene oxide [PPO] middle blocks flanked by polyethylene glycol [PEG] blocks), and poly(hydroxypropyl methacrylamide-lactate) (p(HPMAm-Lacn
)) are among the most frequently studied. Temperature-responsive micelles have been extensively studied in the past years for their applications as drug delivery systems in cancer therapy. Some formulations have reached clinical trials after showing promising results both in vitro
and in vivo
. Fathi and collaborators synthetized chitosan micelles grafted with PNIPAm as temperature-sensitive moiety and oleic acid as hydrophobic monomer. Micelles were targeted with folic acid and loaded with erlotinib, a tyrosine kinase inhibitor [115
]. The micelle solution was transparent at 25 °C, which was below the LCST (35 °C). However, with an increased temperature to 37 °C, the solution became opaque and around 90% of the loaded drug molecules were released within 48 h, demonstrating the thermoresponsive behavior of the self-assembled micelles. In a recent study, a biocompatible, degradable and thermo-sensitive amphiphilic polymer PNIPAm-co
-poly[ethylene glycol] methyl ether acrylate)-block-poly(epsilon-caprolactone) was synthetized and used to produce self-assembled thermo-sensitive micelles loaded with the chromophore cyanine dye IR-780 and heat-shock protein (HSPs, cause of thermotolerance in cancer cells) inhibitors [116
]. The controlled drug release during laser irradiation and change of temperature was demonstrated both in vitro
and in vivo
in a human colorectal adenocarcinoma cell line.
3.2.3. Core-Shell Nanodevices
Thermoresponsive core-shell nanosystems are a unique class of materials widely studied for drug delivery applications. In general, the thermoresponsive molecule is located on the surface and the core can be constituted by either a hard metallic (gold, magnetic) or a soft (dendrimers, chitosan NPs, silica NPs, nanogels) nanoparticle.
Among the hard metallic NPs, gold nanodevices are the most used due to their photo-inducible heat-generating properties as a consequence of localized surface plasmon resonance (SPR). The heat generated on the surface of the plasmonic NPs causes the thermoresponsive polymer to collapse and the release of the drug. Fathi and co-workers recently designed chitosan copolymer-gold hybrid NPs loaded with erlotinib (ETB), which was released from the nanosystem in a thermo-responsive manner thanks to the temperature-responsiveness of chitosan copolymer composed of (poly(N
-oleic acid)-g-chitosan ((PNIPAm-co
-CS) that presented an LCST of around 36 °C. The successful cytotoxicity investigation in A549 cells validated their potential as an effective anticancer drug carrier [117
]. Au NPs were combined in another study with the copolymer Poly (NIPAAm-co
-AAm) that was used to create a collapsible thermo-sensitive nanoshell, which exposed targeting ligands (integrin β1) upon NIR irradiation, enabling cell binding. This nanodevice, which exploited the photothermal properties of Au NPs to control NP binding to cell, could be used for targeted photothermal therapy and drug delivery [118
]. Magnetic NPs have also been used in combination with different thermoresponsive polymers for drug delivery. Gui and collaborators designed a complex nanodevice constituted of Fe3
NPs/CdTe quantum dots dual-embedded mesoporous silica nanocomposites (MQ-MSN) as cores and P(N
-isopropylacrylamide)-graft-Chitosan microgels (PNIPAm-g
-CS) as shells. The carriers, which possessed outstanding magnetism/fluorescence/thermo/pH-sensitivity, were loaded with the anticancer drug adriamycin (ADM) that was released in a temperature dependent manner above the LCST retaining anticancer activity in HepG2 cells [119
Soft materials like dendrimers have been extensively investigated for the synthesis of thermo-sensitive core-shell NPs. Elastin-mimetic dendrimers were synthetized by conjugating Val-Pro-Gly-Val-Gly repeats, an elastin-like peptide was used as temperature-sensitive biomaterial, to a polyamidoamine (PAMAM) dendrimeric core, though the phase transition temperature presented by the system (48 °C) has to be optimized for biomedical applications [120
]. The combination of two dendrimers in the same nanodevice resulted in being more efficient in terms of temperature responsiveness. Oligo (ethylene glycol) (OEG) side chains show attractive thermo-sensitive behavior because their LCST can be tuned from 33 to 64 °C. A thermo-sensitive codendrimer PAMAM-co
-OEG (PAG) by decorating fourth-generation PAMAM with the second generation OEG dendron was synthetized. This system exhibited high drug (methotrexate) loading capacity and temperature-dependent drug release presenting an LCST of 38.2 °C [121
In recent years, thermoresponsive hydrogels are one of the most intensively investigated thermo-sensitive materials for biomedical applications such as drug delivery and tissue engineering and repair. Temperature-sensitive polymer based hydrogels have an LCST above which they undergo transition from a solution to a gel state, forming three-dimensional cross-linked polymeric networks. Injectable biodegradable hydrogels that can form gels in situ have been widely used for biomedical applications, such as cell/drug delivery as well as tissue engineering since they can provide a sustained and controlled delivery to the target site.
When particles with nanometric sizes are obtained during the hydrogel synthesis, the systems are called nanogels, however, various forms of gels have been studied [122
]. Figure 4
shows a schematic representation of the different dimensions and appearances of hydrogels synthetized by click chemistry, a useful approach in forming gels owing to its high reactivity, superb selectivity, mild reaction conditions, and bio-orthogonal feature, though physical and chemical crosslinking methods, mini-emulsion techniques, as well as self-assembly are also important approaches.
Due to the impact of hydrogels on thermo-sensitive biomedical applications, we will review both nanosized hydrogels and hydrogels with other forms and dimensions that are used as thermo-sensitive platforms for the transport of different types of NPs or cells.
Several natural polymers that demonstrate thermally sensitive properties have been used for the synthesis of hydrogels such as polysaccharides (cellulose, chitosan and xiloglucan) [123
] and proteins (gelatin) [126
]. Different polymers such as poly(ethylene glycol)-poly(3-caprolactone)-poly(ethylene glycol) [127
], poly n
-isopropylacrylamide/polyacrylic acid (PNIPAm/PAA) [128
-lactide)-block poly(ethylene glycol)-block-poly(d
-lactide) (PDLLA-PEG-PDLLA) [129
], poly(ethylene oxide)-b
-poly(ethylene oxide) (PEO-PPO-PEO) [130
], and PNIPAm-based hydrogels have also been reported in drug delivery [131
] (see Table 1
Maiti et al. developed a chemically cross-linked poly-N
′-dimethyl aminoethyl methacrylate (PDMAEMA) smart nanogel loaded with both an anticancer drug, doxorubicine, and a radioisotope, 13
I-labeled albumin, for enhanced chemo-radioisotope therapy. The nanogel in solution form was injected into the tumor where it was transformed into a gel at body temperature. This thermogelling behavior led to the sustained release of the drug and the retention of the radionuclide within the tumor achieving excellent therapeutic in vivo
results in mice bearing 4T1 tumors [134
]. A novel water in water thermo-nanoprecipitation technique for the synthesis of thermoresponsive nanogels composed of dendritic polyglycerol (dPG) and linear thermoresponsive polyglycerol (tPG) as building blocks was recently published. The nanogel was used as a carrier of etanercept (ETR), a protein approved for the treatment of psoriasis and arthritis by subcutaneous injection. Nanogels were topically administered to inflammatory skin equivalents or tape striped human skin, resulting in temperature triggered and efficient ETR delivery and anti-inflammatory effects [135
Thermal responsive microgels can carry a drug [136
] or drug-loaded NPs [138
]. The combination of various drugs in a single microgel can be achieved by encapsulating them directly in the hydrogel [140
] or by designing hydrogel composites such as the dual drug loaded thermo-sensitive hydrogel composite recently published by Xu and co-workers [141
]. In this study, the anti-tumor effect of the combination of cisplatin-containing thermo-sensitive hydrogel and paclitaxel-loaded polymeric micelles in a single composite (called PDMT) was investigated in an in vivo
cervical cancer model. Methoxypoly(ethylene glycol)–poly(caprolactone) (MPEG-PCL) was used for the synthesis of the micelles and Poly(ethylene glycol)-poly (epsilon-caprolactone)-poly(ethylene glycol) (PEG-PCL-PEG) for the hydrogel. PDMTs were effective in inhibiting tumor growth and prolonging the survival time of treated mice.
Heat sensitive hydrogels carrying drugs, NPs or cells have been extensively investigated for their application in tissue engineering and regeneration. Recently, vascular endothelial growth factor (VEGF)-loaded poly (lactic-co
-glycolic acid) (PLGA)-NPs embedded thermo-sensitive gels have been used to promote bladder tissue regeneration in a rabbit model exhibiting favorable performance [142
Thermo-responsive hydrogels have shown great potential for bone tissue regeneration. Rosuvastatin-loaded chitosan/chondroitin sulfate NPs incorporated into a thermo-sensitive hydrogel provided positive results in vitro
] and methylcellulose hydrogel containing bioactive calcium phosphate NPs showed a higher new bone formation in vivo
than the pure hydrogel in a rabbit calvaria defect model [144
There are many [145
] in vitro
and in vivo
studies demonstrating the effectiveness of hydrogels for central nervous system (CNS) regeneration. Their three-dimensional porous structure is commonly used to load and deliver drugs and growth factors (such as heparin) or they can be injected, successfully inducing bridging of post-traumatic cystic cavities in the spinal cord, as demonstrated by Hong and collaborators. They studied an imidazole-poly (organophosphazenes) (I-5) hydrogel with thermo-sensitive sol-gel transition behavior for the treatment of cystic cavities that develop following injuries to the brain or spinal cord in a clinically relevant rat spinal cord injury model. The dynamic interaction of the hydrogel with the inflammatory cells induced extracellular matrix remodelling to stimulate tissue repair. An improved coordinated locomotion that was accompanied by preservation of myelinated white matter and motor neurons and an increase in axonal reinnervation of the lumbar motor neurons were observed.
Moreover, other thermo-sensitive hydrogels have been investigated as candidates for cardiac regeneration therapy [149
]. A system based on thermo-sensitive hydrogel and oxygen releasing microspheres was developed for the delivery of oxygen to heart tissue [151
]. The system was able to continuously release oxygen for four weeks leading to a significant increase in cardiac function of infarcted rats. Furthermore, different kinds of hydrogels have been developed for skin wound dressings [152
Cartilage repair is a great challenge due to the limited capacity for self-healing. Hydrogels with smart sol-gel response for altering environmental temperature have been investigated to promote cartilage regeneration. For instance, a transforming growth factor (TGF)-β1-loaded poly(ε-caprolactone)–poly(ethylene glycol)–poly(ε-caprolactone) (PCEC) hydrogel was fabricated and studied, which demonstrated it to be biodegradable and capable of in vivo
cartilage repair [155
]. Moreover, the regeneration of hyaline-like cartilage with reduced fibrous tissue formation in vivo
was achieved by Liu and co-workers by increasing the phenylalanine content into a poly(l
-phenylalanine) (PAF-PEG-PAF) thermo-sensitive hydrogel encapsulating bone marrow mesenchymal stem cells (BMMSCs). The increased phenylalanine unit content resulted in an enlarged pore size and enhanced mechanical strength [156
]. These features provided better permeability and cell-cell communication, nutrient transportation and cell proliferation, migration, and differentiation that lead to better regeneration of cartilage tissue with reduced fibrous tissue formation (Figure 5