Exploiting the layer-by-layer nanoarchitectonics for the fabrication of polymer capsules: A toolbox to provide multifunctional properties to target complex pathologies

Polymer capsules fabricated via the layer-by-layer (LbL) approach have attracted a great deal of attention for biomedical applications thanks to their tunable architecture. Compared to alternative methods, in which the precise control over the final properties of the systems is usually limited, the intrinsic versatility of the LbL approach allows the functionalization of all the constituents of the polymeric capsules following relatively simple protocols. In fact, the final properties of the capsules can be adjusted from the inner cavity to the outer layer through the polymeric shell, resulting in therapeutic, diagnostic, or theranostic (i.e., combination of therapeutic and diagnostic) agents that can be adapted to the particular characteristics of the patient and face the challenges encountered in complex pathologies. The biomedical industry demands novel biomaterials capable of targeting several mechanisms and/or cellular pathways simultaneously while being tracked by minimally invasive techniques, thus highlighting the need to shift from monofunctional to multifunctional polymer capsules. In the present review, those strategies that permit the advanced functionalization of polymer capsules are accordingly introduced. Each of the constituents of the capsule (i.e., cavity, multilayer membrane and outer layer) is thoroughly analyzed and a final overview of the combination of all the strategies toward the fabrication of multifunctional capsules is presented. Special emphasis is given to the potential biomedical applications of these multifunctional capsules, including particular examples of the performed in vitro and in vivo validation studies. Finally, the challenges in the fabrication process and the future perspective for their safe translation into the clinic are summarized.

The Influence of Defects on the Luminescence of Trivalent Terbium in Nanocrystalline Yttrium Orthovanadate

Terbium-doped YVO₄ has been considered a nonluminescent solid since the first classic studies on rare-earth-doped phosphors in the 1960s. However, we demonstrate that defect engineering of YVO₄:Tb³⁺ nanoparticles overcomes the metal-metal charge transfer (MMCT) process which is responsible for the quenching of the Tb³⁺ luminescence. Tetragonal (Y_1-xTb_x)VO₄ nanoparticles obtained by colloidal precipitation showed expanded unit cells, high defect densities, and intimately mixed carbonates and hydroxides, which contribute to a shift of the MMCT states to higher energies. Consequently, we demonstrate unambiguously for the first time that Tb³⁺ luminescence can be excited by VO₄^3- → Tb³⁺ energy transfer and by direct population of the ⁵D₄ state in YVO₄. We also discuss how thermal treatment removes these effects and shifts the quenching MMCT state to lower energies, thus highlighting the major consequences of defect density and microstructure in nanosized phosphors. Therefore, our findings ultimately show nanostructured YVO₄:Tb³⁺ can be reclassified as a UV-excitable luminescent material.

Smart Layer-by-Layer Polymeric Microreactors: pH-Triggered Drug Release and Attenuation of Cellular Oxidative Stress as Prospective Combination Therapy

Polymer capsules fabricated via the layer-by-layer (LbL) approach have emerged as promising biomedical systems for the release of a wide variety of therapeutic agents, owing to their tunable and controllable structure and the possibility to include several functionalities in the polymeric membrane during the fabrication process. However, the limitation of the capsules with a single functionality to overcome the challenges involved in the treatment of complex pathologies denotes the need to develop multifunctional capsules capable of targeting several mediators and/or mechanisms. Oxidative stress is caused by the accumulation of reactive oxygen species [e.g., hydrogen peroxide (H2O2), hydroxyl radicals (•OH), and superoxide anion radicals (•O2-)] in the cellular microenvironment and is a key modulator in the pathology of a broad range of inflammatory diseases. The disease microenvironment is also characterized by the presence of proinflammatory cytokines, increased levels of matrix metalloproteinases, and acidic pH, all of which could be exploited to trigger the release of therapeutic agents. In the present work, multifunctional capsules were fabricated via the LbL approach. Capsules were loaded with an antioxidant enzyme (catalase) and functionalized with a model drug (doxorubicin), which was conjugated to an amine-containing dendritic polyglycerol through a pH-responsive linker. These capsules efficiently scavenge H2O2 from solution, protecting cells from oxidative stress, and release the model drug in acidic microenvironments. Accordingly, in this work, a polymeric microplatform is presented as an unexplored combinatorial approach applicable for multiple targets of inflammatory diseases, in order to perform controlled spatiotemporal enzymatic reactions and drug release in response to biologically relevant stimuli.

Cholesterol and Morpholine Grafted Cationic Amphiphilic Copolymers for miRNA-34a Delivery

miR-34a is a master tumor suppressor playing a key role in the several signaling mechanisms involved in cancer. However, its delivery to the cancer cells is the bottleneck in its clinical translation. Herein we report cationic amphiphilic copolymers grafted with cholesterol (chol), N, N-dimethyldipropylenetriamine (cation chain) and 4-(2-aminoethyl)morpholine (morph) for miR-34a delivery. The copolymer interacts with miR-34a at low N/P ratios (∼2/1) to form nanoplexes of size ∼108 nm and a zeta potential ∼ +39 mV. In vitro studies in 4T1 and MCF-7 cells indicated efficient transfection efficiency. The intracellular colocalization suggested that the copolymer effectively transported the FAM labeled siRNA into the cytoplasm within 2 h and escaped from the endo-/lysosomal environment. The developed miR-34a nanoplexes inhibited the breast cancer cell growth as confirmed by MTT assay wherein 28% and 34% cancer cell viability was observed in 4T1 and MCF-7 cells, respectively. Further, miR-34a nanoplexes possess immense potential to induce apoptosis in both cell lines.

Immuno-silent polymer capsules encapsulating nanoparticles for bioimaging applications

PEGylated polymer capsules encapsulating LaVO₄:Tb³⁺, GdVO₄:Tb³⁺, Gd₂O₃:Tb³⁺, GdF₃:Tb³⁺, YVO₄:Tb³⁺ and iron oxide nanoparticles are promising new fluorescence, magnetic and magnetofluorescence imaging agents. Recently, we have reported their in vitro and in vivo level toxicity profiles which show the non-toxic nature of these polymer capsules encapsulating nanoparticles. However, prior to clinical use, it is essential to ensure that these agents are unlikely to activate immune responses. Herein, we investigated the immunocompatibility of polymer capsules with dendritic cells (DCs), macrophages (MOs), and major antigen presenting cell (APC) subsets required for the activation of innate and adaptive immunity. The capsules were efficiently internalized by both DCs and MOs in vitro. Importantly, despite the presence of intracellular capsules, there was no significant impact on the viability of the cells. We studied the impact of different capsules on the cytokine profiles of the DCs and MOs, which is known to be important for the polarization of T-cell immunity. None of the capsules elicited a change in cytokine secretion from the DCs. Furthermore, the capsules did not alter the polarization of either M1 or M2 MO subsets as determined by the balance of IL-12 and IL-10 secretion. These data support the notion that PEGylated polymer capsules loaded with nanoparticles have the potential to remain immunologically silent as they do not activate APCs nor do they hinder the response of DCs or MOs to pathogen activating signals. These systems, therefore, exhibit promising characteristics for bioimaging applications.