Recent Developments in Layer-by-Layer Assembly for Drug Delivery and Tissue Engineering Applications

Surfaces with biological functionalities are of great interest for biomaterials, tissue engineering, biophysics, and for controlling biological processes. The layer-by-layer (LbL) assembly is a highly versatile methodology introduced 30 years ago, which consists of assembling complementary polyelectrolytes or biomolecules in a stepwise manner to form thin self-assembled films. In view of its simplicity, compatibility with biological molecules, and adaptability to any kind of supporting material carrier, this technology has undergone major developments over the past decades. Specific applications have emerged in different biomedical fields owing to the possibility to load or immobilize biomolecules with preserved bioactivity, to use an extremely broad range of biomolecules and supporting carriers, and to modify the film's mechanical properties via crosslinking. In this review, the focus is on the recent developments regarding LbL films formed as 2D or 3D objects for applications in drug delivery and tissue engineering. Possible applications in the fields of vaccinology, 3D biomimetic tissue models, as well as bone and cardiovascular tissue engineering are highlighted. In addition, the most recent technological developments in the field of film construction, such as high-content liquid handling or machine learning, which are expected to open new perspectives in the future developments of LbL, are presented.

A journey from the endothelium to the tumor tissue: distinct behavior between PEO-PCL micelles and polymersomes nanocarriers

Polymeric nanocarriers must overcome several biological barriers to reach the vicinity of solid tumors and deliver their encapsulated drug. This study assessed the in vitro and in vivo passage through the blood vessel wall to tumors of two well-characterized polymeric nanocarriers: poly(ethyleneglycol-b-ε-caprolactone) micelles and polymersomes charged with a fluorescent membrane dye (DiO: 3,3'-dioctadecyloxacarbo-cyanine perchlorate). The internalization and translocation from endothelial (human primary endothelial cells HUVEC) to cancer cells (human tumor cell line HCT-116) was studied in conventional 2D monolayers, 3D tumor spheroids, or in an endothelium model based on transwell assay. Micelles induced a faster DiO internalization compared to polymersomes but the latter crossed the endothelial monolayer more easily. Both translocation rates were enhanced by the addition of a pro-inflammatory factor or in the presence of tumor cells. These results were confirmed by early in vivo experiments. Overall, this study pointed out the room for the improvement of polymeric nanocarriers design to avoid drug losses when crossing the blood vessel walls.

[Development of innovative three-dimensional tissues and their application for pharmaceutical assays]

Three-dimensional (3D) cardiac myoblast tissues derived from iPS cells were constructed by cell coating technology with nanometer-sized extracellular matrix. Vascularized 3D-cardiac tissues were also fabricated by employing cardiac endothelial cells. These 3D-cardiac tissues are expected to apply to pharmaceutical assays.

Stromal barriers to nanomedicine penetration in the pancreatic tumor microenvironment

Pancreatic cancer is known for its dismal prognosis despite efforts to improve therapeutic outcome. Recently, cancer nanomedicine, application of nanotechnology to cancer diagnosis and treatment, has gained interest for treatment of pancreatic cancer. The enhanced permeability and retention (EPR) effect that promotes selective accumulation of nanometer‐sized molecules within tumors is the theoretical rationale of treatment. However, it is clear that EPR may be insufficient in pancreatic cancer as a result of stromal barriers within the tumor microenvironment (TME). These limit intratumoral accumulation of macromolecules. The TME and stromal barriers inside it consist of various stromal cell types which interact both with each other and with tumor cells. We are only beginning to understand the complexities of the stromal barriers within the TME and its functional consequences for nanomedicine. Understanding the complex crosstalk between barrier stromal cells is challenging because of the difficulty of modeling pancreatic cancer TME. Here we provide an overview of stromal barriers within the TME. We also describe the preclinical models, both in vivo and in vitro, developed to study them. We furthermore discuss the critical gaps in our understanding, and how we might formulate a better strategy for using nanomedicine against pancreatic cancer.

Application of 3D cultured multicellular spheroid tumor models in tumor-targeted drug delivery system research

Tumor-targeted drug delivery systems are promising for their advantages in enhanced tumor accumulation and reduced toxicity towards normal organs. However, few nanomedicines have been successfully translated into clinical application. One reason is the gap between current pre-clinical and clinical studies. The prevalent in vitro models utilized in pre-clinical phase are mainly based on the two-dimensional (2D) cell culture and are limited by the difficulty of simulating three-dimensional physiological conditions in human body, such as three-dimensional (3D) architecture, cell heterogeneity, nutrient gradients and the interaction between cells and the extracellular matrix (ECM). In addition, traditional animal models have drawbacks such as high-cost, long periods and physiological differences between animal and human. On the other hand, the employment of 3D tumor cell culture models, especially multicellular tumor spheroids (MCTS), has increased significantly in recent decades. These models have been shown to simulate 3D structures of tumors in vitro with relatively low cost and simple protocols. Currently, MCTS have also been widely exploited in drug delivery system research for comprehensive study of drug efficacy, drug penetration, receptor targeting, and cell recruitment abilities. This review summarizes the delivery barriers for nano-carriers presented in tumor microenvironment, the characteristics and formation methods for applicable multicellular tumor spheroid culture models and recent studies related to their applications in tumor-targeted drug delivery system research.

Desmoplastic Reaction in 3D-Pancreatic Cancer Tissues Suppresses Molecular Permeability

The survival rate of pancreatic ductal adenocarcinoma is still the lowest among all types of cancers, primarily as a consequence of an important desmoplastic reaction. Although the presence of thick stromal tissues in pancreatic tumors has been reported, in vivo animal studies do not enable a clear understanding of the crosstalk between cancer cells and fibroblasts. Accordingly, this paper reports the design and characterization of an in vitro pancreatic cancer-stromal 3D-tissue model, which enhances the understanding of the interactions between cancer cells and fibroblasts and their influence on the secretion of extracellular matrix (ECM). 3D-tissue models comprising fibroblasts and pancreatic cancer cells (MiaPaCa-2 cell line) or colon cancer cells (HT29 cell line, used as a control) show decreased molecular permeability with increased cancer cell ratios. The 3D-MiaPaCa-2 tissues display an increase in the secretion of collagen as a function of the cancer cell ratio, whereas 3D-HT29 tissues do not show a significant difference. Notably, the secretion of ECM proteins from single fibroblasts in 3D-tissue models containing 90% MiaPaCa-2 cells is ten times higher than that under 10% cancer cell conditions. In vitro pancreatic cancer 3D-tissues will be a valuable tool to obtain information on the interactions between cancer and stromal cells.