Microcapsules with intrinsic barium radiopacity for immunoprotection and X-ray/CT imaging of pancreatic islet cells

X-Ray Visible Protein Scaffolds by Bulk Iodination

Protein-based biomaterial use is expanding within medicine, together with the demand to visualize their placement and behavior in vivo. However, current medical imaging techniques struggle to differentiate between protein-based implants and surrounding tissue. Here a fast, simple, and translational solution for tracking transplanted protein-based scaffolds is presented using X-ray CT-facilitating long-term, non-invasive, and high-resolution imaging. X-ray visible scaffolds are engineered by selectively iodinating tyrosine residues under mild conditions using readily available reagents. To illustrate translatability, a clinically approved hernia repair mesh (based on decellularized porcine dermis) is labeled, preserving morphological and mechanical properties. In a mouse model of mesh implantation, implants retain marked X-ray contrast up to 3 months, together with an unchanged degradation rate and inflammatory response. The technique's compatibility is demonstrated with a range of therapeutically relevant protein formats including bovine, porcine, and jellyfish collagen, as well as silk sutures, enabling a wide range of surgical and regenerative medicine uses. This solution tackles the challenge of visualizing implanted protein-based biomaterials, which conventional imaging methods fail to differentiate from endogenous tissue. This will address previously unanswered questions regarding the accuracy of implantation, degradation rate, migration, and structural integrity, thereby accelerating optimization and safe translation of therapeutic biomaterials.

In Vivo Imaging of Naked and Microencapsulated Islet Cell Transplantation

We describe step-by-step methods to label human pancreatic islet cells and murine insulinoma cells and their subsequent transplantation into type I diabetic mouse models with a focus on in vivo imaging using clinically applicable scanners. We also cover islets that are microencapsulated within alginate hydrogels loaded with imaging agents. By following these methods, it is possible to image cell grafts using T1-weighted and T2/T2*-weighted 1H magnetic resonance imaging (MRI), 19F MRI, computed tomography, ultrasound imaging, and bioluminescence imaging in vivo. Considering a myriad of factors that may affect the outcome of proper in vivo detection, we discuss potential issues that may be encountered during and after the process of labeling. The ultimate goal is to use these in vivo imaging approaches to determine and optimize naked and encapsulated islet cell survival, therapeutic function, and engraftment procedures.

Use of metal-based contrast agents for in vivo MR and CT imaging of phagocytic cells in neurological pathologies

The activation of phagocytic cells is a hallmark of many neurological diseases. Imaging them in their 3-dimensional cerebral environment over time is crucial to better understand their role in disease pathogenesis and to monitor their potential therapeutic effects. Phagocytic cells have the ability to internalize metal-based contrast agents both in vitro and in vivo and can thus be tracked by magnetic resonance imaging (MRI) or computed tomography (CT). In this review article, we summarize the different labelling strategies, contrast agents, and in vivo imaging modalities that can be used to monitor cells with phagocytic activity in the central nervous system using MRI and CT, with a focus on clinical applications. Metal-based nanoparticle contrast agents such as gadolinium, gold and iron are ideal candidates for these applications as they have favourable magnetic and/or radiopaque properties and can be fine-tuned for optimal uptake by phagocytic cells. However, they also come with downsides due to their potential toxicity, especially in the brain where they might accumulate. We therefore conclude our review by discussing the pitfalls, safety and potential for clinical translation of these metal-based neuroimaging techniques. Early results in patients with neuropathologies such as multiple sclerosis, stroke, trauma, cerebral aneurysm and glioblastoma are promising. If the challenges represented by safety issues are overcome, phagocytic cells imaging will be a very valuable tool for studying and understanding the inflammatory response and evaluating treatments that aim at mitigating this response in patients with neurological diseases.

In Vivo Imaging of Pancreatic Islet Grafts in Diabetes Treatment

Transplantation of pancreatic islets has potential to offer life-long blood glucose management in type I diabetes and severe type II diabetes without the need of exogenous insulin administration. However, islet cell therapy suffers from autoimmune and allogeneic rejection as well as non-immune related factors. Non-invasive techniques to monitor and evaluate the fate of cell implants in vivo are essential to understand the underlying causes of graft failure, and hence to improve the precision and efficacy of islet therapy. This review describes how imaging technology has been employed to interrogate the distribution, number or volume, viability, and function of islet implants in vivo. To date, fluorescence imaging, PET, SPECT, BLI, MRI, MPI, and ultrasonography are the many imaging modalities being developed to fulfill this endeavor. We outline here the advantages, limitations, and clinical utility of each particular imaging approach.

Bioprinting of radiopaque constructs for tissue engineering and understanding degradation behavior by use of Micro-CT

Bioprinting has emerged as a potential technique to fabricate tissue engineering constructs and in vitro models directly using living cells as a raw material for fabrication, conforming to the heterogeneity and architectural complexity of the tissues. In several of tissue engineering and in vitro disease modelling or surgical planning applications, it is desirable to have radiopaque constructs for monitoring and evaluation. In the present work, enhanced radiopaque constructs are generated by substituting Calcium ions with Barium ions for crosslinking of alginate hydrogels. The constructs are characterized for their structural integrity and followed by cell culture studies to evaluate their biocompatibility. This was followed by the radiopacity evaluation. The radiological images obtained by micro-CT technique was further applied to investigate the degradation behavior of the scaffolds. In conclusion, it is observed that barium crosslinking can provide a convenient means to obtain radiopaque constructs with potential for multi-faceted applications.

Radio-metal cross-linking of alginate hydrogels for non-invasive in vivo imaging

Alginate hydrogels are cross-linked polymers with high water content, tuneable chemical and material properties, and a range of biomedical applications including drug delivery, tissue engineering, and cell therapy. However, their similarity to soft tissue often renders them undetectable within the body using conventional bio-medical imaging techniques. This leaves much unknown about their behaviour in vivo, posing a challenge to therapy development and validation. To address this, we report a novel, fast, and simple method of incorporating the nuclear imaging radio-metal 111In into the structure of alginate hydrogels by utilising its previously-undescribed capacity as an ionic cross-linking agent. This enabled non-invasive in vivo nuclear imaging of hydrogel delivery and retention across the whole body, over time, and across a range of model therapies including: nasal and oral drug delivery, stem cell transplantation, and cardiac tissue engineering. This information will facilitate the development of novel therapeutic hydrogel formulations, encompassing alginate, across disease categories.

Functionalization of Alginate with Extracellular Matrix Peptides Enhances Viability and Function of Encapsulated Porcine Islets

Translation of transplanted alginate-encapsulated pancreatic islets to treat type 1 diabetes has been hindered by inconsistent long-term efficacy. This loss of graft function can be partially attributed to islet dysfunction associated with the destruction of extracellular matrix (ECM) interactions during the islet isolation process as well as immunosuppression-associated side effects. This study aims at recapitulating islet-ECM interactions by the direct functionalization of alginate with the ECM-derived peptides RGD, LRE, YIGSR, PDGEA, and PDSGR. Peptide functionalization is controlled in a concentration-dependent manner and its presentation is found to be homogeneous across the microcapsule environment. Preweaned porcine islets are encapsulated in peptide-functionalized alginate microcapsules, and those encapsulated in RGD-functionalized alginate displays enhanced viability and glucose-stimulated insulin release. Effects are RGD-specific and not observed with its scrambled control RDG nor with LRE, YIGSR, PDGEA, and PDSGR. This study supports the sustained presentation of ECM-derived peptides in helping to maintain health of encapsulated pancreatic islets and may aid in prolonging longevity of encapsulated islet grafts.

Fluorocapsules allow in vivo monitoring of the mechanical stability of encapsulated islet cell transplants

Clinical trials that have used encapsulated islet cell therapy have been few and overall disappointing. This is due in part to the lack of suitable methods to monitor the integrity vs. rupture of transplanted microcapsules over time. Fluorocapsules were synthesized by embedding emulsions of perfluoro-15-crown-5-ether (PFC), a bioinert compound detectable by 19F MRI, into dual-alginate layer, Ba2+-gelled alginate microcapsules. Fluorocapsules were spherical with an apparent smooth surface and an average diameter of 428 ± 52 μm. After transplantation into mice, the 19F MRI signal of capsules remained stable for up to 90 days, corresponding to the total number of intact fluorocapsules. When single-alginate layer capsules were ruptured with alginate lyase, the 19F MRI signal dissipated within 4 days. For fluoroencapsulated luciferase-expressing mouse βTC6 insulinoma cells implanted into autoimmune NOD/ShiLtJ mice and subjected to alginate-lyase induced capsule rupture in vivo, the 19F MRI signal decreased sharply over time along with a decrease in bioluminescence imaging signal used as a measure of cell viability in vivo. These results indicate that maintenance of capsule integrity is essential for preserving transplanted cell survival, where a decrease in 19F MRI signal may serve as a predictive imaging surrogate biomarker for impending failure of encapsulated islet cell therapy.

Fabrication of inherently radiopaque BaSO4@BaAlg microspheres by a one-step electrospraying method for embolization

Inherently radiopaque BaSO₄@BaAlg microspheres were fabricated by a one-step electrospraying method for embolization and noninvasive examination after operations.

Noninvasive Tracking of Alginate-Microencapsulated Cells

This chapter presents a description of standardized techniques used routinely in our laboratory to encapsulate different cell types using the alginate-PLL-alginate immunoisolation system. Given the importance of noninvasive tracking of encapsulated cell transplants, we present a detailed guidance to achieve maximum efficiency and functionality of the capsule preparations for optimal tracking posttransplantation. The provided protocols cover tracking of encapsulated cells using magnetic resonance (MR), X-ray, computed tomography (CT), and ultrasound (US) imaging. Practical suggestions to optimize each method with specific references to recommended suppliers are included.

Use of Nanoparticle Contrast Agents for Cell Tracking with Computed Tomography

Efforts to develop novel cell-based therapies originated with the first bone marrow transplant on a leukemia patient in 1956. Preclinical and clinical examples of cell-based treatment strategies have shown promising results across many disciplines in medicine, with recent advances in immune cell therapies for cancer producing remarkable response rates, even in patients with multiple treatment failures. However, cell-based therapies suffer from inconsistent outcomes, motivating the search for tools that allow monitoring of cell delivery and behavior in vivo. Noninvasive cell imaging techniques, also known as cell tracking, have been developed to address this issue. These tools can allow real-time, quantitative, and long-term monitoring of transplanted cells in the recipient, providing insight on cell migration, distribution, viability, differentiation, and fate, all of which play crucial roles in treatment efficacy. Understanding these parameters allows the optimization of cell choice, delivery route, and dosage for therapy and advances cell-based therapy for specific clinical uses. To date, most cell tracking work has centered on imaging modalities such as MRI, radionuclide imaging, and optical imaging. However, X-ray computed tomography (CT) is an emerging method for cell tracking that has several strengths such as high spatial and temporal resolution, and excellent quantitative capabilities. The advantages of CT for cell tracking are enhanced by its wide availability and cost effectiveness, allowing CT to become one of the most popular clinical imaging modalities and a key asset in disease diagnosis. In this review, we will discuss recent advances in cell tracking methods using X-ray CT in various applications, in addition to predictions on how the field will progress.

Recent progress in the use and tracking of transplanted islets as a personalized treatment for type 1 diabetes

INTRODUCTION

Type 1 diabetes mellitus (T1DM) is an autoimmune disease in which the pancreas produces insufficient amounts of insulin. T1DM patients require exogenous sources of insulin to maintain euglycemia. Transplantation of naked or microencapsulated pancreatic islets represents an alternative paradigm to obtain an autonomous regulation of blood glucose levels in a controlled and personalized fashion. However, once transplanted, the fate of these personalized cellular therapeutics is largely unknown, justifying the development of non-invasive tracking techniques.

AREAS COVERED

In vivo imaging of naked pancreatic islet transplantation, monitoring of microencapsulated islet transplantation, visualizing pancreatic inflammation, imaging of molecular-genetic therapeutics, imaging of beta cell function.

EXPERT COMMENTARY

There are still several hurdles to overcome before (microencapsulated) islet cell transplantation will become a mainstay therapy. Non-invasive imaging methods that can track graft volume, graft rejection, graft function (insulin secretion) microcapsule engraftment, microcapsule rupture, and pancreatic inflammation are currently being developed to design the best experimental transplantation paradigms.

Magnetoencapsulated human islets xenotransplanted into swine: a comparison of different transplantation sites

BACKGROUND

The fate of magnetically labeled, barium-gelled alginate/protamine sulfate/alginate microcapsules (APSA magnetocapsules) following xenotransplantation was assessed by magnetic resonance imaging (MRI) and histopathology.

METHODS

Magnetocapsules with and without human islets were transplanted into five different clinically accessible sites: portal vein, subcutaneous tissue, skeletal muscle, the liver and the kidney subcapsular space. The surface of APSA magnetocapsules was modified using clinical-grade heparin to mitigate an instant blood-mediated inflammatory reaction.

RESULTS

The accuracy of site-specific delivery was confirmed using a clinical 1.5T MRI setup, where the magnetocapsules appeared as distinct hypointense entities after transplantation. As proven by the Lee-White blood coagulation test, heparin-treated APSA magnetocapsules did not induce blood clotting for more than 48 h in vitro. Heparinized magnetocapsules induced innate and adaptive immune responses in vivo regardless of the transplantation sites.

CONCLUSION

We have demonstrated the feasibility of using a clinical 1.5T MRI to non-invasively detect the accuracy of APSA magnetocapsule injection into various clinically accessible transplantation sites. Among the investigated transplantation sites, the liver and kidney subcapsular space were found to be the least immuno-responsive toward xenografted magneto-encapsulated human islets.

Dendritic Polymers for Theranostics

Dendritic polymers are highly branched polymers with controllable structures, which possess a large population of terminal functional groups, low solution or melt viscosity, and good solubility. Their size, degree of branching and functionality can be adjusted and controlled through the synthetic procedures. These tunable structures correspond to application-related properties, such as biodegradability, biocompatibility, stimuli-responsiveness and self-assembly ability, which are the key points for theranostic applications, including chemotherapeutic theranostics, biotherapeutic theranostics, phototherapeutic theranostics, radiotherapeutic theranostics and combined therapeutic theranostics. Up to now, significant progress has been made for the dendritic polymers in solving some of the fundamental and technical questions toward their theranostic applications. In this review, we briefly summarize how to control the structures of dendritic polymers, the theranostics-related properties derived from their structures and their theranostics-related applications.

Labeling monocytes with gold nanoparticles to track their recruitment in atherosclerosis with computed tomography

Monocytes are actively recruited from the circulation into developing atherosclerotic plaques. In the plaque, monocytes differentiate into macrophages and eventually form foam cells. Continued accumulation of foam cells can lead to plaque rupture and subsequent myocardial infarction. X-ray computed tomography (CT) is the best modality to image the coronary arteries non-invasively, therefore we have sought to track the accumulation of monocytes into atherosclerotic plaques using CT. Gold nanoparticles were synthesized and stabilized with a variety of ligands. Select formulations were incubated with an immortalized monocyte cell line in vitro and evaluated for cytotoxicity, effects on cytokine release, and cell uptake. These data identified a lead formulation, 11-MUDA capped gold nanoparticles, to test for labeling primary monocytes. The formulation did not the affect the viability or cytokine release of primary monocytes and was highly taken up by these cells. Gold labeled primary monocytes were injected into apolipoprotein E deficient mice kept on Western diet for 10 weeks. Imaging was done with a microCT scanner. A significant increase in attenuation was measured in the aorta of mice receiving the gold labeled cells as compared to control animals. Following the experiment, the biodistribution of gold was evaluated in major organs. Additionally, plaques were sectioned and examined with electron microscopy. The results showed that gold nanoparticles were present inside monocytes located within plaques. This study demonstrates the feasibility of using gold nanoparticles as effective cell labeling contrast agents for non-invasive imaging of monocyte accumulation within plaques with CT.

Swelling, mechanical strength, and release properties of probucol microcapsules with and without a bile acid, and their potential oral delivery in diabetes

We have demonstrated a permeation-enhancing effect of deoxycholic acid (DCA), the bile acid, in diabetic rats. In this study, we designed DCA-based microcapsules for the oral delivery of the antilipidemic drug probucol (PB), which has potential antidiabetic effects. We aimed to further characterize these microcapsules and examine their pH-dependent release properties, as well as the effects of DCA on their stability and mechanical strength at various pH and temperature values. Using the polymer sodium alginate (SA), we prepared PB-SA (control) and PB-DCA-SA (test) microcapsules. The microcapsules were examined for drug content, size, surface composition, release, Micro-CT cross-sectional imaging, stability, Zeta potential, mechanical strength, and swelling characteristics at different pH and temperature values. The microencapsulation efficiency and production yield were also examined. The addition of DCA resulted in microcapsules with a greater density and with reduced swelling at a pH of 7.8 and at temperatures of 25°C and 37°C (p < 0.01). The size, surface composition, production yield, and microencapsulation efficiency of the microcapsules remained similar after DCA addition. PB-SA microcapsules produced multiphasic PB release, while PB-DCA-SA microcapsules produced monophasic PB release, suggesting more controlled PB release in the presence of DCA. The PB-DCA-SA microcapsules showed good stability and a pH-sensitive uniphasic release pattern, which may suggest potential applications in the oral delivery of PB in diabetes.

Re-engineering islet cell transplantation

We are living exciting times in the field of beta cell replacement therapies for the treatment of diabetes. While steady progress has been recorded thus far in clinical islet transplantation, novel approaches are needed to make cell-based therapies more reproducible and leading to long-lasting success. The multiple facets of diabetes impose the need for a transdisciplinary approach to attain this goal, by targeting immunity, promoting engraftment and sustained functional potency. We discuss herein the emerging technologies applied to this rapidly evolving field.

Self-assembled gold coating enhances X-ray imaging of alginate microcapsules

Therapeutic biomolecules produced from cells encapsulated within alginate microcapsules (MCs) offer a potential treatment for a number of diseases. However the fate of such MCs once implanted into the body is difficult to establish. Labelling the MCs with medical imaging contrast agents may aid their detection and give researchers the ability to track them over time thus aiding the development of such cellular therapies. Here we report the preparation of MCs with a self-assembled gold nanoparticle (AuNPs) coating which results in distinctive contrast and enables them to be readily identified using a conventional small animal X-ray micro-CT scanner. Cationic Reversible Addition-Fragmentation chain Transfer (RAFT) homopolymer modified AuNPs (PAuNPs) were coated onto the surface of negatively charged alginate MCs resulting in hybrids which possessed low cytotoxicity and high mechanical stability in vitro. As a result of their high localized Au concentration, the hybrid MCs exhibited a distinctive bright circular ring even with a low X-ray dose and rapid scanning in post-mortem imaging experiments facilitating their positive identification and potentially enabling them to be used for in vivo tracking experiments over multiple time-points.

Functionalized synchrotron in-line phase-contrast computed tomography: a novel approach for simultaneous quantification of structural alterations and localization of barium-labelled alveolar macrophages within mouse lung samples

Functionalized computed tomography (CT) in combination with labelled cells is virtually non-existent due to the limited sensitivity of X-ray-absorption-based imaging, but would be highly desirable to realise cell tracking studies in entire organisms. In this study we applied in-line free propagation X-ray phase-contrast CT (XPCT) in an allergic asthma mouse model to assess structural changes as well as the biodistribution of barium-labelled macrophages in lung tissue. Alveolar macrophages that were barium-sulfate-loaded and fluorescent-labelled were instilled intratracheally into asthmatic and control mice. Mice were sacrificed after 24 h, lungs were kept in situ, inflated with air and scanned utilizing XPCT at the SYRMEP beamline (Elettra Synchrotron Light Source, Italy). Single-distance phase retrieval was used to generate data sets with ten times greater contrast-to-noise ratio than absorption-based CT (in our setup), thus allowing to depict and quantify structural hallmarks of asthmatic lungs such as reduced air volume, obstruction of airways and increased soft-tissue content. Furthermore, we found a higher concentration as well as a specific accumulation of the barium-labelled macrophages in asthmatic lung tissue. It is believe that XPCT will be beneficial in preclinical asthma research for both the assessment of therapeutic response as well as the analysis of the role of the recruitment of macrophages to inflammatory sites.

Magnetization transfer contrast MRI for non-invasive assessment of innate and adaptive immune responses against alginate-encapsulated cells

By means of physical isolation of cells inside semi-permeable hydrogels, encapsulation has been widely used to immunoprotect transplanted cells. While spherical alginate microcapsules are now being used clinically, there still is little known about the patient's immune system response unless biopsies are obtained. We investigated the use of Magnetization Transfer (MT) imaging to non-invasively detect host immune responses against alginate capsules containing xenografted human hepatocytes in four groups of animals, including transplanted empty capsules (-Cells/-IS), capsules with live cells with (+LiveCells/+IS) and without immunosuppression (+LiveCells/-IS), and capsules with apoptotic cells in non-immunosuppressed animals (+DeadCells/-IS). The highest MT ratio (MTR) was found in +LiveCells/-IS, which increased from day 0 by 38% and 53% on days 7 and 14 after transplantation respectively, and corresponded to a distinctive increase in cell infiltration on histology. Furthermore, we show that macromolecular ratio maps based on MT data are more sensitive to cell infiltration and fibrosis than conventional MTR maps. Such maps showed a significant difference between +LiveCells/-IS (0.18 ± 0.02) and +DeadCells/-IS (0.13 ± 0.02) on day 7 (P < 0.01) existed, which was not observed on MTR imaging. We conclude that MT imaging, which is clinically available, can be applied for non-invasive monitoring of the occurrence of a host immune response against encapsulated cells.

Application of cell encapsulation for controlled delivery of biological therapeutics

Cell microencapsulation technology is likely to have an increasingly important role in new approaches rather than the classical and pioneering organ replacement. Apart from becoming a tool for protein and morphogen release and long-term drug delivery, it is becoming a new three-dimensional platform for stem cell research. Recent progress in the field has resulted in biodegradable scaffolds that are able to retain and release the cell content in different anatomical locations. Additional advances include the use biomimetic scaffolds that provide greater control over material-cell interactions and the development of more precise encapsulated cell-tracking systems. This review summarises the state of the art of cell microencapsulation and discusses the main directions and challenges of this field towards the controlled delivery of biological therapeutics.

Advances in biocompatibility and physico-chemical characterization of microspheres for cell encapsulation

Cell encapsulation has already shown its high potential and holds the promise for future cell therapies to enter the clinics as a large scale treatment option for various types of diseases. The advancement in cell biology towards this goal has to be complemented with functional biomaterials suitable for cell encapsulation. This cannot be achieved without understanding the close correlation between cell performance and properties of microspheres. The ongoing challenges in the field of cell encapsulation require a critical view on techniques and approaches currently utilized to characterize microspheres. This review deals with both principal subjects of microspheres characterization in the cell encapsulation field: physico-chemical characterization and biocompatibility. The up-to-day knowledge is summarized and discussed with the focus to identify missing knowledge and uncertainties, and to propose the mandatory next steps in characterization of microspheres for cell encapsulation. The primary conclusion of this review is that further success in development of microspheres for cell therapies cannot be accomplished without careful selection of characterization techniques, which are employed in conjunction with biological tests.

Improving stability and biocompatibility of alginate/chitosan microcapsule by fabricating bi-functional membrane

Cell encapsulation technology holds promise for the cell-based therapy. But poor mechanical strength and biocompatibility of microcapsule membrane are still obstacles for the clinical applications. A novel strategy is presented to prepare AC₁ C₂ A microcapsules with bi-functional membrane (that is, both desirable biocompatibility and membrane stability) by sequentially complexing chitosans with higher deacetylation degree (C₁) and lower deacetylation degree (C₂) on alginate (A) gel beads. Both in vitro and in vivo evaluation of AC₁C₂ A microcapsules demonstrate higher membrane stability and less cell adhesion, because the introduction of C₂ increases membrane strength and decreases surface roughness. Moreover, diffusion test of AC₁C₂ A microcapsules displays no inward permeation of IgG protein suggesting good immunoisolation function. The results demonstrate that AC₁C₂ A microcapsules with bi-functional membrane could be a promising candidate for microencapsulated cell implantation with cost effective usage of naturally biocompatible polysaccharides.

Combinatorial biomatrix/cell-based therapies for restoration of host tissue architecture and function

This Progress Report reviews recent advances in the utility of extracellular matrix (ECM)-mimic biomaterials in presenting and delivering therapeutic cells to promote tissue healing. This overview gives a brief introduction of different cell types being used in regenerative medicine and tissue engineering while addressing critical issues that must be overcome before cell-based approaches can be routinely employed in the clinic. A selection of five commonly used cell-associated, biomaterial platforms (collagen, hyaluronic acid, fibrin, alginate, and poly(ethylene glycol)) are reviewed for treatment of a number of acute injury or diseases with emphasis on animal models and clinical trials. This article concludes with current challenges and future perspectives regarding foreign body host response to biomaterials and immunological reactions to allogeneic or xenogeneic cells, vascularization and angiogenesis, matching mechanical strength and anisotropy of native tissues, as well as other non-technical issues regarding the clinical translation of biomatrix/cell-based therapies.

Microencapsulated cell tracking

Microencapsulation of therapeutic cells has been widely pursued to achieve cellular immunoprotection following transplantation. Initial clinical studies have shown the potential of microencapsulation using semi-permeable alginate layers, but much needs to be learned about the optimal delivery route, in vivo pattern of engraftment, and microcapsule stability over time. In parallel with noninvasive imaging techniques for 'naked' (i.e. unencapsulated) cell tracking, microcapsules have now been endowed with contrast agents that can be visualized by (1) H MRI, (19) F MRI, X-ray/computed tomography and ultrasound imaging. By placing the contrast agent formulation in the extracellular space of the hydrogel, large amounts of contrast agents can be incorporated with negligible toxicity. This has led to a new generation of imaging biomaterials that can render cells visible with multiple imaging modalities.

Personalized nanomedicine advancements for stem cell tracking

Recent technological developments in biomedicine have facilitated the generation of data on the anatomical, physiological and molecular level for individual patients and thus introduces opportunity for therapy to be personalized in an unprecedented fashion. Generation of patient-specific stem cells exemplifies the efforts toward this new approach. Cell-based therapy is a highly promising treatment paradigm; however, due to the lack of consistent and unbiased data about the fate of stem cells in vivo, interpretation of therapeutic effects remains challenging hampering the progress in this field. The advent of nanotechnology with a wide palette of inorganic and organic nanostructures has expanded the arsenal of methods for tracking transplanted stem cells. The diversity of nanomaterials has revolutionized personalized nanomedicine and enables individualized tailoring of stem cell labeling materials for the specific needs of each patient. The successful implementation of stem cell tracking will likely be a significant driving force that will contribute to the further development of nanotheranostics. The purpose of this review is to emphasize the role of cell tracking using currently available nanoparticles.