Tunable PEG Hydrogels for Discerning Differential Tumor Cell Response to Biomechanical Cues.

Increased extracellular matrix (ECM) density in the tumor microenvironment has been shown to influence aspects of tumor progression such as proliferation and invasion. Increased matrix density means cells experience not only increased mechanical properties, but also a higher density of bioactive sites. Traditional in vitro ECM models like Matrigel and collagen do not allow these properties to be investigated independently. In this work, a poly(ethylene glycol)-based scaffold is used which modifies with integrin-binding sites for cell attachment and matrix metalloproteinase 2 and 9 sensitive sites for enzyme-mediated degradation. The polymer backbone density and binding site concentration are independently tuned and the effect each of these properties and their interaction have on the proliferation, invasion, and focal complex formation of two different tumor cell lines is evaluated. It is seen that the cell line of epithelial origin (Hs 578T, triple negative breast cancer) proliferates more, invades less, and forms more mature focal complexes in response to an increase in matrix adhesion sites. Conversely, the cell line of mesenchymal origin (HT1080, fibrosarcoma) proliferates more in 2D culture but less in 3D culture, invades less, and forms more mature focal complexes in response to an increase in matrix stiffness.

Reductionist Three-Dimensional Tumor Microenvironment Models in Synthetic Hydrogels.

The tumor microenvironment (TME) plays a determining role in everything from disease progression to drug resistance. As such, in vitro models which can recapitulate the cell-cell and cell-matrix interactions that occur in situ are key to the investigation of tumor behavior and selecting effective therapeutic drugs. While naturally derived matrices can retain the dimensionality of the native TME, they lack tunability and batch-to-batch consistency. As such, many synthetic polymer systems have been employed to create physiologically relevant TME cultures. In this review, we discussed the common semi-synthetic and synthetic polymers used as hydrogel matrices for tumor models. We reviewed studies in synthetic hydrogels which investigated tumor cell interactions with vasculature and immune cells. Finally, we reviewed the utility of these models as chemotherapeutic drug-screening platforms, as well as the future directions of the field.

Chemically Orthogonal Protein Ligation Domains for Independent Control of Hydrogel Modification with Adhesive Ligands and Growth Factors.

The twin, chemically orthogonal protein ligation domains, SpyCatcher and SnoopCatcher, were used to link two engineered proteins into poly(ethylene glycol) (PEG) hydrogels in order to control both endothelial cell adhesion and material-mediated pro-mitotic stimulation. SpyCatcher was appended with an N-terminal adhesion ligand RGDS to form RGDS-SC, and SnoopCatcher was appended with the vascular endothelial growth factor (VEGF)-mimetic peptide QK to form QK-SnpC. QK-SnpC formed a spontaneous covalent bond with SnoopTag peptide with 40% reaction efficiency, both in solution, in a PEG gel containing SnoopTag peptide, and in a PEG gel with both SnoopTag and SpyTag sites. QK-SnpC added to cell culture media enhanced endothelial cell proliferation compared to a negative control, and was statistically indistinguishable from the positive control of 130 pM VEGF165. Endothelial cells seeded onto PEG gels presenting both RGDS-SC and QK-SnpC showed ∼50% of cells actively proliferating (defined as Ki67+), compared to ∼31% of cells seeded on gels presenting RGDS-SC alone. These results show that complementary nondiffusing biochemical signals can be linked into PEG-DA hydrogels simultaneously using 'Catcher-based ligation strategies, thereby inducing more nuanced cell-material interactions.

Gold nanoshell-localized photothermal ablation of prostate tumors in a clinical pilot device study.

Biocompatible gold nanoparticles designed to absorb light at wavelengths of high tissue transparency have been of particular interest for biomedical applications. The ability of such nanoparticles to convert absorbed near-infrared light to heat and induce highly localized hyperthermia has been shown to be highly effective for photothermal cancer therapy, resulting in cell death and tumor remission in a multitude of preclinical animal models. Here we report the initial results of a clinical trial in which laser-excited gold-silica nanoshells (GSNs) were used in combination with magnetic resonance-ultrasound fusion imaging to focally ablate low-intermediate-grade tumors within the prostate. The overall goal is to provide highly localized regional control of prostate cancer that also results in greatly reduced patient morbidity and improved functional outcomes. This pilot device study reports feasibility and safety data from 16 cases of patients diagnosed with low- or intermediate-risk localized prostate cancer. After GSN infusion and high-precision laser ablation, patients underwent multiparametric MRI of the prostate at 48 to 72 h, followed by postprocedure mpMRI/ultrasound targeted fusion biopsies at 3 and 12 mo, as well as a standard 12-core systematic biopsy at 12 mo. GSN-mediated focal laser ablation was successfully achieved in 94% (15/16) of patients, with no significant difference in International Prostate Symptom Score or Sexual Health Inventory for Men observed after treatment. This treatment protocol appears to be feasible and safe in men with low- or intermediate-risk localized prostate cancer without serious complications or deleterious changes in genitourinary function.

Harnessing Macrophages for Vascularization in Tissue Engineering.

In this review, we explore the roles of macrophages both in vessel development and in vascularization of tissue engineered constructs. Upon the implantation of tissue engineered constructs into the body, macrophages respond, invade and orchestrate the host's immune response. By altering their phenotype, macrophages can adopt a variety of roles. They can promote inflammation at the site of the implanted construct; they can also promote tissue repair. Macrophages support tissue repair by promoting angiogenesis through the secretion of pro-angiogenic cytokines and by behaving as support cells for nascent vasculature. Thus, the ability to manipulate the macrophage phenotype may yield macrophages capable of supporting vessel development. Moreover, macrophages are an easily isolated autologous cell source. For the generation of vascularized constructs outside of the body, these isolated macrophages can also be skewed to adopt a pro-angiogenic phenotype and enhance blood vessel development in the presence of endothelial cells. To assess the influence of macrophages on vessel development, both in vivo and in vitro models have been developed. Additionally, several groups have demonstrated the pro-angiogenic roles of macrophages in vascularization of tissue engineered constructs through the manipulation of macrophage phenotypes. This review comments on the roles of macrophages in promoting vascularization within these contexts.

A comparative analysis of EGFR-targeting antibodies for gold nanoparticle CT imaging of lung cancer.

Computed tomography (CT) is the standard imaging test used for the screening and assessment of suspected lung cancer, but distinguishing malignant from benign nodules by CT is an ongoing challenge. Consequently, a large number of avoidable invasive procedures are performed on patients with benign nodules in order to exclude malignancy. Improving cancer discrimination by non-invasive imaging could reduce the need for invasive diagnostics. In this work we focus on developing a gold nanoparticle contrast agent that targets the epidermal growth factor receptor (EGFR), which is expressed on the cell surface of most lung adenocarcinomas. Three different contrast agents were compared for their tumor targeting effectiveness: non-targeted nanoparticles, nanoparticles conjugated with full-sized anti-EGFR antibodies (cetuximab), and nanoparticles conjugated with a single-domain llama-derived anti-EGFR antibody, which is smaller than the cetuximab, but has a lower binding affinity. Nanoparticle targeting effectiveness was evaluated in vitro by EGFR-binding assays and in cell culture with A431 cells, which highly express EGFR. In vivo CT imaging performance was evaluated in both C57BL/6 mice and in nude mice with A431 subcutaneous tumors. The cetuximab nanoparticles had a significantly shorter blood residence time than either the non-targeted or the single-domain antibody nanoparticles. All of the nanoparticle contrast agents demonstrated tumor accumulation; however, the cetuximab-targeted group had significantly higher tumor gold accumulation than the other two groups, which were statistically indistinguishable from one another. In this study we found that the relative binding affinity of the targeting ligands had more of an effect on tumor accumulation than the circulation half life of the nanoparticles. This study provides useful insight into targeted nanoparticle design and demonstrates that nanoparticle contrast agents can be used to detect tumor receptor overexpression. Combining receptor status data with traditional imaging characteristics has the potential for better differentiation of malignant lung tumors from benign lesions.

Hyaluronic acid based low viscosity hydrogel as a novel carrier for Convection Enhanced Delivery of CAR T cells.

Convection Enhanced Delivery (CED) infuses therapeutic agents directly into the intracranial area continuously under pressure. The convection improves the distribution of therapeutics such as those aimed at brain tumors. Although CED successfully delivers small therapeutic agents, this technique fails to effectively deliver cells largely due to cell sedimentation during delivery. To overcome this limitation, we have developed a low viscosity hydrogel (LVHydrogel), which is capable of retaining cells in suspension. In this study, we evaluated whether LVHydrogel can effectively act as a carrier for the CED of tumor-specific chimeric antigen receptor (CAR) T cells. CAR T cells were resuspended in saline or LVHydrogel carriers, loaded into syringes, and passed through the CED system for 5 h. CAR T cells submitted to CED were counted and the efficiency of delivery was determined. In addition to delivery, the ability of CAR T cells to migrate and induce cytotoxicity was evaluated. Our studies demonstrate that LVHydrogel is a superior carrier for CED in comparison to saline. The efficiency of cell delivery in saline carrier was only ∼3-5% of the total cells whereas delivery by the LVHydrogel carrier was much higher, reaching ∼45-75%. Migration and Cytotoxicity was similar in both carriers in non-infused samples but we found superior cytotoxicity in LVHydrogel group post-infusion. We demonstrate that LVHydrogel, a biodegradable biomaterial which does not cause acute toxicity on preclinical animal models, prevents cellular sedimentation during CED and presents itself as a superior carrier to the current carrier, saline, for the CED of CAR T cells.

Dual-Energy CT Imaging of Tumor Liposome Delivery After Gold Nanoparticle-Augmented Radiation Therapy.

Gold nanoparticles (AuNPs) are emerging as promising agents for both cancer therapy and computed tomography (CT) imaging. AuNPs absorb x-rays and subsequently release low-energy, short-range photoelectrons during external beam radiation therapy (RT), increasing the local radiation dose. When AuNPs are near tumor vasculature, the additional radiation dose can lead to increased vascular permeability. This work focuses on understanding how tumor vascular permeability is influenced by AuNP-augmented RT, and how this effect can be used to improve the delivery of nanoparticle chemotherapeutics. Methods: Dual-energy CT was used to quantify the accumulation of both liposomal iodine and AuNPs in tumors following AuNP-augmented RT in a mouse model of primary soft tissue sarcoma. Mice were injected with non-targeted AuNPs, RGD-functionalized AuNPs (vascular targeting), or no AuNPs, after which they were treated with varying doses of RT. The mice were injected with either liposomal iodine (for the imaging study) or liposomal doxorubicin (for the treatment study) 24 hours after RT. Increased tumor liposome accumulation was assessed by dual-energy CT (iodine) or by tracking tumor treatment response (doxorubicin). Results: A significant increase in vascular permeability was observed for all groups after 20 Gy RT, for the targeted and non-targeted AuNP groups after 10 Gy RT, and for the vascular-targeted AuNP group after 5 Gy RT. Combining targeted AuNPs with 5 Gy RT and liposomal doxorubicin led to a significant tumor growth delay (tumor doubling time ~ 8 days) compared to AuNP-augmented RT or chemotherapy alone (tumor doubling time ~3-4 days). Conclusions: The addition of vascular-targeted AuNPs significantly improved the treatment effect of liposomal doxorubicin after RT, consistent with the increased liposome accumulation observed in tumors in the imaging study. Using this approach with a liposomal drug delivery system can increase specific tumor delivery of chemotherapeutics, which has the potential to significantly improve tumor response and reduce the side effects of both RT and chemotherapy.

Lung Adenocarcinoma Cell Responses in a 3D in Vitro Tumor Angiogenesis Model Correlate with Metastatic Capacity.

Many tools from the field of tissue engineering can be used to develop novel model systems to study cancer. We have utilized biomimetic synthetic hydrogels, based on poly(ethylene glycol) (PEG) modified with cell adhesive peptides (RGDS) and peptides sensitive to degradation by matrix metalloproteinases 2 and 9 (GGGPQGIWGQGK), as highly controlled 3D substrates for cell culture. We have previously shown that this hydrogel can support growth of tumor cells and also growth and assembly of microvascular networks. Based on this technology, a 3D in vitro tumor angiogenesis model was developed using a dual layer PEG-based hydrogel comprised of vascular cells (endothelial cells, pericytes) and lung adenocarcinoma cells in separate layers to support recapitulation of the vessel recruitment process as it occurs in vivo. This model was previously used to study highly metastatic murine 344SQ cells and in this paper was used to investigate 2 additional types of lung adenocarcinoma cells: nonmetastatic murine 393P cells and somewhat metastatic human A549 cells. All three cell types readily formed spheroid structures in the 3D hydrogels. When cultured in the dual layer format, where tumor cell spheroids were adjacent to a hydrogel layer with microvascular tubule networks, all three tumor cell types recruited vascular cells into the cancer cell layer. Interactions between vessels invading the cancer layer and the cancer cell structures was nearly twice as high for the highly metastatic 344SQ cells as for the other two cell types. Secretion of angiogenic growth factors by the tumor cells was evaluated. 344SQ cells produced the greatest amount of VEGF and FGFb, which probably accounts for the greater degree of vessel recruitment observed. Upon interaction with vessel structures, the 344SQ spheroids underwent a dramatic change in morphology, increasing in size and adopting highly irregular shapes, suggestive of invasive phenotype. This behavior was observed to a much lesser degree for A549 cells and 393P cells.

Stiffness of Protease Sensitive and Cell Adhesive PEG Hydrogels Promotes Neovascularization In Vivo.

Materials that support the assembly of new vasculature are critical for regenerative medicine. Controlling the scaffold's mechanical properties may help to optimize neovascularization within implanted biomaterials. However, reducing the stiffness of synthetic hydrogels usually requires decreasing polymer densities or increasing chain lengths, both of which accelerate degradation. We synthesized enzymatically-degradable poly(ethylene glycol) hydrogels with compressive moduli from 2 to 18 kPa at constant polymer density, chain length, and proteolytic degradability by inserting an allyloxycarbonyl functionality into the polymer backbone. This group competes with acrylates during photopolymerization to alter the crosslink network structure and reduce the hydrogel's stiffness. Hydrogels that incorporated (soft) or lacked (stiff) this group were implanted subcutaneously in rats to investigate the role of stiffness on host tissue interactions. Changes in tissue integration were quantified after 4 weeks via the hydrogel area replaced by native tissue (tissue area fraction), yielding 0.136 for softer vs. 0.062 for stiffer hydrogels. Including soluble FGF-2 and PDGF-BB improved these responses to 0.164 and 0.089, respectively. Softer gels exhibited greater vascularization with 8.6 microvessels mm-2 compared to stiffer gels at 2.4 microvessels mm-2. Growth factors improved this to 11.2 and 4.9 microvessels mm-2, respectively. Softer hydrogels tended to display more sustained responses, promoting neovascularization and tissue integration in synthetic scaffolds.

A 3D Poly(ethylene glycol)-based Tumor Angiogenesis Model to Study the Influence of Vascular Cells on Lung Tumor Cell Behavior.

Tumor angiogenesis is critical to tumor growth and metastasis, yet much is unknown about the role vascular cells play in the tumor microenvironment. In vitro models that mimic in vivo tumor neovascularization facilitate exploration of this role. Here we investigated lung adenocarcinoma cancer cells (344SQ) and endothelial and pericyte vascular cells encapsulated in cell-adhesive, proteolytically-degradable poly(ethylene) glycol-based hydrogels. 344SQ in hydrogels formed spheroids and secreted proangiogenic growth factors that significantly increased with exposure to transforming growth factor beta 1 (TGF-ß1), a potent tumor progression-promoting factor. Vascular cells in hydrogels formed tubule networks with localized activated TGF-ß1. To study cancer cell-vascular cell interactions, we engineered a 2-layer hydrogel with 344SQ and vascular cell layers. Large, invasive 344SQ clusters (area > 5,000 µm(2), circularity < 0.25) developed at the interface between the layers, and were not evident further from the interface or in control hydrogels without vascular cells. A modified model with spatially restricted 344SQ and vascular cell layers confirmed that observed cluster morphological changes required close proximity to vascular cells. Additionally, TGF-ß1 inhibition blocked endothelial cell-driven 344SQ migration. Our findings suggest vascular cells contribute to tumor progression and establish this culture system as a platform for studying tumor vascularization.

Poly(ethylene glycol) Hydrogel Scaffolds Containing Cell-Adhesive and Protease-Sensitive Peptides Support Microvessel Formation by Endothelial Progenitor Cells.

The development of stable, functional microvessels remains an important obstacle to overcome for tissue engineered organs and treatment of ischemia. Endothelial progenitor cells (EPCs) are a promising cell source for vascular tissue engineering as they are readily obtainable and carry the potential to differentiate towards all endothelial phenotypes. The aim of this study was to investigate the ability of human umbilical cord blood-derived EPCs to form vessel-like structures within a tissue engineering scaffold material, a cell-adhesive and proteolytically degradable poly(ethylene glycol) (PEG) hydrogel. EPCs in co-culture with angiogenic mural cells were encapsulated in hydrogel scaffolds by mixing with polymeric precursors and using a mild photocrosslinking process to form hydrogels with homogeneously dispersed cells. EPCs formed 3D microvessels networks that were stable for at least 30 days in culture, without the need for supplemental angiogenic growth factors. These 3D EPC microvessels displayed aspects of physiological microvasculature with lumen formation, expression of endothelial cell proteins (connexin 32, VE-cadherin, eNOS), basement membrane formation with collagen IV and laminin, perivascular investment of PDGFR-ß and α-SMA positive cells, and EPC quiescence (<1% proliferating cells) by 2 weeks of co-culture. Our findings demonstrate the development of a novel, reductionist system that is well-defined and reproducible for studying progenitor cell-driven microvessel formation.

Cancer-Associated Fibroblasts Induce a Collagen Cross-link Switch in Tumor Stroma.

UNLABELLED: Intratumoral collagen cross-links heighten stromal stiffness and stimulate tumor cell invasion, but it is unclear how collagen cross-linking is regulated in epithelial tumors. To address this question, we used Kras(LA1) mice, which develop lung adenocarcinomas from somatic activation of a Kras(G12D) allele. The lung tumors in Kras(LA1) mice were highly fibrotic and contained cancer-associated fibroblasts (CAF) that produced collagen and generated stiffness in collagen gels. In xenograft tumors generated by injection of wild-type mice with lung adenocarcinoma cells alone or in combination with CAFs, the total concentration of collagen cross-links was the same in tumors generated with or without CAFs, but coinjected tumors had higher hydroxylysine aldehyde-derived collagen cross-links (HLCC) and lower lysine-aldehyde-derived collagen cross-links (LCCs). Therefore, we postulated that an LCC-to-HLCC switch induced by CAFs promotes the migratory and invasive properties of lung adenocarcinoma cells. To test this hypothesis, we created coculture models in which CAFs are positioned interstitially or peripherally in tumor cell aggregates, mimicking distinct spatial orientations of CAFs in human lung cancer. In both contexts, CAFs enhanced the invasive properties of tumor cells in three-dimensional (3D) collagen gels. Tumor cell aggregates that attached to CAF networks on a Matrigel surface dissociated and migrated on the networks. Lysyl hydroxylase 2 (PLOD2/LH2), which drives HLCC formation, was expressed in CAFs, and LH2 depletion abrogated the ability of CAFs to promote tumor cell invasion and migration. IMPLICATIONS: CAFs induce a collagen cross-link switch in tumor stroma to influence the invasive properties of tumor cells.

Studying the influence of angiogenesis in in vitro cancer model systems.

Tumor angiogenesis is a hallmark of cancer that has been identified as a critical component of cancer progression, facilitating rapid tumor growth and metastasis. Anti-angiogenic therapies have exhibited only modest clinical success, highlighting a need for better models that can be used to gain a more thorough understanding of tumor angiogenesis and screen potential therapeutics more accurately. This review explores how recent progress in in vitro cancer and vascular models individually can be applied to the development of in vitro tumor angiogenesis models. Current in vitro tumor angiogenesis models are also discussed, with a focus on aspects of the process that have been successfully recapitulated and opportunities for applying new technologies to expand model complexity to better represent the tumor microenvironment. Continued advances in vascularized tumor models will provide tools to identify novel therapeutic targets and validate their therapeutic benefit.

Biomimetic Surface Patterning Promotes Mesenchymal Stem Cell Differentiation.

Both chemical and mechanical stimuli can dramatically influence cell behavior. By optimizing the signals cells experience, it may be possible to control the behavior of therapeutic cell populations. In this work, biomimetic geometries of adhesive ligands, which recapitulate the morphology of mature cells, are used to direct human mesenchymal stem cell (HMSC) differentiation toward a desired lineage. Specifically, adipocytes cultured in 2D are imaged and used to develop biomimetic virtual masks used in laser scanning lithography to form patterned fibronectin surfaces. The impact of adipocyte-derived pattern geometry on HMSC differentiation is compared to the behavior of HMSCs cultured on square and circle geometries, as well as adipocyte-derived patterns modified to include high stress regions. HMSCs on adipocyte mimetic geometries demonstrate greater adipogenesis than HMSCs on the other patterns. Greater than 45% of all HMSCs cultured on adipocyte mimetic patterns underwent adipogenesis as compared to approximately 19% of cells on modified adipocyte patterns with higher stress regions. These results are attributed to variations in cytoskeletal tension experienced by cells on the different protein micropatterns. The effects of geometry on adipogenesis are mitigated by the incorporation of a cytoskeletal protein inhibitor; exposure to this inhibitor leads to increased adipogenesis on all patterns examined.

Bioactive poly(ethylene glycol) hydrogels to recapitulate the HSC niche and facilitate HSC expansion in culture.

Hematopoietic stem cells (HSCs) have been used therapeutically for decades, yet their widespread clinical use is hampered by the inability to expand HSCs successfully in vitro. In culture, HSCs rapidly differentiate and lose their ability to self-renew. We hypothesize that by mimicking aspects of the bone marrow microenvironment in vitro we can better control the expansion and differentiation of these cells. In this work, derivatives of poly(ethylene glycol) diacrylate hydrogels were used as a culture substrate for hematopoietic stem and progenitor cell (HSPC) populations. Key HSC cytokines, stem cell factor (SCF) and interferon-γ (IFNγ), as well as the cell adhesion ligands RGDS and connecting segment 1 were covalently immobilized onto the surface of the hydrogels. With the use of SCF and IFNγ, we observed significant expansion of HSPCs, ∼97 and ∼104 fold respectively, while maintaining c-kit(+) lin(-) and c-kit(+) Sca1(+) lin(-) (KSL) populations and the ability to form multilineage colonies after 14 days. HSPCs were also encapsulated within degradable poly(ethylene glycol) hydrogels for three-dimensional culture. After expansion in hydrogels, ∼60% of cells were c-kit(+), demonstrating no loss in the proportion of these cells over the 14 day culture period, and ∼50% of colonies formed were multilineage, indicating that the cells retained their differentiation potential. The ability to tailor and use this system to support HSC growth could have implications on the future use of HSCs and other blood cell types in a clinical setting.

Hydrogel-Coated Near Infrared Absorbing Nanoshells as Light-Responsive Drug Delivery Vehicles.

Nanoparticle drug delivery carriers that can modulate drug release based on an exogenous signal, such as light, are of great interest, especially for improving cancer therapy. A light-activated delivery vehicle was fabricated by synthesizing a thin, thermally responsive poly(N-isopropylacrylamide-co-acrylamide) hydrogel coating directly onto the surfaces of individual near-infrared (NIR) absorbing gold-silica nanoshells. This hydrogel was designed to be in a swollen state under physiological conditions and expel large amounts of water, along with any entrapped drug, at elevated temperatures. The required temperature change can be achieved via NIR absorption by the nanoshell, allowing the hydrogel phase change to be triggered by light, which was observed by monitoring changes in particle sizes as water was expelled from the hydrogel network. The phase change was reversible and repeatable. As a model drug, the chemotherapeutic doxorubicin was loaded into this delivery vehicle, and rapid release of doxorubicin occurred upon NIR exposure. Further, colon carcinoma cells exposed to the irradiated platform displayed nearly 3 times as much doxorubicin uptake as cells exposed to nonirradiated particles or free drug, which in turn resulted in a higher loss of cell viability. We hypothesize these effects are because the NIR-mediated heating results in a transient increase in cell membrane permeability, thus aiding in cellular uptake of the drug.

3-Dimensional spatially organized PEG-based hydrogels for an aortic valve co-culture model.

Physiologically relevant in vitro models are needed to study disease progression and to develop and screen potential therapeutic interventions for disease. Heart valve disease, in particular, has no early intervention or non-invasive treatment because there is a lack of understanding the cellular mechanisms which lead to disease. Here, we establish a novel, customizable synthetic hydrogel platform that can be used to study cell-cell interactions and the factors which contribute to valve disease. Spatially localized cell adhesive ligands bound in the scaffold promote cell growth and organization of valve interstitial cells and valve endothelial cells in 3D co-culture. Both cell types maintained phenotypes, homeostatic functions, and produced zonally localized extracellular matrix. This model extends the capabilities of in vitro research by providing a platform to perform direct contact co-culture with cells in their physiologically relevant spatial arrangement.

Improved Angiogenesis in Response to Localized Delivery of Macrophage-Recruiting Molecules.

Successful engineering of complex organs requires improved methods to promote rapid and stable vascularization of artificial tissue scaffolds. Toward this goal, tissue engineering strategies utilize the release of pro-angiogenic growth factors, alone or in combination, from biomaterials to induce angiogenesis. In this study we have used intravital microscopy to define key, dynamic cellular changes induced by the release of pro-angiogenic factors from polyethylene glycol diacrylate hydrogels transplanted in vivo. Our data show robust macrophage recruitment when the potent and synergistic angiogenic factors, PDGFBB and FGF2 were used as compared with VEGF alone and intravital imaging suggested roles for macrophages in endothelial tip cell migration and anastomosis, as well as pericyte-like behavior. Further data from in vivo experiments show that delivery of CSF1 with VEGF can dramatically improve the poor angiogenic response seen with VEGF alone. These studies show that incorporating macrophage-recruiting factors into the design of pro-angiogenic biomaterial scaffolds is a key strategy likely to be necessary for stable vascularization and survival of implanted artificial tissues.

Optically modulated cancer therapeutic delivery: past, present and future.

Chemotherapeutic regimens are often restricted by dose-limiting toxicities that arise from drug exposure to off-site tissues. Nanoparticle drug carriers that specifically deliver therapeutics to the site of malignant tissue are being actively researched today. One strategy is to utilize materials that are light-responsive, such that the carrier can be triggered to release its drug payload at the distinct time and location of light exposure. This review discusses recent advances in the development of such light-responsive drug carriers. With continued optimization and in vivo validation, these approaches may offer novel treatment options for cancer management.