Development of a Smart Portable Hypoxic Chamber with Accurate Sensing, Control and Visualization of In Vitro Cell Culture for Replication of Cancer Microenvironment.

Clinical resistance towards treatment is a major concern in cancer therapy. This is due to in vitro studies lacking essential microenvironmental aspects. Tumor-hypoxia is an important pathophysiological phenomenon in numerous malignant tumors. Various studies have shown the importance of a hypoxic microenvironment (HME) in cancer drug resistance and its effects on cellular signaling and metabolism pathways. Most drugs fail in transition from a laboratory to clinical trials because of the variability in the testing microenvironment conditions. It is, thus, very crucial that research work needs to replicate these conditions in vitro to test the drugs and/or drug carriers for cancer therapy. Previous works have used a portable hypoxia chamber to reduce the cell microenvironment to hypoxic conditions. These techniques lack reliability and consistency due to a lack of control and visualization. In this research, we developed a smart portable hypoxia chamber that could accurately control the oxygen inside the portable chamber and have a global visualization. The proposed hypoxia chamber provided ease of use with the ranges of 1% to 20% oxygen with increments of 0.5%, as well as reproducibility and accuracy. The chamber displayed great precision on reaching the set oxygen limit and a high stability in maintaining that set level of oxygen compared to the uncontrolled setup for extended durations (24 h). For instance, at a 2% oxygen level, our automated system maintained this level over 1400 min, whereas the oxygen level fluctuated up to 4.5% in the conventional hypoxic chamber. We have also demonstrated the pitfalls of uncontrolled and non-visualized hypoxia chamber setup and the dire need for our system. The hypoxia-induced factor (HIF-1α) expression in cancer cell lines was tested and compared between the conventional hypoxia setup and our automated hypoxia chamber. We observed that there was a twofold increase in HIF-1α expression in the automated controlled chamber compared to the conventional device. The device also provided real-time sensing, visualization and control of the chamber conditions, which could aid in complex in vitro studies.

Lung Cancer Targeted Chemoradiotherapy via Dual-Stimuli Responsive Biodegradable Core-Shell Nanoparticles.

Lung cancer is one of the major causes of cancer-related deaths worldwide, primarily because of the limitations of conventional clinical therapies such as chemotherapy and radiation therapy. Side effects associated with these treatments have made it essential for new modalities, such as tumor targeting nanoparticles that can provide cancer specific therapies. In this research, we have developed novel dual-stimuli nanoparticles (E-DSNPs), comprised of two parts; (1) Core: responsive to glutathione as stimuli and encapsulating Cisplatin (a chemo-drug), and (2) Shell: responsive to irradiation as stimuli and containing NU7441 (a radiation sensitizer). The targeting moieties on these nanoparticles are Ephrin transmembrane receptors A2 (EphA2) that are highly expressed on the surfaces of lung cancer cells. These nanoparticles were then evaluated for their enhanced targeting and therapeutic efficiency against lung cancer cell lines. E-DSNPs displayed very high uptake by lung cancer cells compared to healthy lung epithelial cells. These nanoparticles also demonstrated a triggered release of both drugs against respective stimuli and a subsequent reduction in in vitro cancer cell survival fraction compared to free drugs of equivalent concentration (survival fraction of about 0.019 and 0.19, respectively). Thus, these nanoparticles could potentially pave the path to targeted cancer therapy, while overcoming the side effects of conventional clinical therapies.

Enhanced Endothelial Cell Delivery for Repairing Injured Endothelium via Pretargeting Approach and Bioorthogonal Chemistry.

Arterial wall injury often leads to endothelium cell activation, endothelial detachment, and atherosclerosis plaque formation. While abundant research efforts have been placed on treating the end stages of the disease, no cure has been developed to repair injured and denude endothelium often occurred at an early stage of atherosclerosis. Here, a pretargeting cell delivery strategy using combined injured endothelial targeting nanoparticles and bioorthogonal click chemistry approach was developed to deliver endothelial cells to replenish the injured endothelium via a two-step process. First, nanoparticles bearing glycoprotein 1b α (Gp1bα) proteins and tetrazine (Tz) were fabricated to provide a homogeneous nanoparticle coating on an injured arterial wall via the interactions between Gp1bα and von Willebrand factor (vWF), a ligand that is present on denuded endothelium. Second, transplanted endothelium cells bearing transcyclooctene (TCO) would be quickly immobilized on the surfaces of nanoparticles via TCO:Tz reactions. In vitro binding studies under both static and flow conditions confirmed that our novel Tz-labeled Gp1bα-conjugated poly(lactic-co-glycolic acid) (PLGA) nanoparticles can successfully pretargeted toward the injured site and support rapid adhesion of endothelial cells from the circulation. Ex vivo results also confirm that such an approach is highly efficient in mediating the local delivery of endothelial cells at the sites of arterial injury. The results support that this pretargeting cell delivery approach may be used for repairing injured endothelium in situ at its early stage.

Engineering Delivery of Nonbiologics Using Poly(lactic-co-glycolic acid) Nanoparticles for Repair of Disrupted Brain Endothelium.

Traumatic brain injury (TBI) is known to alter the structure and function of the blood-brain barrier (BBB). Blunt force or explosive blast impacting the brain can cause neurological sequelae through the mechanisms that remain yet to be fully elucidated. For example, shockwaves propagating through the brain have been shown to create a mechanical trauma that may disrupt the BBB. Indeed, using tissue engineering approaches, the shockwave-induced mechanical injury has been shown to modulate the organization and permeability of the endothelium tight junctions. Because an injury to the brain endothelium typically induces a high expression of E-selectin, we postulated that upregulation of this protein after an injury can be exploited for diagnosis and potential therapy through targeted nanodelivery to the injured brain endothelium. To test this hypothesis, we engineered poly(lactic-co-glycolic acid) (PLGA) nanoparticles to encapsulate therapeutic nonbiologics and decorated them with ligands to specifically target the E-selectin. A high level of the conjugated nanoparticles was found inside the injured cells. Repair of the injury site was then quantitatively measured and analyzed. To summarize, exploiting the tunable properties of PLGA, a targeted drug delivery strategy has been developed and validated, which combines the specificity of ligand/receptor interaction with therapeutic reagents. Such a strategy could be used to provide a potential theragnostic approach for the treatment of modulated brain endothelium associated with TBI.

Targeting of PP2Cδ By a Small Molecule C23 Inhibits High Glucose-Induced Breast Cancer Progression In Vivo.

Aims: Epidemiologic evidence indicates that diabetes may increase risk of breast cancer (BC) and mortality in patients with cancer. The pathophysiological relationships between diabetes and cancer are not fully understood, and personalized treatments for diabetes-associated BC are urgently needed. Results: We observed that high glucose (HG), via activation of nuclear phosphatase PP2Cδ, suppresses p53 function, and consequently promotes BC cell proliferation, migration, and invasion. PP2Cδ expression is higher in tumor tissues from BC patients with hyperglycemia than those with normoglycemia. The mechanisms underlying HG stimulation of PP2Cδ involve classical/novel protein kinase-C (PKC) activation and GSK3ß phosphorylation. Reactive oxygen species (ROS)/NF-κB pathway also mediates HG induction of PP2Cδ. Furthermore, we identified a 1,5-diheteroarylpenta-1,4-dien-3-one (Compound 23, or C23) as a novel potent PP2Cδ inhibitor with a striking cytotoxicity on MCF-7 cells through cell-based screening assay for growth inhibition and activity of a group of curcumin mimics. Beside directly inhibiting PP2Cδ activity, C23 blocks HG induction of PP2Cδ expression via heat shock protein 27 (HSP27) induction and subsequent ablation of ROS/NF-κB activation. C23 can thus significantly block HG-triggered inhibition of p53 activity, leading to the inhibition of cancer cell proliferation, migration, and invasion. In addition, hyperglycemia promotes BC development in diabetic nude mice, and C23 inhibits the xenografted BC tumor growth. Conclusions and Innovation: Our findings elucidate mechanisms that may have contributed to diabetes-associated BC progression, and provide the first evidence to support the possible alternative therapeutic approach to BC patients with diabetes. Antioxid. Redox Signal. 30, 1983-1998.

Study on the Antimicrobial Properties of Citrate-Based Biodegradable Polymers.

Citrate-based polymers possess unique advantages for various biomedical applications since citric acid is a natural metabolism product, which is biocompatible and antimicrobial. In polymer synthesis, citric acid also provides multiple functional groups to control the crosslinking of polymers and active binding sites for further conjugation of biomolecules. Our group recently developed a number of citrate-based polymers for various biomedical applications by taking advantage of their controllable chemical, mechanical, and biological characteristics. In this study, various citric acid derived biodegradable polymers were synthesized and investigated for their physicochemical and antimicrobial properties. Results indicate that citric acid derived polymers reduced bacterial proliferation to different degrees based on their chemical composition. Among the studied polymers, poly(octamethylene citrate) showed ~70-80% suppression to microbe proliferation, owing to its relatively higher ratio of citric acid contents. Crosslinked urethane-doped polyester elastomers and biodegradable photoluminescent polymers also exhibited significant bacteria reduction of ~20 and ~50% for Staphylococcus aureus and Escherichia coli, respectively. Thus, the intrinsic antibacterial properties in citrate-based polymers enable them to inhibit bacteria growth without incorporation of antibiotics, silver nanoparticles, and other traditional bacteria-killing agents suggesting that the citrate-based polymers are unique beneficial materials for wound dressing, tissue engineering, and other potential medical applications where antimicrobial property is desired.

Biophysical assessment of single cell cytotoxicity: diesel exhaust particle-treated human aortic endothelial cells.

Exposure to diesel exhaust particles (DEPs), a major source of traffic-related air pollution, has become a serious health concern due to its adverse influences on human health including cardiovascular and respiratory disorders. To elucidate the relationship between biophysical properties (cell topography, cytoskeleton organizations, and cell mechanics) and functions of endothelial cells exposed to DEPs, atomic force microscope (AFM) was applied to analyze the toxic effects of DEPs on a model cell line from human aortic endothelial cells (HAECs). Fluorescence microscopy and flow cytometry were also applied to further explore DEP-induced cytotoxicity in HAECs. Results revealed that DEPs could negatively impair cell viability and alter membrane nanostructures and cytoskeleton components in a dosage- and a time-dependent manner; and analyses suggested that DEPs-induced hyperpolarization in HAECs appeared in a time-dependent manner, implying DEP treatment would lead to vasodilation, which could be supported by down-regulation of cell biophysical properties (e.g., cell elasticity). These findings are consistent with the conclusion that DEP exposure triggers important biochemical and biophysical changes that would negatively impact the pathological development of cardiovascular diseases. For example, DEP intervention would be one cause of vasodilation, which will expand understanding of biophysical aspects associated with DEP cytotoxicity in HAECs.

Enhanced endothelialization on surface modified poly(L-lactic acid) substrates.

Improved biodegradable vascular grafts and stents are in demand, particularly for pediatric patients. Poly(L-lactic acid) (PLLA) is an FDA-approved biodegradable polymer of potential use for such applications. However, tissue culture studies have shown that endothelial cell (EC) attachment and growth occurs relatively slowly on PLLA surfaces. This slow growth has been attributed to the fact that PLLA represents a hydrophobic substrate, relatively devoid of active functional groups. As a result, the slow EC recovery on the luminal side of PLLA stents provides an increased risk of induced thrombosis. In the present study, surface modification of PLLA substrates has been examined as a potential route to enhance EC growth. For this purpose, PLLA surfaces were modified via pulsed plasma deposition of thin films of poly(vinylacetic acid). The -COOH surface groups, introduced by the plasma deposition, were employed to conjugate fibronectin (FN), followed by attachment of vascular endothelial growth factor to FN. Pig Aorta ECs (PAE) and kinase-insert domain-containing receptor (KDR)-transfected PAE showed increased cell adhesion and proliferation, as well as substantially improved cell retention under fluidic shear stress on surface-modified PLLA compared with untreated PLLA. Although KDR-transfected PAE exhibited better cell proliferation than PAE, normal EC functions, including EC morphology, nitric oxide production, and KDR expression, were observed when cells were grown on surface-modified PLLA. The results obtained clearly indicate that this combined surface modification technique using poly(vinylacetic acid) deposition, FN conjugation, and vascular endothelial growth factor surface delivery can enhance endothelialization on PLLA, particularly when employed in conjunction with the growth of KDR-transfected ECs.

Crosslinked urethane doped polyester biphasic scaffolds: Potential for in vivo vascular tissue engineering.

In vivo tissue engineering uses the body as a bioreactor for tissue regeneration, thus placing stringent requirements on tissue scaffolds, which should be mechanically robust for immediate implantation without a long in vitro cell culture time. In addition to mechanical strength, vascular grafts fabricated for in vivo tissue engineering approach must have matching mechanical properties to the target tissues to avoid compliance mismatch, which is one of the reasons for graft failure. We recently synthesized a new generation of strong and elastic biodegradable crosslinked urethane-doped polyesters (CUPE) to address the challenge of developing soft, elastic yet strong biodegradable polymers. This study evaluated the tensile strength, burst pressure, and suture retention of CUPE biphasic scaffolds to determine if the scaffolds met the requirements for immediate implantation in an in vivo tissue engineering approach. In addition, we also examined the hemocompatibility and inflammatory potential of CUPE to demonstrate its potential in serving as a blood-contacting vascular graft material. Tensile strength of CUPE biphasic scaffolds (5.02 ± 0.70 MPa) was greater than native vessels (1.43 ± 0.60 MPa). CUPE scaffolds exhibited tunable burst pressure ranging from 1500 mmHg to 2600 mmHg, and adequate suture retention values (2.45 ± 0.23 N). CUPE showed comparable leukocyte activation and whole blood clotting kinetics to poly(L-lactic acid) PLLA. However, CUPE incited a lesser release of inflammatory cytokines and was found to be non hemolytic. Combined with the mechanical properties and previously demonstrated anti-thrombogenic nature, CUPE may serve as a viable graft material for in vivo blood vessel tissue engineering.

In vitro evaluation of novel polymer-coated magnetic nanoparticles for controlled drug delivery.

Previously uncharacterized poly(N-isopropylacrylamide-acrylamide-allylamine)-coated magnetic nanoparticles (MNPs) were synthesized using silane-coated MNPs as a template for radical polymerization of N-isopropylacrylamide, acrylamide, and allylamine. Properties of these nanoparticles such as size, biocompatibility, drug loading efficiency, and drug release kinetics were evaluated in vitro for targeted and controlled drug delivery. Spherical core-shell nanoparticles with a diameter of 100 nm showed significantly lower systemic toxicity than did bare MNPs, as well as doxorubicin encapsulation efficiency of 72%, and significantly higher doxorubicin release at 41°C compared with 37°C, demonstrating their temperature sensitivity. Released drugs were also active in destroying prostate cancer cells (JHU31). Furthermore, the nanoparticle uptake by JHU31 cells was dependent on dose and incubation time, reaching saturation at 500 µg/mL and 4 hours, respectively. In addition, magnetic resonance imaging capabilities of the particles were observed using agarose platforms containing cells incubated with nanoparticles. Future work includes investigation of targeting capability and effectiveness of these nanoparticles in vivo using animal models. FROM THE CLINICAL EDITOR: In this paper, previously uncharacterized magnetic nanoparticles were synthesized using silane-coated MNPs as a template for radical polymerization of N-isopropylacrylamide, acrylamide, and allylamine. Various properties of these nanoparticles were evaluated in vitro for targeted drug delivery.

Development of multiple-layer polymeric particles for targeted and controlled drug delivery.

The purpose of this work was to develop multilayered particles consisting of a magnetic core and two encompassing shells made up of poly(N-isopropylacrylamide) (PNIPAAm) and poly(D,L-lactide-co-glycolide) (PLGA) for targeted and controlled drug delivery. Transmission electron microscopy confirmed that multilayered particles were obtained with PNIPAAm magnetic nanoparticles embedded within the PLGA shell. Factorial analysis studies also showed that the particle size was inversely proportional to the surfactant concentration and sonication power and directly proportional to the PLGA concentration. Drug-release results demonstrated that these multilayer particles produced an initial burst release and a subsequent sustained release of both bovine serum albumin (BSA) and curcumin loaded into the core and shell of the particle, respectively. BSA release was also affected by changes in temperature. In conclusion, our results indicate that the multilayered magnetic particles could be synthesized and used for targeted and controlled delivery of multiple drugs with different release mechanisms. FROM THE CLINICAL EDITOR: Authors demonstrate the synthesis of multilayered particles consisting of a magnetic core and two encompassing shells made up of poly (N-isopropylacrylamide) (PNIPAAm) and poly(D, L-lactide-co-glycolide) (PLGA) for targeted and controlled drug delivery. The presented results indicate successful synthesis and application for targeted and controlled delivery of multiple drugs with different release mechanisms.

Effects of cyclic strain and growth factors on vascular smooth muscle cell responses.

Under physiological and pathological conditions, vascular smooth muscle cells (SMC) are exposed to different biochemical factors and biomechanical forces. Previous studies pertaining to SMC responses have not investigated the effects of both factors on SMCs. Thus, in our research we investigated the combined effects of growth factors like Bfgf (basic fibroblast growth factor), TGF-beta (transforming growth factor beta) and PDGF (platelet-derived growth factor) along with physiological cyclic strain on SMC responses. Physiological cyclic strain (10% strain) significantly reduced SMC proliferation compared to static controls while addition of growth factors bFGF, TGF-beta or PDGF-AB had a positive influence on SMC growth compared to strain alone. Microarray analysis of SMCs exposed to these growth factors and cyclic strain showed that several bioactive genes (vascular endothelial growth factor, epidermal growth factor receptor, etc.) were altered upon exposure. Further work involving biochemical and pathological cyclic strain stimulation will help us better understand the role of cyclic strain and growth factors in vascular functions and development of vascular disorders.

Studies of the cellular uptake of hydrogel nanospheres and microspheres by phagocytes, vascular endothelial cells, and smooth muscle cells.

Intensive research efforts have been placed on the development of nanospheres for targeted drug delivery for treating a variety of diseases, including coronary restenosis, cancer, and inflammatory reactions. Although most of these drug-bearing spheres are delivered via intravenous administration, little is known about the effect of sphere physical characteristics on the responses of vascular and blood cells. To find the answer, this work was aimed to investigate the cellular uptake of nanosized (100 nm) and microsized hydrogel spheres (1 microm) made of poly(N-isopropylacrylamide) by vascular cells and phagocytes under various flow conditions in vitro. We found that the cellular uptake of nanospheres depended on incubation times and sphere concentrations as well as on the introduced shear stress levels of the medium. Measurements of the intracellular-released fluorescence and confocal fluorescence microscopy revealed that nanospheres were internalized by endothelial cells and smooth muscle cells more than microspheres, whereas microspheres were rapidly taken up by phagocytes, especially at high concentration. Our results strongly suggest that hydrogel nanospheres are more effective as an intravascular delivery system compared to microspheres in the terms of vascular cellular uptake and biocompatibility.

A review of materials, fabrication methods, and strategies used to enhance bone regeneration in engineered bone tissues.

Over the last decade, bone engineered tissues have been developed as alternatives to autografts and allografts to repair and reconstruct bone defects. This article provides a review of the current technologies in bone tissue engineering. Factors used for fabrication of three-dimensional bone scaffolds such as materials, cells, and biomolecular signals, as well as required properties for ideal bone scaffolds, are reviewed. In addition, current fabrication techniques including rapid prototyping are elaborated upon. Finally, this review article further discusses some effective strategies to enhance cell ingrowth in bone engineered tissues; for example, nanotopography, biomimetic materials, embedded growth factors, mineralization, and bioreactors. In doing so, it suggests that there is a possibility to develop bone substitutes that can repair bone defects and promote new bone formation for orthopedic applications.

Cellular and molecular responses of smooth muscle cells to surface nanotopography.

Recent studies have demonstrated that surface nano-topography affect cell responses and activities. However, the molecular mechanism of the nano-structures on cellular behavior is yet to be determined. To bridge this gap, the present study was aimed to investigate the cellular and molecular responses of smooth muscle cells (SMCs) to surface nano-topography in vitro using nano-porous alumina membranes with different sizes (200 nm- and 20 nm-pores). Cellular responses such as cell adhesion, morphology, and proliferation were assessed using scanning electron microscopy (SEM), hematoxylin and eosin (HE) staining, and cell counting. The molecular cell responses were also investigated using cDNA microarrays. Results from these studies showed an unchanged response in cell adhesion, an alteration in cell morphology, and an increase in cell proliferation for cells grown on 200 nm-pore surfaces than on 20 nm-pore surfaces. In addition, exposure of SMCs to larger nano-pores induced the expression of various genes involved in cell cycle, DNA replication, cell proliferation, and signaling transduction pathways. These findings demonstrated that cellular responses of SMCs are dependent on the underlying nano-topography, and thereby suggesting nano-dimensional surface is one of the most important considerations to design of the next generation of medical devices and tissue engineering scaffolds.

Curcumin impregnation improves the mechanical properties and reduces the inflammatory response associated with poly(L-lactic acid) fiber.

We investigated poly(L-lactic acid) (PLLA) fibers and coils, simulating stents and the influence of impregnation with curcumin, a non-steroidal anti-inflammatory drug, intended to reduce the pro-inflammatory property of these implants. Fibers obtained by melt extrusion of 137 kDa PLLA resin containing 10% curcumin (C-PLLA) exhibited a stable curcumin release rate for periods up to 36 days. Curcumin increased the fiber tensile strength at break and decreased embrittlement vs. controls in 36 day 37 degrees C saline incubation. A mouse peritoneal phagocyte model was employed to test the anti-inflammatory properties of C-PLLA fibers in vitro. Myeloperoxidase and non-specific esterase activity assays were performed for adherent cells (polymorphonuclear leukocytes (PMN) and macrophages (MPhi), respectively). PMN and MPhi adhesion to C-PLLA fibers were significantly reduced compared to control PLLA fibers (2.6 +/- 0.91) x 10(5) vs. (5.6 +/- 0.67) x 10(5) PMN/cm2 and (3.9 +/- 0.23) x 10(3) vs. (9.1 +/- 0.7) x 10(3) MPhi/cm2 (P < 0.05), respectively. In addition, superoxide release in the phagocyte pool contacting C-PLLA fibers was 97% less than that for PLLA controls. A fresh human whole blood recirculation system was employed to analyze cell adhesion under flow conditions, employing scanning electron microscopy (SEM). Reduced adhesion of cells on C-PLLA fiber coils vs. controls was observed. These in vitro studies demonstrate that bulk curcumin impregnation can reduce the inflammatory response to bioresorbable PLLA fibers, whilst improving mechanical properties, thereby suggesting curcumin loading may benefit PLLA-based implants.

Molecular responses of vascular smooth muscle cells to paclitaxel-eluting bioresorbable stent materials.

We studied the influence of paclitaxel, eluted from poly(L-lactic acid) (PLLA), on cultured vascular smooth muscle cell (VSMC) proliferation as a model of bioresorbable stent-induced restenosis. We blended paclitaxel in cast PLLA films (P-PLLA), demonstrating controlled release of the drug, then studied VSMC adhesion, proliferation, and gene expression profiles. No difference in cell adhesion was found between P-PLLA and PLLA controls (105 +/- 12% of PLLA controls). However, P-PLLA significantly reduced VSMC proliferation (40 +/- 15% of PLLA controls, p < 0.05). Using cDNA microarray technology, we identified major effects of P-PLLA, including: upregulation of genes related to apoptosis, anti-proliferation and antioxidation; and suppression of cell cycle regulators and cell survival markers. The expression patterns indicate that P-PLLA regulates gene expression and cell functions via new pathways, including receptor tyrosine kinase (RTKs), mitogen-activated protein kinase (MAPKs), and protein kinase (PKs, e.g., PKA) pathways, in addition to the stabilization of polymerized-microtubules.

Bioresorbable polymeric stents: current status and future promise.

Metal stents and, more recently, polymer-coated metal stents are used to stabilize dissections, eliminate vessel recoil, and guide remodeling after balloon angioplasty and other treatments for arterial disease. Bioresorbable polymeric stents are being developed to improve the biocompatibility and the drug reservoir capacity of metal stents, and to offer a transient alternative to the permanent metallic stent implant. Following a brief review of metal stent technology, the emerging class of expandable, bioresorbable polymeric stents is described, with emphasis on developments in the authors' laboratory.

Photopolymerizable hydrogels for tissue engineering applications.

Photopolymerized hydrogels are being investigated for a number of tissue engineering applications because of the ability to form these materials in situ in a minimally invasive manner such as by injection. In addition, hydrogels, three-dimensional networks of hydrophilic polymers that are able to swell large amounts of water, can be made to resemble the physical characteristics of soft tissues. Hydrogel materials also generally exhibit high permeability and good biocompatibility making, these materials attractive for use in cell encapsulation and tissue engineering applications. A number of hydrogel materials can be formed via photopolymerization processes mild enough to be carried out in the presence of living cells. This allows one to homogeneously seed cells throughout the scaffold material and to form hydrogels in situ. This review presents advantages of photopolymerization of hydrogels and describes the photoinitiators and materials in current use. Applications of photopolymerized hydrogels in tissue engineering that have been investigated are summarized.