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Interactions between epithelial and immune cells with the gut microbiota have wide-ranging effects on many aspects of human health. Therefore, there is value in developing in vitro models capable of performing highly controlled studies of such interactions. However, several critical factors that enable long term homeostasis between bacterial and mammalian cultures have yet to be established. In this study, we explored a model consisting of epithelial and immune cells, as well as four different bacterial species (Bacteroides fragilis KLE1958, Escherichia coli MG1655, Lactobacillus rhamnosus KLE2101, or Ruminococcus gnavus KLE1940), over a 50 hour culture period. Interestingly, both obligate and facultative anaerobes grew to similar extents in aerobic culture environments during the co-culture period, likely due to measured microaerobic oxygen levels near the apical surface of the epithelia. It was demonstrated that bacteria elicited reactive oxygen species (ROS) production, and that the resulting oxidative damage heavily contributed to observed epithelial barrier damage in these static cultures. Introduction of a ROS scavenger significantly mitigated oxidative damage, improving cell monolayer integrity and reducing lipid peroxidation, although not to control (bacteria-free culture) levels. These results indicate that monitoring and mitigating ROS accumulation and oxidative damage can enable longer term bacteria-intestinal epithelial cultures, while also highlighting the significance of additional factors that impact homeostasis in mammalian cell-bacteria systems.

Intestinal homeostasis is tightly regulated by the orchestrated actions of a multitude of cell types, including enterocytes, goblet cells and immune cells. Disruption of intestinal barrier function can increase susceptibility to pathogen invasion and destabilize commensal microbial-epithelial-immune interaction, manifesting in various intestinal and systemic pathologies. However, a quantitative understanding of how these cell types communicate and collectively contribute to tissue function in health and disease is lacking. Here, we utilized a human intestinal epithelial-dendritic cell model and multivariate analysis of secreted factors to investigate the cellular crosstalk in response to physiological and/or pathological cues (e.g., endotoxin, non-steroidal anti-inflammation drug (NSAID)). Specifically, we demonstrated that treatment with diclofenac (DCF), an NSAID commonly used to treat inflammation associated with acute infection and other conditions, globally suppressed cytokine secretion when dosed in isolation. However, the disruption of barrier function induced by DCF allowed for luminal lipopolysaccharide (LPS) translocation and activation of resident immune cells that overrode the anti-inflammatory influence of DCF. DCF-facilitated inflammation in the presence of LPS was in part mediated by upregulation of macrophage migration inhibitory factor (MIF), an important regulator of innate immunity. However, while neutralization of MIF activity normalized inflammation, it did not lead to intestinal healing. Our data suggest that systems-wide suppression of inflammation alone is insufficient to achieve mucosal healing, especially in the presence of DCF, the target of which, the COX-prostaglandin pathway, is central to mucosal homeostasis. Indeed, DCF removal post-injury enabled partial recovery of intestinal epithelium functions, and this recovery phase was associated with upregulation of a subset of cytokines and chemokines, implicating their potential contribution to intestinal healing. The results highlight the utility of an intestinal model capturing immune function, coupled with multivariate analysis, in understanding molecular mechanisms governing response to microbial factors, supporting application in studying host-pathogen interactions.

Some species successfully repair retinal injuries in contrast to non-regenerative mammalian retina. We show here that the Mexican axolotl salamander regrows its excised retina even in adulthood. During early regeneration, cell proliferation occurred in the retinal pigment epithelium (RPE). All dividing cells expressed Vimentin, and some also expressed Müller glia and neural progenitor cell marker Glast (Slc1a3), suggesting that regeneration is driven by RPE-derived retinal progenitor cells. Bulk RNA sequencing showed that genes associated with the extracellular matrix and angiogenesis were upregulated in early-to-mid retinal regeneration. The fully regenerated retina re-established nerve projections to the brain and contained all the original retinal cell types, including Müller glia. Regeneration of cellular diversity may be modulated by Notch signaling, as inhibiting Notch signaling in early regeneration promoted production of rod photoreceptors. Our study highlights the axolotl salamander as an advantageous model of adult tetrapod retinal regeneration and provides insights into its mechanisms.SummaryWe demonstrate that adult Mexican axolotl salamanders regenerate retinas after a retinectomy. We also show some cellular and molecular mechanisms that drive axolotl retinal regeneration.

BackgroundThere is a growing appreciation for the significance of the gut microbiome in health and disease. Specifically, considerable effort has been put forth to understand mechanisms by which the microbiota modulates and responds to inflammation. Here, we explored whether oxidation metabolites produced by the host during inflammation, sodium nitrate and trimethylamine oxide, impact the composition of a human stool bacterial population in a gut simulator. We then assessed whether an immune-competent in vitro intestinal model responded differently to spent medium from bacteria exposed to these cues compared to spent medium from a control bacterial population. ResultsThe host-derived oxidation products were found to decrease levels of Bacteroidaceae and overall microbiota metabolic potential, while increasing levels of pro-inflammatory Enterobacteriaceae and lipopolysaccharide in bacterial cultures, reflecting shifts that occur in vivo in inflammation. Spent medium, with or without sodium nitrate and trimethylamine oxide, induced elevated intracellular mucin levels and reduced intestinal monolayer integrity as reflected in trans-epithelial electrical resistance relative to fresh medium controls. However, multiplexed cytokine analysis revealed markedly different cytokine signatures from intestinal cultures exposed to spent medium with added oxidation products relative to spent control medium, while cytokine signatures of cultures exposed to fresh media were similar regardless of addition of host-derived cues. Further, the presence of immune cells in the intestinal model was required for this differentiation of cytokine signatures. ConclusionsThis study indicates that simple in vitro immune-competent intestinal models can capture bacterial-mammalian cross-talk in response to host-derived oxidation products and supports utility of these systems for mechanistic studies of interactions between the gut microbiome and host in inflammation.

Considerable effort has been put forth to understand mechanisms by which the microbiota modulates and responds to inflammation. Here, we explored whether oxidation metabolites produced by the host during inflammation, sodium nitrate and trimethylamine oxide, impact the composition of a human stool bacterial population in a gut simulator. We then assessed whether an immune-competent

The gut microbiome and its interactions with epithelial and immune cells have wide-ranging effects on many aspects of human health. Thus, in vitro models that enable highly controlled studies of these interactions are of value, yet critical parameters enabling long term homeostasis between bacteria and mammalian cultures have not been established. In this study, we developed a model incorporating epithelial and immune cells as well as different bacterial species (B. fragilis, E. coli, L. rhamnosus, or R. gnavus) over a 50 hour culture period. Interestingly, both obligate and facultative anaerobes grew to similar extents in aerobic culture environments in co-culture with epithelial and immune cells, potentially due to measured microaerobic oxygen levels near the epithelial apical surface. It was demonstrated that bacteria elicited reactive oxygen species (ROS) production, and that these species heavily contribute to observed epithelial barrier damage in these static cultures. Introduction of a ROS scavenger significantly mitigated ROS-mediated damage, improving cell monolayer integrity and reducing lipid peroxidation, although not to control (bacteria-free culture) levels. These results indicate that monitoring and mitigating ROS concentrations can enable longer term bacteria-intestinal epithelial cultures, but also highlight the significance of additional factors that impact homeostasis in mammalian cell-bacteria systems.

Retinal degenerative diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP), are major causes of blindness worldwide. Humans cannot regenerate retina, however, axolotl (Ambystoma mexicanum), a laboratory-bred salamander, can regenerate retinal tissue throughout adulthood. Classic signaling pathways, including fibroblast growth factor (FGF), are involved in axolotl regeneration. Glycosaminoglycan (GAG) interaction with FGF is required for signal transduction in this pathway. GAGs are anionic polysaccharides in extracellular matrix (ECM) that have been implicated in limb and lens regeneration of amphibians, however, GAGs have not been investigated in the context of retinal regeneration. GAG composition is characterized native and decellularized axolotl and porcine retina using liquid chromatography mass spectrometry. Pig was used as a mammalian vertebrate model without the ability to regenerate retina. Chondroitin sulfate (CS) was the main retinal GAG, followed by heparan sulfate (HS), hyaluronic acid, and keratan sulfate in both native and decellularized axolotl and porcine retina. Axolotl retina exhibited a distinctive GAG composition pattern in comparison with porcine retina, including a higher content of hyaluronic acid. In CS, higher levels of 4- and 6- O-sulfation were observed in axolotl retina. The HS composition was greater in decellularized tissues in both axolotl and porcine retina by 7.1% and 15.4%, respectively, and different sulfation patterns were detected in axolotl. Our findings suggest a distinctive GAG composition profile of the axolotl retina set foundation for role of GAGs in homeostatic and regenerative conditions of the axolotl retina and may further our understanding of retinal regenerative models.

Necrotizing enterocolitis (NEC) is a devastating gastrointestinal disease of incompletely understood pathophysiology predominantly affecting premature infants. While NEC is associated with microbial invasion of intestinal tissues, and mucus modulates interactions between microbes and underlying tissues, variations in mucus barrier properties with NEC-associated risk factors have not been investigated. This study explored differences in mucus composition (total protein, DNA, mucin content, sialic acid, and immunoregulatory proteins), as well as structural and transport properties, assessed by tracking of particles and bacteria (E. coli and E. cloacae) with developmental age and exposure to NEC stressors in Sprague Dawley rats. Early developmental age (5 day old) was characterized by a more permeable mucus layer relative to 21 day old pups, suggesting immaturity may contribute to exposure of the epithelium to microbes. Exposure to NEC stressors was associated with reduced mucus permeability, which may aid in survival. Feeding with breastmilk as opposed to formula reduces incidence of NEC. Thus, NEC-stressed (N-S) rat pups were orally dosed with breastmilk components lysozyme (N-S-LYS) or docosahexaenoic acid (N-S-DHA). N-S-LYS and N-S-DHA pups had a less permeable mucus barrier relative to N-S pups, which suggests the potential of these factors to strengthen the mucus barrier and thus protect against disease.

The consumption of generally regarded as safe emulsifiers has increased, and has been associated with an increased prevalence of inflammatory bowel and metabolic diseases, as well as an altered microbiome. The mucus barrier, which selectively controls the transport of particulates and microorganisms to the underlying epithelial layer, has been previously shown to be altered by dietary salts and lipids. However, the potential impact of emulsifiers on the protective mucus barrier, its permeability, and associated structural changes are not clear. In this study, we analyzed changes in the mucus barrier to both passively diffusing nanoparticles and actively swimming E. coli upon exposure to two emulsifiers, carboxymethylcellulose (CMC) and polysorbate 80 (Tween). When exposed to CMC, mucus pore size decreased, which resulted in significantly slower E. coli speed and particle diffusion rates through mucus. Tween exposure minimally impacted mucus microstructure and particle diffusion, but increased E. coli speed in mucus. Moreover, both emulsifiers appeared to alter mucus amount and thickness in rat intestinal tissue and mucus-producing cell cultures. These results indicate that acute exposure to emulsifiers impacts barrier and structural properties of intestinal mucus, modulating interactions between intestinal lumen contents, microbes, and underlying tissue, which may contribute to development of intestinal inflammation.

The utilization of polymers to stabilize drug supersaturation and enhance oral drug absorption has recently garnered considerable interest. The potential role of intestinal mucus in stabilizing drug supersaturation, however, has not been previously explored. The ability for intestinal mucus to stabilize drug supersaturation and delay drug precipitation is potentially useful in enhancing the absorption of orally dosed compounds from drug delivery systems that generate supersaturation within the gastrointestinal tract (e.g., solid dispersions, lipid-based drug delivery systems). This work aims to evaluate the precipitation-delaying abilities of intestinal mucus using carvedilol (CVDL) and piroxicam (PXM) as model drugs. In supersaturation-precipitation (S-P) experiments, CVDL and PXM supersaturation were induced in test media (0, 0.1, 0.2, 0.4 %w/v mucin and 8 %w/v native pig intestinal mucus (PIM)) via the solvent-shift method at supersaturation ratios (SSR) of 5 and 6, respectively. Time to drug precipitation was assessed using ion-selective electrodes and HPLC. The S-P experiments showed that increasing mucin concentration led to increasingly delayed CVDL precipitation, while PXM precipitation was prevented at all mucin concentrations studied. The ability of mucus-stabilized CVDL supersaturation to translate into enhanced CVDL absorption was evaluated in transport experiments using mucus-producing (90% Caco-2:10% HT29-MTX-E12 co-cultures) vs. non-mucus-producing intestinal monolayers (100% Caco-2 cultures). The absorption enhancement of CVDL (SSR = 5 relative to SSR = 1) was higher across mucus-producing than non-mucus-producing intestinal monolayers. This work demonstrates for the first time the potential for intestinal mucus to delay the precipitation and enhance the absorption of poorly water-soluble compounds, suggesting that drug supersaturation can be stabilized in close proximity to the absorptive site, thereby presenting a possible novel approach for targeted supersaturating drug delivery systems.

Microphysiological systems (MPSs) are in vitro models that capture facets of in vivo organ function through use of specialized culture microenvironments, including 3D matrices and microperfusion. Here, we report an approach to co-culture multiple different MPSs linked together physiologically on re-useable, open-system microfluidic platforms that are compatible with the quantitative study of a range of compounds, including lipophilic drugs. We describe three different platform designs – “4-way”, “7-way”, and “10-way” – each accommodating a mixing chamber and up to 4, 7, or 10 MPSs. Platforms accommodate multiple different MPS flow configurations, each with internal re-circulation to enhance molecular exchange, and feature on-board pneumatically-driven pumps with independently programmable flow rates to provide precise control over both intra- and inter-MPS flow partitioning and drug distribution. We first developed a 4-MPS system, showing accurate prediction of secreted liver protein distribution and 2-week maintenance of phenotypic markers. We then developed 7-MPS and 10-MPS platforms, demonstrating reliable, robust operation and maintenance of MPS phenotypic function for 3 weeks (7-way) and 4 weeks (10-way) of continuous interaction, as well as PK analysis of diclofenac metabolism. This study illustrates several generalizable design and operational principles for implementing multi-MPS “physiome-on-a-chip” approaches in drug discovery.

Gut-on-a-chip in vitro modeling is an emerging field, as the human gut epithelium and gut microbiome have been recently identified as novel drug targets for a wide variety of diseases. Realistic in vitro gut models require a variety of precise environmental cues, such as chemical and gas gradients, in combination with substrates like mucus that support the growth of microbial communities. This technical brief describes a microfluidic architecture capable of developing a physiologically relevant oxygen gradient that emulates the oxygen profile proximal to the epithelial inner lining of the human colon. The device generates stable and repeatable defined oxygen gradients from 0% to 4 % partial pressure O2 over a length scale of hundreds of microns, and was applied to study the effects of oxygenation on the structure of native mucus that lines the colon wall. Using simulation as a design tool for hybrid gas-liquid microfluidic devices enables on-chip creation of defined, physiologically oxygen gradients. These microfluidic architectures have powerful potential applications for gut physiology, including providing optimal oxygenation conditions for the culture of mammalian epithelial cells in the gut lining, as well as creating a realistic mimic of the oxygen gradient found in the intestinal lumen for complex microbiome cultures.

Intestinal mucus provides a significant barrier to transport of orally delivered drug carriers, as well as other particulates (e.g. food, microbes). The relative significance of particle size, surface chemistry, and dosing medium to mucus barrier properties is not well characterized, but important in designing delivery systems targeted to the intestinal mucosa. In this study, multiple particle tracking (MPT) was used to study diffusion of 20–500 nm diameter carboxylate- and polyethylene glycol-(PEG-)functionalized polystyrene model carriers through intestinal mucus. The impact of exposure to mucus in buffer versus a partially digested triglyceride mixture was explored. Effective diffusivity of particles in intestinal mucus decreased with an increasing particle size less than and more than theoretically (Stokes–Einstein) expected in a homogenous medium when dosed in buffer and model-fed state intestinal contents, respectively. For example, effective diffusivity decreased 2.9- versus 20-fold with increase in the particle size from 100 to 500 nm when dosed to mucus in buffer versus lipid-containing medium. Functionalization with PEG dramatically decreased sensitivity to lipids in a dosing medium. The results indicate that reduction of particle size may increase particle transport through intestinal mucus barriers, but these effects are strongly dependent on intestinal contents and particle surface chemistry.

Mucus forms a protective hydrogel layer over the intestinal epithelium, presenting a selective and robust barrier to the uptake of particulates and microbe invasion. Disease can alter mucus production and composition, thus potentially modifying mucosal barrier properties. Hirschsprung’s disease (HD) is a developmental abnormality of the nervous system often complicated by intestinal infection. An investigation of colonic mucus barrier properties in an HD animal model, endothelin receptor B mutant mice, revealed significantly reduced microsphere (passive) and microbe (active) transport rates (7-fold and 3.6-fold, respectively, in proximal colonic mucus) relative to wild-type. Transport differences were evident in both the ganglionic and aganglionic colon segments, in agreement with the risk of Hirschsprung’s disease-associated enterocolitis after surgery to remove aganglionic colon segments. The development of therapies aimed at altering colonic mucus barrier properties could be explored towards preventing the onset of enterocolitis in Hirschsprung’s disease. Keywords: mucus, Hirschsprung’s disease, nanoparticle diffusion, microbe transport, intestine 1. Introduction Mucus, a highly complex hydrogel composed of mucin glycoproteins with a pore size reported to be approximately 10–500 nm,[1] provides a barrier against particulate materials and biological pathogens. The barrier properties of mucus are a combined result of physical effects, such as size exclusion and hydrodynamic drag, and intermolecular interactions between diffusing entities and mucus components.[1–5] The significance of the mucus barrier in controlling access to the underlying epithelium raises the question of whether mucosal structural and/or chemical changes occur in disease states, potentially contributing to pathogen invasion. Hirschsprung’s disease (HD), also known as intestinal aganglionosis, is a developmental abnormality of the enteric nervous system that affects 1 in 5,000 live births.[6, 7] HD is characterized by the lack of ganglion cells along a variable length of the distal intestine, resulting in the absence of peristalsis in the aganglionic segment and dilation of the colon proximally.[6, 7] HD is associated with the development of Hirschsprung’s-associated enterocolitis (HAEC), a severe inflammation of the intestinal mucosa that presents with abdominal distension, diarrhea, and fever and can progress to sepsis and death.[6, 7] HAEC, the leading cause of death in HD, occurs in up to 50% of untreated HD cases, and the risk persists even after surgical removal of the aganglionic intestine, suggesting that the abnormality associated with HAEC extends beyond the region of aganglionosis.[6] It is thought that HD leads to changes in colonic microbiota, innate immunity, and epithelial barrier properties,[8] any of which may contribute to the risk of developing enterocolitis. Although these theories are being investigated, little attention has been focused on the potential contributory role of mucus barrier properties in this disease. Recent insight into changes in HD mucosal properties has been gained by investigating mucus turnover rate, mucin gene expression, and histological structure.[8–13] These studies have shown that HD is associated with lowered mucus turnover rate, lowered mucin concentrations, abnormal mucin ratios (neutral mucins:acidic-sulphomucins),[13] and differences in mucin producing goblet cell size and proliferation compared to the healthy state.[14] These results therefore support differences in mucus quantity and composition in HD, but do not specifically test potential changes in mucus barrier properties that may play a role in microbe invasion and in the development of enterocolitis. Given the important role of mucus in protecting underlying epithelia from microbial invasion, we hypothesize that the inherent barrier properties of colonic mucus are altered in HD. To test this hypothesis, transport of passively diffusing entities (polystyrene microspheres) and actively moving microbes (Escherichia coli) through colonic mucus in an HD animal model were investigated. In this work, live imaging and multiple particle tracking (MPT),[15–21] a powerful technique enabling non-disruptive investigation of transport across the mucosal layer, was employed. Particle and microbe trajectories were used to calculate time-averaged mean squared displacements (MSDs) and average velocities to quantitatively characterize transport across mucus layers. Results supported differences in colonic mucus barriers between HD and wild-type (WT) animals. A deeper understanding of the mechanisms underlying mucus barrier differences could lead to therapeutic treatments to modulate mucus barrier properties and potentially prevent the onset of HAEC.

Mucus is a complex hydrogel that acts as a natural barrier to drug delivery at different mucosal surfaces including the respiratory, gastrointestinal, and vaginal tracts. To elucidate the role mucus plays in drug delivery, different in vitro, in vivo, and ex vivo mucus models and techniques have been utilized. Drug and drug carrier diffusion can be studied using various techniques in either isolated mucus gels or mucus present on cell cultures and tissues. The species, age, and potential disease state of the animal from which mucus is derived can all impact mucus composition and structure, and therefore impact drug and drug carrier diffusion. This review provides an overview of the techniques used to characterize drug and drug carrier diffusion, and discusses the advantages and disadvantages of the different models available to highlight the information they can afford.

A capability for analyzing complex cellular communication among tissues is important in drug discovery and development, and in vitro technologies for doing so are required for human applications. A prominent instance is communication between the gut and the liver, whereby perturbations of one tissue can influence behavior of the other. Here, we present a study on human gut-liver tissue interactions under normal and inflammatory contexts, via an integrative multi-organ platform comprising human liver (hepatocytes and Kupffer cells), and intestinal (enterocytes, goblet cells, and dendritic cells) models. Our results demonstrated long-term (>2 weeks) maintenance of intestinal (e.g., barrier integrity) and hepatic (e.g., albumin) functions in baseline interaction. Gene expression data comparing liver in interaction with gut, versus isolation, revealed modulation of bile acid metabolism. Intestinal FGF19 secretion and associated inhibition of hepatic CYP7A1 expression provided evidence of physiologically relevant gut-liver crosstalk. Moreover, significant non-linear modulation of cytokine responses was observed under inflammatory gut-liver interaction; for example, production of CXCR3 ligands (CXCL9,10,11) was synergistically enhanced. RNA-seq analysis revealed significant upregulation of IFNα/β/γ signaling during inflammatory gut-liver crosstalk, with these pathways implicated in the synergistic CXCR3 chemokine production. Exacerbated inflammatory response in gut-liver interaction also negatively affected tissue-specific functions (e.g., liver metabolism). These findings illustrate how an integrated multi-tissue platform can generate insights useful for understanding complex pathophysiological processes such as inflammatory organ crosstalk., National Institutes of Health (U.S.) (grant UH3TR00069), United States. Defense Advanced Research Projects Agency (grant Microphysiological Systems Program (W911NF-12-2-00))

In vitro human small intestine models play a crucial part in preclinical drug development. Although conventional 2D systems possess many advantages, such as facile accessibility and high-throughput capability, they can also provide misleading results due to their relatively poor recapitulation of in vivo physiology. Significant progress has recently been made in developing 3D human small intestine models, suggesting that more-reliable preclinical results could be obtained by recreating the 3D intestinal microenvironment in vitro. Although there are still many challenges, 3D human small intestine models have the potential to facilitate drug screening and drug development.

Poly(2-hydroxyethyl methacrylate) (pHEMA) is a widely utilized biomaterial due to lack of toxicity and suitable mechanical properties; conformal thin pHEMA films produced via chemical vapor deposition (CVD) would thus have broad biomedical applications. Thin films of pHEMA were deposited using photoinitiated CVD (piCVD). Incorporation of ethylene glycol diacrylate (EGDA) into the pHEMA polymer film as a crosslinker, confirmed via Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, resulted in varied swelling and degradation behavior. 2-Hydroxyethyl methacrylate-only films showed significant thickness loss (up to 40%), possibly due to extraction of low-molecular-weight species or erosion, after 24 h in aqueous solution, whereas films crosslinked with EGDA (9.25-12.4%) were stable for up to 21 days. These results differ significantly from those obtained with plasma-polymerized pHEMA, which degraded steadily over a 21-day period, even with crosslinking. This suggests that the piCVD films differ structurally from those fabricated via plasma polymerization (plasma-enhanced CVD). piCVD pHEMA coatings proved to be good cell culture materials, with Caco-2 cell attachment and viability comparable to results obtained on tissue-culture polystyrene. Thus, thin film CVD pHEMA offers the advantage of enabling conformal coating of a cell culture substrate with tunable properties depending on method of preparation and incorporation of crosslinking agents.

Significant effort and resource expenditure is dedicated to enabling low-solubility oral drug delivery using solubilization technologies. Cyclodextrins (CD) are cyclic oligosaccharides which form inclusion complexes with many drugs and are often used as solubilizing agents. It is not clear prior to developing a drug delivery device with CD what level of absorption enhancement might be achieved; modeling can provide useful guidance in formulation and minimize resource intensive iterative formulation development. A model was developed to enable quantitative, dynamic prediction of the influence of CD on oral absorption of low solubility drug administered as a pre-formed complex. The predominant effects of CD considered were enhancement of dissolution and slowing of precipitation kinetics, as well as binding of free drug in solution. Simulation results with different parameter values reflective of typical drug and CD properties indicate a potential positive (up to five times increase in drug absorption), negative (up to 50% decrease in absorption) or lack of effect of CD. Comparison of model predictions with in vitro and in vivo experimental results indicate that a systems-based dynamic model incorporating CD complexation and key process kinetics may enable quantitative prediction of impact of CD delivered as a pre-formed complex on drug bioavailability. Biotechnol. Bioeng. 2013; 110:2536–2547. © 2013 Wiley Periodicals, Inc.

While cell transplantation therapies show great promise as treatments for retinal degeneration, the challenge of low cell survival upon transplantation motivates exploration of materials that may serve as cell delivery vehicles and promote survival and differentiation. In this study, we explored the native matrix that surrounds the outer segments of photoreceptors and promotes their homeostasis, interphotoreceptor matrix (IPM), as a substrate for human retinal progenitor cells (hRPCs). Bovine IPM was characterized to determine its structure and biochemical composition, and processed to develop substrates for cells. Cell viability, morphology, proliferation and expression of photoreceptors marker genes were studied on IPM-based substrates in vitro. We explored different preparations of IPM as a scaffold. Lectin staining revealed that a distinct honeycomb structure of native IPM is lost during centrifugation to prepare a more concentrated suspension of matrix. Biochemical analysis of bovine IPM indicated presence of glycosaminoglycans and proteins. IPM mediated hRPC attachment and spreading with no signs of cytotoxicity. Cells proliferated more on native IPM substrates compared to IPM that was centrifuged to create a concentrated suspension. Cells cultured on IPM substrates expressed markers of photoreceptors: rhodopsin, NRL and ROM1. Together this data supports further exploration of IPM as a tool for retinal tissue engineering. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 891-899, 2018.

The interaction with cervicovaginal mucus presents the potential to impact the performance of drug nanocarriers. These systems must migrate through this biological fluid in order to deliver their drug payload to the underlying mucosal surface. We studied the ability of dapivirine-loaded polycaprolactone (PCL)-based nanoparticles (NPs) to interact with a simulated vaginal fluid (SVF) incorporating mucin. Different surface modifiers were used to produce NPs with either negative (poloxamer 338 NF and sodium lauryl sulfate) or positive (cetyltrimethylammonium bromide) surface charge. Studies were performed using the mucin particle method, rheological measurements, and real-time multiple particle tracking. Results showed that SVF presented rheological properties similar to those of human cervicovaginal mucus. Analysis of NP transport indicated mild interactions with mucin and low adhesive potential. In general, negatively charged NPs underwent subdiffusive transport in SVF, i.e., hindered as compared to their diffusion in water, but faster than for positively charged NPs. These differences were increased when the pH of SVF was changed from 4.2 to 7.0. Diffusivity was 50- and 172-fold lower in SVF at pH 4.2 than in water for negatively charged and positively charged NPs, respectively. At pH 7.0, this decrease was around 20- and 385-fold, respectively. The estimated times required to cross a layer of SVF were equal to or lower than 1.7 h for negatively charged NPs, while for positively charged NPs these values were equal to or higher than 7 h. Overall, our results suggest that negatively charged PCL NPs may be suitable to be used as carriers in order to deliver dapivirine and potentially other antiretroviral drugs to the cervicovaginal mucosal lining. Also, they further reinforce the importance in characterizing the interactions of nanosystems with mucus fluids or surrogates when considering mucosal drug delivery.

The basement membrane of small intestinal epithelium possesses complex topography at multiple scales ranging from the mesoscale to nanoscale. Specifically, intestinal crypt-villus units are comprised of hundred-micron-scale well-like invaginations and finger-like projections; intestinal cell phenotype is related to location on this crypt-villus unit. A biomimetic intestinal cell culture system composed of type I collagen based permeable cell culture membranes incorporating both micron-scale intestinal crypt-like topography and nanometer scale topography was fabricated. Membranes were pre-incubated with either laminin (Ln) or fibronectin (Fn), inoculated with intestinal epithelial Caco-2 cells and cultured for 1-21 days to study the relative significance of influence of crypt-like topography and biomimetic substrate chemistry on cell phenotype. Crypt-like topography inhibited Caco-2 differentiation during early culture, as evidenced by slower cell spreading and lower brush border enzyme activity. For example, alanine aminopeptidase activity was lower on Ln-coated patterned collagen ( approximately 3.4+/-0.24mU/mg) compared to flat collagen (10.84+/-0.55mU/mg) at day 7. Caco-2 cultured on Fn-coated collagen started to spread earlier (1 day vs 3 days) and formed longer protrusions than on Ln-coated collagen. Pre-coating of Ln enhanced cell differentiation, as the maximum activity of a cell differentiation marker (alkaline phosphatase) was 2-3 times higher than on Fn-coated collagen, and maintained differentiated phenotype in long term (up to 21 days) culture. In general, compared to substrate topography, coating with ECM protein had more prominent and longer effect on cell behavior. Crypt-like topography affected Caco-2 spreading and differentiation during early culture, however the effect diminished as culture progressed. This information will benefit intestinal tissue engineering scaffold design, and modification of in vitro intestinal cell models.

Gastrointestinal mucus, a complex network of highly branched glycoproteins and macromolecules, is the first barrier through which orally delivered drug and gene vectors must traverse. The diffusion of such vectors can be restricted by the high adhesivity and viscoelasticity of mucus. In this investigation, the barrier properties of gastrointestinal mucus to particle transport were explored using real-time multiple particle tracking. The influence of surface chemistry on particle transport rates was examined using amine-, carboxylate-, and sulfate-modified polystyrene nanoparticles. A strong dependence of particle mobility in gastrointestinal mucus on surface charge was observed, with anionic particles diffusing 20-30 times faster than cationic particles. Comparison of diffusion coefficients calculated for gastrointestinal mucus with significantly varying values previously reported in the literature for other mucus sources, including cervicovaginal mucus and cystic fibrosis sputum, highlight the dependence of mucus barrier properties on the anatomical source. A significant degree of transport rate heterogeneity was also observed in native gastrointestinal mucus, suggesting a highly heterogeneous distribution of pore sizes. Furthermore, the suitability of purified mucin as a model system for transport studies was assessed by comparing particle transport rates between native intestinal mucus and purified porcine gastric mucin. Particle transport rates were approximately threefold lower in native mucus compared to purified mucin for anionic particles, yet comparable for cationic particles. Differences between barrier properties of the purified mucin preparation and native mucus depended on specific carrier properties, indicating that the purified mucin preparation does not provide an accurate model system for native mucus.

Lipid based drug delivery systems, and in particular self-emulsifying drug delivery systems (SEDDS), show great potential for enhancing oral bioavailability but have not been broadly applied, largely due to lack of general formulation guidance. To help understand how formulation design influences physicochemical emulsion properties and associated function in the gastrointestinal environment, a range of twenty-seven representative self-emulsifying formulations were investigated. Two key functions of emulsion-based drug delivery systems, permeability enhancement and drug release, were studied and statistically related to three formulation properties - oil structure, surfactant hydrophilic liphophilic balance (HLB) values, and surfactant-to-oil ratio. Three surfactants with HLB values ranging from 10 to 15 and three structurally different oils (long chain triglyceride, medium chain triglyceride, and propylene glycol dicaprylate/dicaprate) were combined at three different weight ratios (1:1, 5:1, 9:1). Unstable formulations of low HLB surfactant (HLB=10) had a toxic effect on cells at high (1:1) surfactant concentrations, indicating the importance of formulation stability for minimizing toxicity. Results also indicate that high HLB surfactant (Tween 80) loosens tight junction at high (1:1) surfactant concentrations. Release coefficients for each emulsion system were calculated. Incorporation of a long chain triglyceride (Soybean oil) as the oil phase increased the drug release rate constant. These results help establish an initial foundation for relating emulsion function to formulation design and enabling bioavailability optimization across a broad, representative range of SEDDS formulations.

A microfabrication approach was utilized to create topographic analogs of intestinal crypts on a polymer substrate. It was hypothesized that biomimetic crypt-like micro-architecture may induce changes in small intestinal cell (i.e. Caco-2 cell) phenotype. A test pattern of micro-well features with similar dimensions (50, 100, and 500 microm diameter, 50 microm spacing, 120 microm in depth) to the crypt structures found in native basal lamina was produced in the surface of a poly(dimethylsiloxane) (PDMS) substrate. PDMS surfaces were coated with fibronectin, seeded with intestinal-epithelial-cell-like Caco-2 cells, and cultured up to fourteen days. The cells were able to crawl along the steep side walls and migrated from the bottom to the top of the well structures, completely covering the surface by 4-5 days in culture. The topography of the PDMS substrates influenced cell spreading after seeding; cells spread faster and in a more uniform fashion on flat surfaces than on those with micro-well structures, where cell protrusions extending to micro-well side walls was evident. Substrate topography also affected cell metabolic activity and differentiation; cells had higher mitochondrial activity but lower alkaline phosphatase activity at early time points in culture (2-3 days post-seeding) when seeded on micro-well patterned PDMS substrates compared to flat substrates. These results emphasize the importance of topographical design properties of a scaffolds used for tissue engineered intestine.

Plasma Enhanced Chemical Vapor Deposition (PECVD) of poly-2-hydroxyethyl methacrylate (pHEMA) biocompatible, biodegradable polymer films were produced alone and cross-linked with ethylene glycol diacrylate (EGDA). Degree of cross-linking was controlled via manipulation of the EGDA flow rate, which influenced the amount of swelling and the extent of degradation of the films in an aqueous solution over time. Noncross-linked pHEMA films swelled 10% more than cross-linked films after 24 h of incubation in an aqueous environment. Increasing degree of film cross-linking decreased degradation over time. Thus, PECVD pHEMA films with variable cross-linking properties enable tuning of gel formation and degradation properties, making these films useful in a variety of biologically significant applications.

Cyclodextrins (CD) have been utilized extensively in pharmaceutical formulations to enhance oral bioavailability. A critical review of the literature in which cyclodextrins were utilized for this purpose was conducted. The goal of this review was to determine if quantitative guidelines for drug and cyclodextrin properties necessary for bioavailability enhancement using cyclodextrins could be extracted. Twenty-eight studies were examined in which the focus was on the use of cyclodextrins as solubilizers to enhance bioavailability. Commonly observed factors included: utilization of pre-formed complex rather than physical mixtures, drug hydrophobicity (logP > 2.5), low drug solubility (typically< 1 mg/ml), moderate binding constant (< 5000 M(-1)), low dose (< 100 mg), and low CD:drug ratio (< 2:1). These general guidelines, however, did not apply to all studies. Quantitative guidelines useful to a formulation scientist considering the use of cyclodextrins were difficult to develop due to missing information and the complicated manner in which drug and cyclodextrin properties interact to influence key drug delivery processes (e.g., dissolution, absorption). The mechanisms by which cyclodextrins influence these processes, again emphasizing solubilization capabilities, are discussed to provide further insight into why cyclodextrins will increase bioavailability in certain cases but not influence or possibly decrease bioavailability in others.

Cardiac muscle with a certain threshold thickness, uniformity of tissue architecture, and functionality would expand the therapeutic options currently available to patients with congenital or acquired cardiac defects. Cardiac constructs cultured in well-mixed medium had an approximately 100-microm-thick peripheral tissue-like region around a relatively cell-free interior, a structure consistent with the presence of concentration gradients within the tissue. We hypothesized that direct perfusion of cultured constructs can reduce diffusional distances for mass transport, improve control of oxygen, pH, nutrients and metabolites in the cell microenvironment, and thereby increase the thickness and spatial uniformity of engineered cardiac muscle. To test this hypothesis, constructs (9.5-mm-diameter, 2-mm-thick discs) based on neonatal rat cardiac myocytes and fibrous polyglycolic acid scaffolds were cultured either directly perfused with medium or in control spinner flasks. Perfusion improved the spatial uniformity of cell distribution and enhanced the expression of cardiac-specific markers, presumably due to the improved control of local microenvironmental conditions within the forming tissue. Medium perfusion could thus be utilized to better mimic the transport conditions within native cardiac muscle and enable in vitro engineering of cardiac constructs with clinically useful thicknesses.

Orally delivered drugs and nutrients must diffuse through mucus to enter the circulatory system, but the barrier properties of mucus and their modulation by physiological factors are generally poorly characterized. The main objective of this study was to examine the impact of physicochemical changes occurring upon food ingestion on gastrointestinal (GI) mucus barrier properties. Lipids representative of postprandial intestinal contents enhanced mucus barriers, as indicated by a 10–142-fold reduction in the transport rate of 200 nm microspheres through mucus, depending on surface chemistry. Physiologically relevant increases in [Ca 2+ ] resulted in a 2–4-fold reduction of transport rates, likely due to enhanced cross-linking of the mucus gel network. Reduction of pH from 6.5 to 3.5 also affected mucus viscoelasticity, reducing particle transport rates approximately 5–10-fold. Macroscopic visual observation and micro-scale lectin staining revealed mucus gel structural changes, including clumping into regions into which particles did not penetrate. Histological examination indicated food ingestion can prevent microsphere contact with and endocytosis by intestinal epithelium. Taken together, these results demonstrate that GI mucus barriers are significantly altered by stimuli associated with eating and potentially dosing of lipid-based delivery systems; these stimuli represent broadly relevant variables to consider upon designing oral therapies.

This study utilizes high-throughput screening to examine human retinal progenitor cell migration in response to biomaterial and chemokine stimuli. Degenerative diseases of the retina impact millions of people worldwide each year, with no known cure available to date. Implantation of retinal progenitor cells in animal models has shown considerable promise but is limited by extremely low (

Retinal degeneration, associated with loss of photoreceptors, is the primary cause of permanent vision impairment, impacting millions of people worldwide. Age-related macular degeneration and retinitis pigmentosa are two common retinal diseases resulting in photoreceptor loss and vision impairment or blindness. Presently, available treatments can only delay the progress of retinal degeneration, and there are no treatments that can restore permanent vision loss. Research is underway to develop methods of regenerating the impaired retina by delivering photoreceptor precursor cells and retinal pigment epithelium to the subretinal space. Challenges to cell transplantation include limited survival upon implantation and the formation of abnormal cell architectures in vivo. Retinal tissue engineering shows immense promise and potential in treatment of retinal degeneration by employing scaffold-based delivery systems of retinal progenitor cells to the subretinal space. Scaffold delivery strategy has been shown to enhance the cell survival and direct cell differentiation in a variety of retinal degenerative models. In this chapter, we summarize the research findings on different scaffold- or substrate-based transplantation techniques used to deliver retinal progenitor/photoreceptor precursors and retinal pigment epithelial cells to the subretinal space.

Tissue engineered cartilage can be grown in vitro if the necessary physical and biochemical factors are present in the tissue culture environment. Cell me- tabolism and tissue composition were studied for engi- neered cartilage cultured for 5 weeks using bovine ar- ticular chondrocytes, polymer scaffolds (5 mm diameter ◊ 2 mm thick fibrous discs), and rotating bioreactors. Medium pH and concentrations of oxygen, carbon diox- ide, glucose, lactate, ammonia, and glycosoaminoglycan (GAG) were varied by altering the exchange rates of gas and medium in the bioreactors. Cell-polymer constructs were assessed with respect to histomorphology, bio- chemical composition and metabolic activity. Low oxy- gen tension (~40 mmHg) and low pH (~6.7) were associ- ated with anaerobic cell metabolism (yield of lactate on glucose, YL/G, of 2.2 mol/mol) while higher oxygen ten- sion (~80 mmHg) and higher pH (~7.0) were associated with more aerobic cell metabolism (YL/G of 1.65-1.79 mol/mol). Under conditions of infrequent medium re- placement (50% once per week), cells utilized more eco- nomical pathways such that glucose consumption and lactate production both decreased, cell metabolism re- mained relatively aerobic (YL/G of 1.67 mol/mol) and the resulting constructs were cartilaginous. More aerobic conditions generally resulted in larger constructs con- taining higher amounts of cartilaginous tissue compo- nents, while anaerobic conditions suppressed chondro- genesis in 3D tissue constructs. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 63: 197-205, 1999.

Intestinal mucus, a viscous secretion that lines the mucosa, is believed to be a barrier to absorption of many therapeutic compounds and carriers, and is known to play an important physiological role in controlling pathogen invasion. Nevertheless, there is as yet no clear understanding of the barrier properties of mucus, such as the nature of the molecular interactions between drug molecules and mucus components as well as those that govern gel formation. Secretory mucins, large and complex glycoprotein molecules, are the principal determinants of the viscoelastic properties of intestinal mucus. Despite the important role that mucins play in controlling transport and in diseases such as cystic fibrosis, their structures remain poorly characterized. The major intestinal secretory mucin gene, MUC2, has been identified and fully sequenced. The present study was undertaken to determine a detailed structure of the cysteine-rich region within the C-terminal end of human intestinal mucin (MUC2) via homology modeling, and explore possible configurations of a dimer of this cysteine-rich region, which may play an important role in governing mucus gel formation. Based on sequence-structure alignments and three-dimensional modeling, a cystine knot tertiary structure homologous to that of human chorionic gonadotropin (HCG) is predicted at the C-terminus of MUC2. Dimers of this C-terminal cystine knot (CTCK) were modeled using sequence alignment based on HCG and TGF-beta, followed by molecular dynamics and simulated annealing. Results support the formation of a cystine knot dimer with a structure analogous to that of HCG.

Lipids, either derived from food or used as drug delivery agents, can have a significant effect on orally delivered drug dissolution, absorption, and bioavailability. Despite numerous studies about fat-rich food/drug interactions, there is still an incomplete understanding of the influence of ingested lipids on oral bioavailability, and a lack of a general in vitro model that is able to predict a priori the in vivo performance of drug-lipid systems. In order to determine the impact of lipid and lipid digestion on drug dissolution and absorption, the choice of a bio-relevant in vitro model is the first and crucial point. A suitable in vitro model should include a medium that mimics as much as possible the gastrointestinal (GI) tract contents after food intake. The goal of this review is to provide an updated overview of (i) in vivo measurements examining stomach and intestine contents in fasted and fed states and (ii) the wide variety of bio-relevant media used in dissolution testing and in vitro lipid digestion studies. These approaches are compared and discussed in light of their capability to model physiological and physicochemical properties of GI tract contents in the fasted and fed states, and in particular when the lipid digestion process occurs.

It is recognized that topographical features such as ridges and grooves can dramatically influence cell phenotype, motivating the development of substrates with precisely biomimetic topography for study of the influence on cultured cells. Intestinal basement membrane topography has been precisely replicated using plasma enhanced chemical vapor deposition (CVD) of poly(2-hydroxyethyl methacrylate) (pHEMA) on native tissue. The ability for CVD pHEMA to coat and retain the complex architecture of the intestinal basement membrane at the micrometer scale was demonstrated using electron microscopy and surface chemical analysis (XPS). The suitability of CVD pHEMA as a cell culture substrate was assessed. Caco-2 cells maintained a high (>85%) viability on CVD pHEMA. Cell attachment and proliferation on CVD pHEMA were similar to those observed on materials traditionally used for cell culture and microfabrication purposes. Results indicate that CVD pHEMA is useful for development of precise (micrometer-scale) topographically biomimetic substrates for cell culture.

Polydimethylsiloxane (PDMS) silicone elastomer is extensively used in soft lithography processes to fabricate microscale or nano scale systems for microfluidic or cell culture applications. Though PDMS is biocompatible, it is not an ideal material for cell culture due to its poor cell adhesion properties. In this study, PDMS surfaces were modified to promote intestinal cell adhesion, in the interest of testing feasibility of using microfabricated PDMS systems for high throughput drug screening. Modification techniques included changing chemical composition of PDMS (i.e., varying curing to mixing agent ratio, and oxidization of PDMS surface by oxygen plasma), surface treatment of PDMS by coating with charged molecules (i.e., poly-D-lysine, L-alpha-phosphatidylcholine, and a layer bylayer coating), and deposition of extracellular matrix (ECM) proteins (i.e., laminin, fibronectin, and collagen). The influence of these modifications on PDMS properties, including elastic modulus and surface properties (wettability, chemical composition, topography, and protein adsorption) were characterized. Modification techniques were all found to change PDMS properties and influence the attachment and proliferation of Caco-2 cells over three days of culture to varying degrees. Generally, Caco-2 cells preferred to attach on collagen-coated, fibronectin-coated, and fibronectin-coated oxygen-plasma treated PDMS. The results highlight the importance of considering multiple physical and chemical factors that may be influenced by biomaterial modification and result in altered cell attachment to microfabricated systems, including surface hydrophobicity, chemical composition, stiffness, and topography. This study provides a foundation for further miniaturization, utilizing soft lithography techniques, of Caco-2 cell-based system for high-throughput screening of drug intestinal absorption during lead optimization in drug discovery. The understanding of different surface modifications on adjusting cell adhesion on PDMS allows systemic design of Biomicroelectromechanical Systems (BioMEMS) with tunable cell adhesion properties.

Background Potential routes of nanomaterial exposure include inhalation, dermal contact, and ingestion. Toxicology of inhalation of ultra-fine particles has been extensively studied; however, risks of nanomaterial exposure via ingestion are currently almost unknown. Using enterocyte-like Caco-2 cells as a small intestine epithelial model, the possible toxicity of CdSe quantum dot (QD) exposure via ingestion was investigated. Effect of simulated gastric fluid treatment on CdSe QD cytotoxicity was also studied. Results Commercially available CdSe QDs, which have a ZnS shell and poly-ethylene glycol (PEG) coating, and in-house prepared surfactant coated CdSe QDs were dosed to Caco-2 cells. Cell viability and attachment were studied after 24 hours of incubation. It was found that cytotoxicity of CdSe QDs was modulated by surface coating, as PEG coated CdSe QDs had less of an effect on Caco-2 cell viability and attachment. Acid treatment increased the toxicity of PEG coated QDs, most likely due to damage or removal of the surface coating and exposure of CdSe core material. Incubation with un-dialyzed in-house prepared CdSe QD preparations, which contained an excess amount of free Cd2+, resulted in dramatically reduced cell viability. Conclusion Exposure to CdSe QDs resulted in cultured intestinal cell detachment and death; cytotoxicity depended largely, however, on the QD coating and treatment (e.g. acid treatment, dialysis). Experimental results generally indicated that Caco-2 cell viability correlated with concentration of free Cd2+ ions present in cell culture medium. Exposure to low (gastric) pH affected cytotoxicity of CdSe QDs, indicating that route of exposure may be an important factor in QD cytotoxicity.

Modeling the influence of a technology such as nanoparticle systems on drug delivery is beneficial in rational formulation design. While there are many studies showing drug delivery enhancement by nanoparticles, the literature provides little guidance regarding when nanoparticles are useful for delivery of a given drug. A model was developed predicting intracellular drug concentration in cultured cells dosed with nanoparticles. The model considered drug release from nanoparticles as well as drug and nanoparticle uptake by the cells as the key system processes. Mathematical expressions for these key processes were determined using experiments in which each process occurred in isolation. In these experiments, intracellular delivery of saquinavir, a low solubility drug dosed as a formulation of poly(ethylene oxide)-modified poly(epsilon- caprolactone) (PEO–PCL) nanoparticles, was studied in THP-1 human monocyte/macrophage (Mo/Mac) cells. The model accurately predicted the enhancement in intracellular concentration when drug was administered in nanoparticles compared to aqueous solution. This simple model highlights the importance of relative kinetics of nanoparticle uptake and drug release in determining overall enhancement of intracellular drug concentration when dosing with nanoparticles. Biotechnol. Bioeng. © 2008 Wiley Periodicals, Inc.

The following is a review of the literature that addresses the use of cyclodextrin in solid dosage forms. Care was taken to exclude physical and chemical characteristics of cyclodextrin, which have been discussed in the literature. A flow diagram is provided to outline the decision-making steps that are involved in the development process. Both preparation of physical mixtures and inclusion complexes are considered. Analytical techniques to determine the presence of inclusion complexes, the effect of other excipients on complex formation, the effect of size limitation of solid dosages forms, powder processing, and storage of solid dosage forms are discussed.

A novel wavy-walled bioreactor designed to enhance mixing at controlled shear stress levels was used to culture chondrocytes in suspension. Chondrocyte aggregation in suspensions mixed at 30, 50, and 80 rpm was characterized in the wavy-walled bioreactor and compared with that in conventional smooth-walled and baffled-walled spinner flask bioreactors. Aggregation was characterized in terms of the percentage of cells that aggregated over time, and aggregate size changes over time. The kinetics of chondrocyte aggregation observed in the bioreactors was composed of two phases: early aggregation between 0 and 2 h of culture, and late aggregation between 3 and 24 h of culture. At 50 rpm, the kinetics of early aggregation in the wavy-walled bioreactor was approximately 25% and 65% faster, respectively, than those in the smooth-walled and baffled-walled spinner flask bioreactors. During the late aggregation phase, the kinetics of aggregation in the wavy-walled bioreactor were approximately 45% and 65% faster, respectively, than in the smooth-walled and baffled-walled spinner flasks. The observed improved kinetics of chondrocyte aggregation was obtained at no cost to the cell survival rate. Results of computerized image analysis suggest that chondrocyte aggregation occurred initially by the formation of new aggregates via cell-cell interactions and later by the joining of small aggregates into larger cell clumps. Aggregates appeared to grow for only a couple of hours in culture before reaching a steady size, possibly determined by limitations imposed by the hydrodynamic environment. These results suggest that the novel geometry of the wavy-walled bioreactor generates a hydrodynamic environment distinct from those traditionally used to culture engineered cartilage. Such differences may be useful in studies aimed at distinguishing the effects of the hydrodynamic environment on tissue-engineered cartilage. Characterizing the wavy-walled bioreactor's hydrodynamic environment and its effects on cartilage cell/tissue culture can help establish direct relationships between hydrodynamic forces and engineered tissue properties.

Concentration gradients associated with the in vitro cultivation of engineered tissues that are vascularized in vivo result in the formation of only a thin peripheral tissue-like region (e.g., ∼100 μm for engineered cardiac muscle) around a relatively cell-free interior. We previously demonstrated that diffusional gradients within engineered cardiac constructs can be minimized by direct perfusion of culture medium through the construct. In the present study, we measured the effects of medium perfusion rate and local oxygen concentration (p) on the in vitro reconstruction of engineered cardiac muscle. Neonatal rat cardiomyocytes were seeded onto biodegradable polymer scaffolds (fibrous discs, 1.1 cm diameter × 2 mm thick, made of polyglycolic acid, 24 × 106 cells per scaffold). The resulting cell-polymer constructs were cultured for a total of 12 days in serially connected cartridges (n = 1–8), each containing one construct directly perfused with culture medium at a flow rate of 0.2–3.0 mL/min. In all groups, oxygen concentration decreased due to cell respiration, and depended on construct position in the series and medium flow rate. Higher perfusion rates and higher p correlated with more aerobic cell metabolism, and higher DNA and protein contents. Constructs cultured at p of 160 mm Hg had 50% higher DNA and protein contents, markedly higher expression of sarcomeric α-actin, better organized sarcomeres and cell junctions, and 4.5-fold higher rate of cell respiration as compared to constructs cultured at p of 60 mm Hg. Contraction rates of the corresponding cardiac cell monolayers were 40% higher at p of 160 than 60 mm Hg. The control of oxygen concentration in cell microenvironment can thus improve the structure and function of engineered cardiac muscle. Experiments of this kind can form a basis for controlled studies of the effects of oxygen on the in vitro development of engineered tissues. © 2002 Wiley Periodicals, Inc. Biotechnol Bioeng 78: 617–625, 2002.