Chandipura viral glycoprotein (CNV-G) promotes Gectosome generation and enables delivery of intracellular therapeutics.

Overexpression of vesicular stomatitis virus G protein (VSV-G) elevates the secretion of EVs known as gectosomes, which contain VSV-G. Such vesicles can be engineered to deliver therapeutic macromolecules. We investigated viral glycoproteins from several viruses for their potential in gectosome production and intracellular cargo delivery. Expression of the viral glycoprotein (viral glycoprotein from the Chandipura virus [CNV-G]) from the human neurotropic pathogen Chandipura virus in 293T cells significantly augments the production of CNV-G-containing gectosomes. In comparison with VSV-G gectosomes, CNV-G gectosomes exhibit heightened selectivity toward specific cell types, including primary cells and tumor cell lines. Consistent with the differential tropism between CNV-G and VSV-G gectosomes, cellular entry of CNV-G gectosome is independent of the Low-density lipoprotein receptor, which is essential for VSV-G entry, and shows varying sensitivity to pharmacological modulators. CNV-G gectosomes efficiently deliver diverse intracellular cargos for genomic modification or responses to stimuli in vitro and in the brain of mice in vivo utilizing a split GFP and chemical-induced dimerization system. Pharmacokinetics and biodistribution analyses support CNV-G gectosomes as a versatile platform for delivering macromolecular therapeutics intracellularly.

Mechanics of 3D Cell-Hydrogel Interactions: Experiments, Models, and Mechanisms.

Hydrogels are highly water-swollen molecular networks that are ideal platforms to create tissue mimetics owing to their vast and tunable properties. As such, hydrogels are promising cell-delivery vehicles for applications in tissue engineering and have also emerged as an important base for ex vivo models to study healthy and pathophysiological events in a carefully controlled three-dimensional environment. Cells are readily encapsulated in hydrogels resulting in a plethora of biochemical and mechanical communication mechanisms, which recapitulates the natural cell and extracellular matrix interaction in tissues. These interactions are complex, with multiple events that are invariably coupled and spanning multiple length and time scales. To study and identify the underlying mechanisms involved, an integrated experimental and computational approach is ideally needed. This review discusses the state of our knowledge on cell-hydrogel interactions, with a focus on mechanics and transport, and in this context, highlights recent advancements in experiments, mathematical and computational modeling. The review begins with a background on the thermodynamics and physics fundamentals that govern hydrogel mechanics and transport. The review focuses on two main classes of hydrogels, described as semiflexible polymer networks that represent physically cross-linked fibrous hydrogels and flexible polymer networks representing the chemically cross-linked synthetic and natural hydrogels. In this review, we highlight five main cell-hydrogel interactions that involve key cellular functions related to communication, mechanosensing, migration, growth, and tissue deposition and elaboration. For each of these cellular functions, recent experiments and the most up to date modeling strategies are discussed and then followed by a summary of how to tune hydrogel properties to achieve a desired functional cellular outcome. We conclude with a summary linking these advancements and make the case for the need to integrate experiments and modeling to advance our fundamental understanding of cell-matrix interactions that will ultimately help identify new therapeutic approaches and enable successful tissue engineering.

The effects of prostaglandin E2 on gene expression of IDG-SW3-derived osteocytes in 2D and 3D culture.

Prostaglandin E2 (PGE2) is a key signaling molecule produced by osteocytes in response to mechanical loading, but its effect on osteocytes is less understood. This work examined the effect of PGE2 on IDG-SW3-derived osteocytes in standard 2D culture (collagen-coated tissue culture polystyrene) and in a 3D degradable poly(ethylene glycol) hydrogel. IDG-SW3 cells were differentiated for 35 days into osteocytes in 2D and 3D cultures. 3D culture led to a more mature osteocyte phenotype with 100-fold higher Sost expression. IDG-SW3-derived osteocytes were treated with PGE2 and assessed for expression of genes involved in PGE2, anabolic, and catabolic signaling. In 2D, PGE2 had a rapid (1 h) and sustained (24 h) effect on many PGE2 signaling genes, a rapid stimulatory effect on Il6, and a sustained inhibitory effect on Tnfrsf11b and Bglap. Comparing culture environment without PGE2, osteocytes had higher expression of all four EP receptors and Sost but lower expression of Tnfrsf11b, Bglap, and Gja1 in 3D. Osteocytes were more responsive to PGE2 in 3D. With increasing PGE2, 3D led to increased Gja1 and decreased Sost expressions and a higher Tnfrsf11b/Tnfsf11 ratio, indicating an anabolic response. Further analysis in 3D revealed that EP4, the receptor implicated in PGE2 signaling in bone, was not responsible for the PGE2-induced gene expression changes in osteocytes. In summary, osteocytes are highly responsive to PGE2 when cultured in an in vitro 3D hydrogel model suggesting that autocrine and paracrine PGE2 signaling in osteocytes may play a role in bone homeostasis.

Hydrolytically degradable Poly (ß-amino ester) resins with tunable degradation for 3D printing by projection micro-stereolithography.

Applications of 3D printing that range from temporary medical devices to environmentally responsible manufacturing would benefit from printable resins that yield polymers with controllable material properties and degradation behavior. Towards this goal, poly(ß-amino ester) (PBAE)-diacrylate resins were investigated due to the wide range of available chemistries and tunable material properties. PBAE-diacrylate resins were synthesized from hydrophilic and hydrophobic chemistries and with varying electron densities on the ester bond to provide control over degradation. Hydrophilic PBAE-diacrylates led to degradation behaviors characteristic of bulk degradation while hydrophobic PBAE-diacrylates led to degradation behaviors dominated initially by surface degradation and then transitioned to bulk degradation. Depending on chemistry, the crosslinked PBAE-polymers exhibited a range of degradation times under accelerated conditions, from complete mass loss in 90 min to minimal mass loss at 45 days. Patterned features with 55 µm resolution were achieved across all resins, but their fidelity was dependent on PBAE-diacrylate molecular weight, reactivity, and printing parameters. In summary, simple chemical modifications in the PBAE-diacrylate resins coupled with projection microstereolithography enables high resolution 3D printed parts with similar architectures and initial properties, but widely different degradation rates and behaviors.

Effects of Kinetic Chain Length on the Degradation of Poly(ß-amino ester)-Based Networks and Use in 3D Printing by Projection Microstereolithography.

Poly(ß-amino ester)-diacrylates (PBAE-dAs) are promising resins for three-dimensional (3D) printing. This study investigated the degradation of two PBAEs with different chemistries and kinetic chain lengths. PBAE-dA monomers were synthesized from benzhydrazide and poly(ethylene glycol) (A6) or butanediol (B6) diacrylate and then photopolymerized with pentaerythritol tetrakis(3-mercaptopropionate), which formed thiol-polyacrylate kinetic chains. This tetrathiol acts as a cross-linker and chain-transfer agent that controls the polyacrylate kinetic chain length. A6 networks exhibited bulk degradation, while B6 networks exhibited surface degradation, which transitioned to a combined surface and bulk degradation. Increasing the tetrathiol concentration shortened the polyacrylate kinetic chain and time-to-reverse gelation but degradation mode was unaffected. Hydrolysis occurred primarily through the ß-amino ester. As network hydrophilicity increased, the slower degrading ester in the thiol-polyacrylate chains contributed to degradation. Overall, this work demonstrates control over network degradation rate, mode of degradation, and time-to-reverse gelation in PBAE networks and their application in 3D printing.

The Effects of Stably Tethered BMP-2 on MC3T3-E1 Preosteoblasts Encapsulated in a PEG Hydrogel.

Bone morphogenetic protein-2 (BMP-2) is a clinically used osteoinductive growth factor. With a short half-life and side effects, alternative delivery approaches are needed. This work examines thiolation of BMP-2 for chemical attachment to a poly(ethylene glycol) hydrogel using thiol-norbornene click chemistry. BMP-2 retained bioactivity post-thiolation and was successfully tethered into the hydrogel. To assess tethered BMP-2 on osteogenesis, MC3T3-E1 preosteoblasts were encapsulated in matrix metalloproteinase (MMP)-sensitive hydrogels containing RGD and either no BMP-2, soluble BMP-2 (5 nM), or tethered BMP-2 (40-200 nM) and cultured in a chemically defined medium containing dexamethasone for 7 days. The hydrogel culture supported MC3T3-E1 osteogenesis regardless of BMP-2 presentation, but tethered BMP-2 augmented the osteogenic response, leading to significant increases in osteomarkers, Bglap and Ibsp. The ratio, Ibsp-to-Dmp1, highlighted differences in the extent of differentiation, revealing that without BMP-2, MC3T3-E1 cells showed a higher expression of Dmp1 (low ratio), but an equivalent expression with tethered BMP-2 and more abundant bone sialoprotein. In addition, this work identified that dexamethasone contributed to Ibsp expression but not Bglap or Dmp1 and confirmed that tethered BMP-2 induced the BMP canonical signaling pathway. This work presents an effective method for the modification and incorporation of BMP-2 into hydrogels to enhance osteogenesis.

Synthesis and Characterization of Click Nucleic Acid Conjugated Polymeric Microparticles for DNA Delivery Applications.

Microparticle-mediated nucleic acid delivery is a popular strategy to achieve therapeutic outcomes via antisense gene therapy. However, current methods used to fabricate polymeric microparticles suffer from suboptimal properties such as particle polydispersity and low encapsulation efficiency. Here, a new particulate delivery system based on step-growth thiol-Michael dispersion polymerization is reported in which a low polydispersity microparticle is functionalized with a synthetic nucleic acid mimic, namely, click nucleic acids (CNA). CNA oligomers, exhibiting an average length of approximately four nucleic acid repeat units per chain for both adenine and thymine bases, were successfully conjugated to excess thiols present in the microparticles. Effective DNA loading was obtained by simple mixing, and up to 6 ± 2 pmol of complementary DNA/mg of particle was achieved, depending on the length of DNA used. In addition, DNA loading was orders of magnitude less for noncomplementary sequences and sequences containing an alternating base mismatch. The DNA release properties were evaluated, and it was found that release could be triggered by sudden changes in temperature but was unaffected over a range of pH. Finally, phagocytosis of loaded microparticles was observed by confocal microscopy and corroborated by an increase in cellular metabolic activity up to 90%. Overall, this work suggests that CNA functionalized microparticles could be a promising platform for controlled DNA delivery.

Desire to eat and intake of 'insect' containing food is increased by a written passage: The potential role of familiarity in the amelioration of novel food disgust.

Over two studies we investigated the effect of various written interventions (passages) on the disgust response towards a food (falafels) which supposedly contained mealworm (insect) flour. Actually, participants (Study 1 N = 80, Study 2 N = 78) were given the same non-mealworm containing food in all conditions. Disgust was measured using: tactile sensitivity, food intake, liking and desire to eat. Results of Study 1 showed that a sustainability passage (sustainability advantages of entomophagy), but not a delicacy passage (oro-sensory qualities of insects), was effective in reducing disgust. In Study 2, contrary to prediction, a passage describing the sustainability and nutritional advantages entomophagy failed to reduce disgust - falafel intake, liking and desire to eat were decreased. However, a passage which described how mealworm flour is produced, did significantly reduce disgust. Taken together, these studies demonstrate that written passages can alter the disgust response, notably resulting in a maintenance of food intake. Interventions that increase the perception of familiarity of a novel food, but not logic-based arguments, may be a key driver of the amelioration of disgust. These results also support the suggestion that altering the ideational component of disgust can result in changes of distaste perception.

Death caused by a domestic pig attack.

The primary medico-legal investigation goal in deaths due to animal attacks is distinguishing between animal-related injuries and potential homicidal wounds. We report the case of a 49-year-old male found dead in his farm's pigsty, where a sow and her piglets were present. At the postmortem examination, numerous, severe blunt force injuries were observed on the body, with special regard to the upper extremities where massive injuries involving soft tissues, bones, and regional vessels, tendons, and nerves were present. The death resulted from severe bleeding from massive upper extremities injuries due to a domestic pig attack. Domestic pigs are usually placid but they can become aggressive if disturbed and attack humans producing severe injuries due to trampling, kicking and biting. Knowing the relevant anatomy, pattern of attack, and morphologies of wounds produced by particular animals can distinguish animal attacks from homicides, as well as attempt to identify the type of animal involved in an unwitnessed attack.

Photo-tunable hydrogel mechanical heterogeneity informed by predictive transport kinetics model.

Understanding the three-dimensional (3D) mechanical and chemical properties of distinctly different, adjacent biological tissues is crucial to mimicking their complex properties with materials. 3D printing is a technique often employed to spatially control the distribution of the biomaterials, such as hydrogels, of interest, but it is difficult to print both mechanically robust (high modulus and toughness) and biocompatible (low modulus) hydrogels in a single structure. Moreover, due to the fast diffusion of mobile species during printing and nonequilibrium swelling conditions of low-solids-content hydrogels, it is challenging to form the high-fidelity structures required to mimic tissues. Here a predictive transport and swelling model is presented to model these effects and then is used to compensate for these effects during printing. This model is validated experimentally by photopatterning spatially distinct hydrogel elastic moduli using a single photo-tunable poly(ethylene glycol) (PEG) pre-polymer solution by sequentially patterning and in-diffusing fresh pre-polymer for further polymerization.

Assessment and prevention of cartilage degeneration surrounding a focal chondral defect in the porcine model.

Focal defects in articular cartilage are unable to self-repair and, if left untreated, are a leading risk factor for osteoarthritis. This study examined cartilage degeneration surrounding a defect and then assessed whether infilling the defect prevents degeneration. We created a focal chondral defect in porcine osteochondral explants and cultured them ex vivo with and without dynamic compressive loading to decouple the role of loading. When compared to a defect in a porcine knee four weeks post-injury, this model captured loss in sulfated glycosaminoglycans (sGAGs) along the defect's edge that was observed in vivo, but this loss was not load dependent. Loading, however, reduced the indentation modulus of the surrounding cartilage. After infilling with in situ polymerized hydrogels that were soft (100 kPa) or stiff (1 MPa) and which produced swelling pressures of 13 and 310 kPa, respectively, sGAG loss was reduced. This reduction correlated with increased hydrogel stiffness and swelling pressure, but was not affected by loading. This ex vivo model recapitulates sGAG loss surrounding a defect and, when infilled with a mechanically supportive hydrogel, degeneration is minimized.

A comparison of human mesenchymal stem cell osteogenesis in poly(ethylene glycol) hydrogels as a function of MMP-sensitive crosslinker and crosslink density in chemically defined medium.

This study investigated osteogenesis of human mesenchymal stem cells encapsulated in matrix-metalloproteinase (MMP)-sensitive poly(ethylene glycol) (PEG) hydrogels in chemically defined medium (10 ng/ml bone morphogenic factor-2). Thiol-norbornene photoclick hydrogels were formed with CRGDS and crosslinkers of PEG dithiol (nondegradable), CVPLS-LYSGC (P1) or CRGRIGF-LRTDC (P2; dash indicates cleavage site) at two crosslink densities. Exogenous MMP-2 degraded P1 and P2 hydrogels similarly. MMP-14 degraded P1 hydrogels more rapidly than P2 hydrogels. Cell spreading was greatest in P1 low crosslinked hydrogels and to a lesser degree in P2 low crosslinked hydrogels, but not evident in nondegradable and high crosslinked MMP-sensitive hydrogels. Early osteogenesis (Alkaline phosphatase [ALP] activity) was accelerated in hydrogels that facilitated cell spreading. Contrarily, late osteogenesis (mineralization) was independent of cell spreading. Mineralized matrix was present in P1 hydrogels, but only present in P2 high crosslinked hydrogels and not yet present in nondegradable hydrogels. Overall, the low crosslinked P1 hydrogels exhibited an accelerated early and late osteogenesis with the highest ALP activity (Day 7), greatest calcium content (Day 14), and greatest collagen content (Day 28), concomitant with increased compressive modulus over time. Collectively, this study demonstrates that in chemically defined medium, hydrogel degradability is critical to accelerating early osteogenesis, but other factors are important in late osteogenesis.

Effects of cell adhesion motif, fiber stiffness, and cyclic strain on tenocyte gene expression in a tendon mimetic fiber composite hydrogel.

We recently developed a fiber composite consisting of tenocytes seeded onto discontinuous fibers embedded within a hydrogel, designed to mimic physiological tendon micromechanics of tension and shear. This study examined if cell adhesion peptide (DGEA or YRGDS), fiber modulus (50 or 1300 kPa) and/or cyclic strain (5% strain, 1 Hz) influenced bovine tenocyte gene expression. Ten genes were analyzed and none were sensitive to peptide or fiber modulus in the absence of cyclic tensile strain. Genes associated with tendon (SCX and TNMD), collagens (COL1A1, COL3A1, COL11A1), and matrix remodelling (MMP1, MMP2, and TIMP3) were insensitive to cyclic strain. Contrarily, cyclic strain up-regulated IL6 by 30-fold and MMP3 by 10-fold in soft YRGDS fibers. IL6 expression in soft YRGDS fibers was 5.7 and 3.3-fold greater than in soft DGEA fibers and stiff RGD fibers, respectively, under cyclic strain. Our findings suggest that changes in the surrounding matrix can influence catabolic genes in tenocytes when cultured in a complex strain environment mimicking that of tendon, while having minimal effects on tendon and homeostatic genes.

Cytocompatibility and Cellular Internalization of PEGylated "Clickable" Nucleic Acid Oligomers.

The recently developed synthetic oligonucleotides referred to as "click" nucleic acids (CNAs) are promising due to their relatively simple synthesis based on thiol-X reactions with numerous potential applications in biotechnology, biodetection, gene silencing, and drug delivery. Here, the cytocompatibility and cellular uptake of rhodamine tagged, PEGylated CNA copolymers (PEG-CNA-RHO) were evaluated. NIH 3T3 fibroblast cells treated for 1 h with 1, 10, or 100 µg/mL PEG-CNA-RHO maintained an average cell viability of 86%, which was not significantly different from the untreated control. Cellular uptake of PEG-CNA-RHO was detected within 30 s, and the amount internalized increased over the course of 1 h. Moreover, these copolymers were internalized within cells to a higher degree than controls consisting of either rhodamine tagged PEG or the rhodamine alone. Uptake was not affected by temperature (i.e., 4 or 37 °C), suggesting a passive uptake mechanism. Subcellular colocalization analysis failed to indicate significant correlations between the internalized PEG-CNA-RHO and the organelles examined (mitochondria, endoplasmic reticulum, endosomes and lysosomes). These results indicate that CNA copolymers are cytocompatible and are readily internalized by cells, supporting the idea that CNAs are a promising alternative to DNA in antisense therapy applications.

Zwitterionic PEG-PC Hydrogels Modulate the Foreign Body Response in a Modulus-Dependent Manner.

Reducing the foreign body response (FBR) to implanted biomaterials will enhance their performance in tissue engineering. Poly(ethylene glycol) (PEG) hydrogels are increasingly popular for this application due to their low cost, ease of use, and the ability to tune their compliance via molecular weight and cross-linking densities. PEG hydrogels can elicit chronic inflammation in vivo, but recent evidence has suggested that extremely hydrophilic, zwitterionic materials and particles can evade the immune system. To combine the advantages of PEG-based hydrogels with the hydrophilicity of zwitterions, we synthesized hydrogels with comonomers PEG and the zwitterion phosphorylcholine (PC). Recent evidence suggests that stiff hydrogels elicit increased immune cell adhesion to hydrogels, which we attempted to reduce by increasing hydrogel hydrophilicity. Surprisingly, hydrogels with the highest amount of zwitterionic comonomer elicited the highest FBR. Lowering the hydrogel modulus (165 to 3 kPa), or PC content (20 to 0 wt %), mitigated this effect. A high density of macrophages was found at the surface of implants associated with a high FBR, and mass spectrometry analysis of the proteins adsorbed to these gels implicated extracellular matrix, immune response, and cell adhesion protein categories as drivers of macrophage recruitment. Overall, we show that modulus regulates macrophage adhesion to zwitterionic-PEG hydrogels, and demonstrate that chemical modifications to hydrogels should be studied in parallel with their physical properties to optimize implant design.

Characterization of Somatically-Eliminated Genes During Development of the Sea Lamprey (Petromyzon marinus).

The sea lamprey (Petromyzon marinus) is a basal vertebrate that undergoes developmentally programmed genome rearrangements (PGRs) during early development. These events facilitate the elimination of ∼20% of the genome from the somatic cell lineage, resulting in distinct somatic and germline genomes. Thus far only a handful of germline-specific genes have been definitively identified within the estimated 500 Mb of DNA that is deleted during PGR, although a few thousand germline-specific genes are thought to exist. To improve our understanding of the evolutionary/developmental logic of PGR, we generated computational predictions to identify candidate germline-specific genes within a new transcriptomic dataset derived from adult germline and the early embryonic stages during which PGR occurs. Follow-up validation studies identified 44 germline-specific genes and further characterized patterns of transcription and DNA loss during early embryogenesis. Expression analyses reveal that many of these genes are differentially expressed during early embryogenesis and presumably function in the early development of the germline. Ontology analyses indicate that many of these germline-specific genes play known roles in germline development, pluripotency, and oncogenesis (when misexpressed). These studies provide support for the theory that PGR serves to segregate molecular functions related to germline development/pluripotency in order to prevent their potential misexpression in somatic cells. This larger set of eliminated genes also allows us to extend the evolutionary/developmental breadth of this theory, as some deleted genes (or their gnathostome homologs) appear to be associated with the early development of somatic lineages, perhaps through the evolution of novel functions within gnathostome lineages.

Characterization of the chondrocyte secretome in photoclickable poly(ethylene glycol) hydrogels.

Poly(ethylene glycol) (PEG) hydrogels are highly tunable platforms that are promising cell delivery vehicles for chondrocytes and cartilage tissue engineering. In addition to characterizing the type of extracellular matrix (ECM) that forms, understanding the types of proteins that are secreted by encapsulated cells may be important. Thus, the objectives for this study were to characterize the secretome of chondrocytes encapsulated in PEG hydrogels and determine whether the secretome varies as a function of hydrogel stiffness and culture condition. Bovine chondrocytes were encapsulated in photoclickable PEG hydrogels with a compressive modulus of 8 and 46 kPa and cultured under free swelling or dynamic compressive loading conditions. Cartilage ECM deposition was assessed by biochemical assays and immunohistochemistry. The conditioned medium was analyzed by liquid chromatography-tandem mass spectrometry. Chondrocytes maintained their phenotype within the hydrogels and deposited cartilage-specific ECM that increased over time and included aggrecan and collagens II and VI. Analysis of the secretome revealed a total of 64 proteins, which were largely similar among all experimental conditions. The identified proteins have diverse functions such as biological regulation, response to stress, and collagen fibril organization. Notably, many of the proteins important to the assembly of a collagen-rich cartilage ECM were identified and included collagen types II(α1), VI (α1, α2, and α3), IX (α1), XI (α1 and α2), and biglycan. In addition, many of the other identified proteins have been reported to be present within cell-secreted exosomes. In summary, chondrocytes encapsulated within photoclickable PEG hydrogels secrete many types of proteins that diffuse out of the hydrogel and which have diverse functions, but which are largely preserved across different hydrogel culture environments. Biotechnol. Bioeng. 2017;114: 2096-2108. © 2017 Wiley Periodicals, Inc.

Heterogeneity is key to hydrogel-based cartilage tissue regeneration.

Degradable hydrogels have been developed to provide initial mechanical support to encapsulated cells while facilitating the growth of neo-tissues. When cells are encapsulated within degradable hydrogels, the process of neo-tissue growth is complicated by the coupled phenomena of transport of large extracellular matrix macromolecules and the rate of hydrogel degradation. If hydrogel degradation is too slow, neo-tissue growth is hindered, whereas if it is too fast, complete loss of mechanical integrity can occur. Therefore, there is a need for effective modelling techniques to predict hydrogel designs based on the growth parameters of the neo-tissue. In this article, hydrolytically degradable hydrogels are investigated due to their promise in tissue engineering. A key output of the model focuses on the ability of the construct to maintain overall structural integrity as the construct transitions from a pure hydrogel to engineered neo-tissue. We show that heterogeneity in cross-link density and cell distribution is the key to this successful transition and ultimately to achieve tissue growth. Specifically, we find that optimally large regions of weak cross-linking around cells in the hydrogel and well-connected and dense cell clusters create the optimum conditions needed for neo-tissue growth while maintaining structural integrity. Experimental observations using cartilage cells encapsulated in a hydrolytically degradable hydrogel are compared with model predictions to show the potential of the proposed model.

In Vitro and In Vivo Models for Assessing the Host Response to Biomaterials.

The foreign body response (FBR) occurs ubiquitously to essentially all non-biological materials that are implanted into higher organisms. The FBR is characterized by inflammation followed by fibrosis and is mediated largely by macrophages. While many current medical devices tolerate the FBR, the FBR is responsible for many asceptic device failures and is hindering advancements of new devices that rely on device-host communication to function. To this end, in vitro and in vivo models are critical to studying how a biomaterial, via its chemistry and properties, affect the FBR. This short review highlights the main in vitro and in vivo models that are used to study the FBR. In vitro models that capture macrophage interrogation of a biomaterial and evaluation of macrophage attachment, polarization and fusion are described. In vivo models using rodents, which provide a relatively simple model of the complex FBR process, and human-relevant nonhuman primate models are described. Collectively, the combination of in vitro and in vivo models will help advance our fundmental understanding of the FBR and enable new biomaterials to be developed that can effectively modulate the FBR to achieve a desire device-host outcome.

Tuning tissue growth with scaffold degradation in enzyme-sensitive hydrogels: a mathematical model.

Despite tremendous advances in the field of tissue engineering, a number of obstacles remain that hinder its successful translation to the clinic. One challenge that relates to the use of cells encapsulated in a hydrogel is identifying a hydrogel design that can provide an appropriate environment for cells to successfully synthesize and deposit new matrix molecules while providing a mechanical support that can resist physiological loads at the early stage of implementation. A solution to this problem has been to balance tissue growth and hydrogel degradation. However, identifying this balance is difficult due to the complexity of coupling diffusion, deposition, and degradation mechanisms. Very little is known about the complex behavior of these mechanisms, emphasizing the need for a rigorous mathematical approach that can assist and guide experimental advances. To address this issue, this paper discusses a model for interstitial growth based on mixture theory, that can capture the coupling between cell-mediated hydrogel degradation (i.e., hydrogels containing enzyme-sensitive crosslinks) and the transport of extracellular matrix (ECM) molecules released by encapsulated cells within a hydrogel. Taking cartilage tissue engineering as an example, the model investigates the role of enzymatic degradation on ECM diffusion and its impact on two important outcomes: the extent of ECM transport (and deposition) and the evolution of the hydrogel's mechanical integrity. Numerical results based on finite element analysis show that if properly tuned, enzymatic degradation yields the appearance of a highly localized degradation front propagating away from the cell, which can be immediately followed by a front of growing neotissue. We show that this situation is key to maintaining mechanical properties (e.g., stiffness) while allowing for deposition of new ECM molecules. Overall, our study suggests a hydrogel design that could enable successful tissue engineering (e.g., of cartilage, bone, etc.) where mechanical integrity is important.