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PubMed2001

Tissue engineering research in oral implant surgery.

In this article, we introduce some of the more extensively evaluated technologies using concepts of tissue engineering. We report on hard tissue engineering and soft tissue engineering and their utility for dental implant therapy. For hard tissue engineering, we evaluated human recombinant bone morphogenetic protein-2 and marrow mesenchymal stem cells using a model of sinus augmentation procedure in rabbit. We also describe distraction osteogenesis as another category for hard tissue engineering. In addition, we evaluate soft tissue management using cultured epithelial grafting for soft tissue engineering. The results of our tissue regeneration materials and methods in this study are positive. When the tissue engineering materials are used in clinics in the future, implant surgery could be the leading field.

Absorbable Implants↗
PubMed2000

Combining chondrocytes and smooth muscle cells to engineer hybrid soft tissue constructs.

Engineering new tissues using cell transplantation may provide a valuable tool for reconstructive surgery applications. Chondrocyte transplantation in particular has been successfully used to engineer new tissue masses due to the low metabolic requirements of these cells. However, the engineered cartilaginous tissue is too rigid for many soft tissue applications. We propose that hybrid tissue engineered from chondrocytes and smooth muscle cells could reflect mechanical properties intermediate between these two cell types. In this study, rat aortic smooth muscle cells and pig auricular chondrocytes were co-cultured on polyglycolic acid fiber-based matrices to address this hypothesis. Mixed cell suspensions were seeded by agitating the polymer matrices and a cell suspension with an orbital shaker. After seeding, cell-polymer constructs were cultured in stirred bioreactors for 8 weeks. The cell density and extracellular matrix (collagen, elastin, and glycosaminoglycan) content of the engineered tissues were determined biochemically. After 8 weeks in culture, the hybrid tissue had a high cell density (5.8 x 108 cells/cm(3)), and elastin (519 microg/g wet tissue sample), collagen (272 microg/g wet tissue sample), and glycosaminoglycan (GAG; 10 microg/g wet tissue sample) content. Mechanical testing indicated the compressive modulus of the hybrid tissues after 8 weeks to be 40.8 +/- 4.1 kPa and the equilibrium compressive modulus to be 8.4 +/- 0.8 kPa. Thus, these hybrid tissues exhibited intermediate stiffness; they were less stiff than native cartilage but stiffer than native smooth muscle tissue. This tissue engineering approach may be useful to engineer tissues for a variety of reconstructive surgery applications.

Animals↗
PubMed2002

Three-dimensional tissue engineering of hyaline cartilage: comparison of adult nasal and articular chondrocytes.

Adult chondrocytes are less chondrogenic than immature cells, yet it is likely that autologous cells from adult patients will be used clinically for cartilage engineering. The aim of this study was to compare the postexpansion chondrogenic potential of adult nasal and articular chondrocytes. Bovine or human chondrocytes were expanded in monolayer culture, seeded onto polyglycolic acid (PGA) scaffolds, and cultured for 40 days. Engineered cartilage constructs were processed for histological and quantitative analysis of the extracellular matrix and mRNA. Some engineered constructs were implanted in athymic mice for up to six additional weeks before analysis. Using adult bovine tissues as a cell source, nasal chondrocytes generated a matrix with significantly higher fractions of collagen type II and glycosaminoglycans as compared with articular chondrocytes. Human adult nasal chondrocytes proliferated approximately four times faster than human articular chondrocytes in monolayer culture, and had a markedly higher chondrogenic capacity, as assessed by the mRNA and protein analysis of in vitro-engineered constructs. Cartilage engineered from human nasal cells survived and grew during 6 weeks of implantation in vivo whereas articular cartilage constructs failed to survive. In conclusion, for adult patients nasal septum chondrocytes are a better cell source than articular chondrocytes for the in vitro engineering of autologous cartilage grafts. It remains to be established whether cartilage engineered from nasal cells can function effectively when implanted at an articular site.

Animals↗
PubMed2000

Tissue engineering of artificial organs.

Tissue engineering efforts are currently being undertaken for every type of tissue and organ within the urinary system. Most of the effort expended to engineer genitourinary tissues has occurred within the last decade. Tissue engineering techniques require expertise in growth factor biology, a cell culture facility designed for human application, and personnel who have mastered the techniques of cell harvest, culture, and expansion. Polymer scaffold design and manufacturing resources are essential for the successful application of this technology. In order to apply these engineering techniques to humans, further studies need to be performed with many of the tissues described. The first human application of cell-based tissue engineering technology for urologic applications took place at our institution, with the injection of autologous cells for the correction of vesicoureteral reflux in children. The same technology has been expanded to treat adult patients with urinary incontinence. Trials of urethral tissue replacement with processed collagen matrices are in progress, and bladder replacement using tissue engineering techniques are currently being arranged. Recent progress suggests that engineered urologic tissues may have clinical applicability in the future.

Artificial Organs↗
PubMed2002

Autologous penile corpora cavernosa replacement using tissue engineering techniques.

PURPOSE: The availability of engineered tissues would be beneficial to patients undergoing penile reconstruction. We explored the possibility of replacing an entire cross-sectional segment of both corporal bodies with autologous engineered tissues in rabbits, and investigated the structural and functional integrity of the neo-corpora. MATERIALS AND METHODS: Acellular corporal collagen matrices were obtained from donor rabbit penis. Autologous corpus cavernosal smooth muscle and endothelial cells were harvested, expanded and seeded on the matrices. An entire cross-sectional segment of protruding rabbit phallus was excised, leaving the urethra intact. A total of 26 matrices, including 18 seeded with cells and 8 without cells, were interposed into the excised corporal space. An additional 4 rabbits that did not undergo surgical intervention served as normal controls. Functional and structural parameters (cavernosography, cavernosometry, mating behavior and sperm ejaculation) were followed for 6 months. Gross examination, and histochemical, immunocytochemical and Western blot analyses were performed at 3 and 6 months after implantation. RESULTS: The experimental corporal bodies demonstrated intact structural integrity on cavernosography and decreased maximal intracavernosal pressures on cavernosometry compared to normal controls. Mating activity in animals with engineered corpora normalized by 3 months postoperatively. The presence of sperm was confirmed during mating and was present in all rabbits with engineered corpora but in only 2 with the matrix alone. Histologically sinusoidal spaces and walls lined with endothelial and smooth muscle cells were observed in the engineered grafts. Each cell type was identified immunocytochemically. Grafts without cells contained fibrotic tissue and calcifications with sparse corporal elements. Western blot analysis of engineered grafts showed nitric oxide synthase activity similar to normal controls. CONCLUSIONS: Autologous corpus cavernosal smooth muscle and endothelial cells seeded on collagen matrices can form corpora cavernosa tissue structures in a rabbit model. Engineered corpora cavernosa achieved adequate structural and functional parameters. This technology may be applicable to patients who require additional tissue for phallic reconstruction.

Animals↗
PubMed2002

Bridging tendon defects using autologous tenocyte engineered tendon in a hen model.

Tendon defects remain a major concern in plastic surgery because of the limited availability of tendon autografts. Whereas immune rejection prohibits the use of tendon allografts, most prosthetic replacements also fail to achieve a satisfactory long-term result of tendon repair. The tissue engineering technique, however, can generate different tissues using autologous cells and thus may provide an optimal approach to address this concern. The purpose of this study was to test the feasibility of engineering tendon tissues with autologous tenocytes to bridge a tendon defect in either a tendon sheath open model or a partial open model in the hen. In a total of 40 Leghorn hens, flexor tendons were harvested from the left feet and were digested with 0.25% type II collagenase. The isolated tenocytes were expanded in vitro and mixed with unwoven polyglycolic acid fibers to form a cell-scaffold construct in the shape of a tendon. The constructs were wrapped with intestinal submucosa and then cultured in Dulbecco's Modified Eagle Medium plus 10% fetal bovine serum for 1 week before in vivo transplantation. On the feet, a defect of 3 to 4 cm was created at the second flexor digitorum profundus tendon by resecting a tendon fragment. The defects were bridged either with a cell-scaffold construct in the experimental group ( n= 20) or with scaffold material alone in the control group ( n= 20). Specimens were harvested at 8, 12, and 14 weeks postrepair for gross and histologic examination and for biomechanical analysis. In the experimental group, a cordlike tissue bridging the tendon defect was formed at 8 weeks postrepair. At 14 weeks, the engineered tendons resembled the natural tendons grossly in both color and texture. Histologic examination at 8 weeks showed that the neo-tendon contained abundant tenocytes and collagen; most collagen bundles were randomly arranged. The undegraded polyglycolic acid fibers surrounded by inflammatory cells were also observed. At 12 weeks, tenocytes and collagen fibers became longitudinally aligned, with good interface healing to normal tendon. At 14 weeks, the engineered tendons displayed a typical tendon structure hardly distinguishable from that of normal tendons. Biomechanical analysis demonstrated increased breaking strength of the engineered tendons with time, which reached 83 percent of normal tendon strength at 14 weeks. In the control group, polyglycolic acid constructs were mostly degraded at 8 weeks and disappeared at 14 weeks. However, the breaking strength of the scaffold materials accounted for only 9 percent of normal tendon strength. The results of this study indicated that tendon tissue could be engineered in vivo to bridge a tendon defect. The engineered tendons resembled natural tendons not only in gross appearance and histologic structure but also in biomechanical properties.

Animals↗
PubMed2002

Tissue engineering of the lymphatic system.

The field of tissue engineering has seen tremendous expansion in the last decade. In the last several years, tissue-engineering strategies to treat diseases of skin, cartilage, bone, bladder, blood vessel, tendon, and other tissues have been described. However, tissue-engineering approaches to treat diseases of the lymphatic system are currently nonexistent. We propose that acellular tissues, either native or engineered, could be exploited as a platform for the study of lymphatic biology, and for lymphatic tissue engineering. While speculative, this type of experimental model system could prove powerful for dissecting molecular and cellular events surrounding tumor invasion of lymphatics, as well as lymphangiogenesis. Scaffolds seeded with genetically engineered lymphatic cells could also be implanted to repopulate lymphatic vasculature. In the future, the lymphatic system will surely be added to the list of tissues and organs that prove amenable to tissue-engineering therapies.

Animals↗
PubMed2003

Tissue engineering therapy for cardiovascular disease.

The present treatments for the loss or failure of cardiovascular function include organ transplantation, surgical reconstruction, mechanical or synthetic devices, or the administration of metabolic products. Although routinely used, these treatments are not without constraints and complications. The emerging and interdisciplinary field of tissue engineering has evolved to provide solutions to tissue creation and repair. Tissue engineering applies the principles of engineering, material science, and biology toward the development of biological substitutes that restore, maintain, or improve tissue function. Progress has been made in engineering the various components of the cardiovascular system, including blood vessels, heart valves, and cardiac muscle. Many pivotal studies have been performed in recent years that may support the move toward the widespread application of tissue-engineered therapy for cardiovascular diseases. The studies discussed include endothelial cell seeding of vascular grafts, tissue-engineered vascular conduits, generation of heart valve leaflets, cardiomyoplasty, genetic manipulation, and in vitro conditions for optimizing tissue-engineered cardiovascular constructs.

Animals↗
PubMed2000

Ethical considerations for biomedical scientists and engineers: issues for the rank and file.

Biomedical science and engineering is inextricably linked with the fields of medicine and surgery. Yet, while physicians and surgeons, nurses, and other medical professionals receive instruction in ethics during their training and must abide by certain codes of ethics during their practice, those engaged in biomedical science and engineering typically receive no formal training in ethics. In fact, the little contact that many biomedical science and engineering professionals have with ethics occurs either when they participate in government-funded research or submit articles for publication in certain journals. Thus, there is a need for biomedical scientists and engineers as a group to become more aware of ethics. Moreover, recent advances in biomedical technology and the ever-increasing use of new devices virtually guarantee that biomedical science and engineering will become even more important in the future. Although they are rarely in direct contact with patients, biomedical scientists and engineers must become aware of ethics in order to be able to deal with the complex ethical issues that arise from our society's increasing reliance on biomedical technology. In this brief communication, the need for ethical awareness among workers in biomedical science and engineering is discussed in terms of certain conflicts that arise in the workaday world of the biomedical scientist in a complex, modern society. It is also recognized that inasmuch as workers in the many branches of bioengineering are not regulated like their counterparts in medicine and surgery, perhaps academic institutions and professional societies are best equipped to heighten ethical awareness among workers in this important field.

Authorship↗
PubMed1998

1998 ARRS President's Award. The potential of in vivo vascular tissue engineering for the treatment of vascular thrombosis: a preliminary report. American Roentgen Ray Society.

OBJECTIVE: Current gene therapy and tissue engineering protocols suffer from a number of inherent limitations. In this study, we examine the feasibility of a new approach for the treatment of vascular thrombosis: in vivo tissue engineering. MATERIALS AND METHODS: Rabbit femoral veins were transfected in situ with either a previously characterized adenoviral-construct-expressing tissue plasminogen activator or a viral (adenoviral-construct-expressing beta-galactosidase) or nonviral (buffer) control and used as cross sections (n = 3). Treated veins were then harvested and grafted into the ipsilateral common femoral artery as an interposition vein graft. A potent stimulus for thrombus formation was then introduced into the recipient artery downstream of the graft. Six days later, the rabbits were sacrificed, and the grafts and downstream arteries were harvested. Vessel segments were then examined for thrombus according to defined anatomic zones. Transfection efficiency and presence of smooth muscle cells in the vein graft were also evaluated. RESULTS: The engineered vein graft showed a significant reduction in thrombus formation within both the graft and the downstream artery relative to nonviral (buffer) and viral (adenoviral-Rous sarcoma virus beta-galactosidase [Adv/RSV-betagal]) controls. Underlying endothelial cell transfection efficiency of 90% was observed in viral controls (Adv/RSV-betagal). A 2.4-fold increase in smooth muscle alpha-actin positive cells in the engineered vein graft was seen compared with nonviral (phosphate-buffered saline) controls. A 10-fold increase in smooth muscle alpha-actin-positive cells in the engineered vein graft relative to viral (Adv/RSV-betagal) controls was also observed. CONCLUSION: In vivo tissue engineering is a new paradigm in molecular medicine that is a viable alternative to conventional gene therapy and tissue engineering for the treatment of vascular thrombosis.

Animals↗
PubMed2001

Tissue-engineered heart valve leaflets: an animal study.

BACKGROUND: Tissue-engineered heart valve leaflets are a promising way to overcome the inherent limitations of current prosthetic valves. The aim of this study was to compare the biological responses of an autologous cell seeded scaffold and an acellular scaffold implanted in the pulmonary valve leaflet in the same animal. METHODS: Myofibroblasts and endothelial cells were isolated and cultured from an ovine artery. A synthetic biodegradable scaffold consisting of polyglycolic acid and polylactic acid was initially seeded with the myofibroblasts, then coated with endothelial cells. Cells were seeded using a medium containing collagen and cultured. A tissue-engineered construct and a plain scaffold were implanted as double pulmonary valve leaflet replacement in the same animal in an ovine model (n=3). Additionally, the tissue-engineered construct (n=2) and the plain scaffold (n=2) were implanted as single valve leaflet replacements for long-term analysis. After sacrifice, the implanted valve leaflet tissues were retrieved, analyzed visually and using light microscopy. RESULTS: Three animals that underwent replacement of two valve leaflets with a tissue-engineered construct and a plain scaffold, survived only a short-time (12, 24, 36 hours). The death was attributed to heart failure caused by severe pulmonary insufficiency. Animals that underwent single valve leaflet replacement survived longer and were electively sacrificed at 6 and 9 weeks after operation. The analysis of the leaflets from the short-term survivors showed that the tissue-engineered constructs contained less fibrins and protein exudates than the plain scaffold. In contrast, leaflets obtained from animals surviving 6 and 9 weeks showed similar well organized granulation tissues in the tissue-engineered constructs and the plain scaffolds. CONCLUSION: This animal experiment demonstrates that in the early phase of implantation, the tissue-engineered construct shows a better biological response in terms of antithrombogenicity than the plain scaffold, although both of them have similar results in the later reparative phase.

Animals↗
PubMed2000

[Vegetation and erosion control: exploration on basic principle of slope engineering].

During its interaction with local climate and surface and shallow soil, vegetation appears to be of certain hydro-mechanical effects, and provides with some engineering properties on erosion control and slope protection. Slope eco-engineering integrates these properties with soil for slope protection projects. Along with the increasing awareness of the interactions between vegetation and soil and their significance in erosion control during the past few decades, this biological approach has been widely applied in physical environment improvement and engineered slope protection world wide. Some important contents in slope eco-engineering are the concept of "soil-vegetation system" and the related biological natures and engineering properties. The "soil-vegetation system" concept is a theoretical substance, and its eco-biological natures, engineering properties and some internal regulations are the theoretical hypothesis of the substance, which could be the basic materials for setting up the scientific principles of slope eco-engineering.

Agriculture↗
PubMed2001

[Fabrication of allogeneic cartilage with tissue engineering technique and repair of laryngeal cartilage defects in rabbits].

OBJECTIVE: To investigate the fabrication of allogeneic tissue engineered cartilage in predetermined shapes and its ability to repair laryngeal cartilage defects. METHODS: Tissue engineering technique was used to make allogeneic tissue engineered cartilage in predetermined shapes, and then some of them were utilized to repair cartilage defects in the rabbit larynges(n = 12). Finally, the chondrogenesis and reparative effect were studied grossly and histologically at various periods. RESULTS: 1. The gross specimens of 4 weeks after implantation revealed the presence of new hyaline cartilage of approximately the same shapes as original predetermined shapes, which had fair elasticity and support ability. Histological evaluation showed the characteristics of cartilage tissue. 2. Laryngeal cartilage defects were well repaired using allogeneic tissue engineered cartilage in different periods after operation. A few chondrocytes were observed between the reparative areas with tissue engineered cartilage and normal cartilage. Abundant matrices were also detected in the reparative and interfacial areas. No inflammatory cells were found. CONCLUSION: It is possible to regenerate new cartilage in predetermined shapes using tissue engineering technique in allograft animals with immunity. Allogeneic tissue engineered cartilage can be used as reparative material to repair cartilage defect of rabbit larynx.

Animals↗
PubMed2001

Chromosomal aberrations in peripheral lymphocytes of train engine drivers.

Studies of Swedish railway employees have indicated that railroad engine drivers have an increased cancer morbidity and incidence of chronic lymphatic leukemia. The drivers are exposed to relatively high magnetic fields (MF), ranging from a few to over a hundred microT. Although the possible genotoxic potential of MF is unclear, some earlier studies have indicated that occupational exposure to MF may increase chromosome aberrations in blood lymphocytes. Since an increased level of chromosomal aberrations has been suggested to predict elevated cancer risk, we performed a cytogenetic analysis on cultured (48 h) peripheral lymphocytes of Swedish train engine drivers. A pilot study of 18 engine drivers indicated a significant difference in the frequency of cells with chromosomal aberrations (gaps included or excluded) in comparison with seven concurrent referents (train dispatchers) and a control group of 16 office workers. The engine drivers had about four times higher frequency of cells with chromosome-type aberrations (excluding gaps) than the office workers (P < 0.01) and the dispatchers (P < 0.05). Seventy-eight percent of the engine drivers showed at least one cell per 100 with chromosome-type aberrations compared with 29% among the dispatchers and 31% among the office workers. In a follow-up study, another 30 engine drivers showed an increase (P < 0.05) in the frequency of cells with chromosome-type aberrations (gaps excluded) as compared with 30 referent policemen. Sixty percent of the engine drivers had one or more cells (per 100 cells) with chromosome-type aberrations compared with 30% among the policemen. In conclusion, the results of the two studies support the hypothesis that exposure to MF at mean intensities of 2-15 microT can induce chromosomal damage.

Adult↗
PubMed1998

Videofetoscopically assisted fetal tissue engineering: bladder augmentation.

BACKGROUND/PURPOSE: Treatment of several congenital anomalies is frequently hindered by lack of enough tissue for surgical reconstruction in the neonatal period. Minimally invasive harvest of fetal tissue, which is then processed through tissue engineering techniques in vitro while pregnancy is allowed to continue so that at delivery a newborn with a prenatally diagnosed congenital anomaly can benefit from having autologous, expanded tissue promptly available for surgical reconstruction at birth. This concept was applied to a bladder defect. METHODS: Bladder exstrophy was surgically created in ten 90- to 95-day gestation fetal lambs, which were divided in two groups. In group I, a small fetal bladder specimen was harvested through a minimally invasive technique (videofetoscopy). Urothelial and smooth muscle cells were then separately cultivated and expanded in vitro for 55 to 60 days, resulting in a total of approximately 200 million cells. Seven to 10 days before delivery, the cells were seeded in two layers in a 16- to 20-cm2, 3-mm thick biodegradable polyglycolic acid polymer matrix. One to 4 days after delivery, autologous engineered tissue was used for surgical augmentation of the exstrophic bladder. In group II, no harvest was performed, and the bladder exstrophy was primarily closed after delivery. In both groups, a catheter was left inside the bladder for 3 weeks, at which time a cystogram was performed and the catheter then removed. In all animals, at 60 days, another cystogram was performed and urodynamic studies of the bladder were performed. The bladder was then removed for histological analysis. RESULTS: Fetal survival rate was 100%. One newborn died immediately after the implantation of the engineered bladder from an anesthetic accident. The other nine (four in group I and five in group II) survived. One of the animals from group I lost its bladder catheter prematurely and had a urinary leak detected only at the time of death. There were no other complications. The engineered bladders were more compliant (P < .05) and had greater capacity pressures greater than 20 mm Hg (P < .05) than those closed primarily. Histological analysis of the engineered tissue showed a multilayered urothelial lining on the luminal side and overlying layers of smooth muscle cells surrounded by connective tissue. CONCLUSIONS: Videofetoscopically assisted fetal bladder engineering may be a viable alternative for prompt bladder reconstruction at birth. The architecture of autologous engineered fetal bladder tissue resembles that of native bladder. This concept may prove useful for the treatment of certain human neonatal conditions such as bladder and cloacal exstrophies.

Animals↗
PubMed2000

Analytical evaluation of primer engineered multiplex polymerase chain reaction-restriction fragment length polymorphism for detection of factor V Leiden and prothrombin G20210A.

Factor V Leiden and prothrombin G20210A are clinically relevant genetic risk factors for venous thrombosis. Analysis for both mutations is increasingly being performed on patients exhibiting hypercoagulability. The goal of the current study was to evaluate the performance of primer-engineered multiplex polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) for the simultaneous detection of factor V Leiden and prothrombin G20210A. Primer-engineered multiplex PCR-RFLP methods for the detection of factor V Leiden and prothrombin G20210A from the medical literature were reviewed. A modified method was optimized in which both mutations generate HindIII RFLPs and the prothrombin amplicon contains an invariant HindIII recognition site to assess the completeness of endonuclease digestion. Digested amplification products were analyzed by agarose gel electrophoresis in a single gel lane and visualized by ethidium bromide. Primer-engineered multiplex PCR-RFLP was used to analyze 205 human genomic DNA samples whose factor V Leiden genotypes had been previously determined by MnlI PCR-RFLP. Complete concordance for factor V Leiden genotypes was observed between the two methods in the 205-sample cohort comprising 139 wild-type, 62 heterozygous mutant, and four homozygous mutant individuals. For prothrombin G20210A, primer-engineered multiplex PCR-RFLP identified 196 wild-type and nine heterozygous mutant individuals in the 205-sample cohort. To independently verify prothrombin genotypes, the nine heterozygous mutants and an additional 11 wild-type patient samples (representing 10% of patient samples) were subjected to DNA sequencing. Complete concordance was observed between DNA sequencing and primer-engineered multiplex PCR-RFLP results. In further validation, 123 of the DNA samples consisting of four heterozygous mutant and 119 wild type individuals were genotyped with the Invader Assay for Factor II (prothrombin G20210A). Results showed 100% concordance between the Invader Assay and primer-engineered multiplex PCR-RFLP. A primer-engineered multiplex PCR-RFLP based on single restriction endonuclease digestion has been evaluated and shown to simultaneously and accurately detect factor V Leiden and prothrombin G20210A mutations. The method is robust and readily adaptable to the clinical molecular diagnostic laboratory.

Amino Acid Substitution↗
PubMed2000

Speciated hydrocarbon emissions from small utility engines.

Partially speciated hydrocarbon (HC) emissions data from several small utility engines, as measured by a Fourier Transform Infrared analyzer, are presented. The engines considered have nominal horsepower ratings between 3.7 and 9.3 kW. Both side-valve and overhead-valve engines are studied, and four different fuels are used in the engines. The results indicate that the small HCs present in the exhaust tend to be in the form of either methane or unsaturated HCs. Other small alkanes, such as ethane and propane, are present in only relatively small concentrations. In terms of ozone formation potential, the HCs in the form of methane will lead to little ozone, but the distribution of the C2 and C3 species is not ideal from an ozone reduction stand-point. It is also found that the presence of oxygen in the fuels appears to lead to somewhat more complete combustion, although the effects are not large. Finally, the overhead-valve engines appear to have lower HC emissions than side-valve engines, which is primarily due to higher operating A/F ratios and the engine geometry.

Air Pollution↗
PubMed2000

Scaffolds for engineering smooth muscle under cyclic mechanical strain conditions.

Cyclic mechanical strain has been demonstrated to enhance the development and function of engineered smooth muscle (SM) tissues, but appropriate scaffolds for engineering tissues under conditions of cyclic strain are currently lacking. These scaffolds must display elastic behavior, and be capable of inducing an appropriate smooth muscle cell (SMC) phenotype in response to mechanical signals. In this study, we have characterized several scaffold types commonly utilized in tissue engineering applications in order to select scaffolds that exhibit elastic properties under appropriate cyclic strain conditions. The ability of the scaffolds to promote an appropriate SMC phenotype in engineered SM tissues under cyclic strain conditions was subsequently analyzed. Poly(L-lactic acid)-bonded polyglycolide fiber-based scaffolds and type I collagen sponges exhibited partially elastic mechanical properties under cyclic strain conditions, although the synthetic polymer scaffolds demonstrated significant permanent deformation after extended times of cyclic strain application. SM tissues engineered with type I collagen sponges subjected to cyclic strain were found to contain more elastin than control tissues, and the SMCs in these tissues exhibited a contractile phenotype. In contrast, SMCs in control tissues exhibited a structure more consistent with the nondifferentiated, synthetic phenotype. These studies indicate the appropriate choice of a scaffold for engineering tissues in a mechanically dynamic environment is dependent on the time frame of the mechanical stimulation, and elastic scaffolds allow for mechanically directed control of cell phenotype in engineered tissues.

Animals↗