Biomedical subjects

Robert Langer

Publications and source records attributed to Robert Langer.

At least 73 records · Page 4Linked to original sources

PubMed2005

Stimulation of neurite outgrowth by neurotrophins delivered from degradable hydrogels.

Degradable hydrogels are useful vehicles for the delivery of growth factors to promote the regeneration of diseased or damaged tissue. In the central nervous system, there are many instances where the delivery of neurotrophins has great potential in tissue repair, especially for treatment of spinal cord injury. In this work, hydrogels based on poly(ethylene glycol) that form via a photoinitiated polymerization were investigated for the delivery of neurotrophins. The release kinetics of these factors are controlled by changes in the network crosslinking density, which influences neurotrophin diffusion and subsequent release from the gels with total release times ranging from weeks to several months. The release and activity of one neurotrophic factor, ciliary-neurotrophic factor (CNTF), was assessed with a cell-based proliferation assay and an assay for neurite outgrowth from retinal explants. CNTF released from a degradable hydrogel above an explanted retina was able to stimulate outgrowth of a significantly higher number of neurites than controls without CNTF. Finally, unique microsphere/hydrogel composites were developed to simultaneously deliver multiple neurotrophins with individual release rates.

Animals↗

PubMed2005

In vivo delivery of BCNU from a MEMS device to a tumor model.

A drug delivery micrcoelectromechanical systems (MEMS) device was used to locally deliver a chemotherapeutic agent (BCNU) to an experimental tumor in rats. This MEMS device consists of an array of reservoirs etched into the silicon substrate. The drug release is achieved by the electrochemical dissolution of the gold membranes covering the reservoirs. A new Pyrex package was developed to improve the BCNU release kinetics and enhance device capacity. Co-formulation of BCNU with polyethylene glycol (PEG) led to complete and rapid release of drug in vivo. BCNU delivered from the MEMS device showed dose-dependent inhibiting effect on the tumor growth in the BCNU dosage range of 0.67 approximately 2 mg. BCNU delivered from the activated devices was as effective as equipotent subcutaneous injections of BCNU in inhibiting tumor growth. Further optimization using this MEMS device to deliver BCNU in combination with other therapeutic agents against the tumor challenge is possible because of the unique capability of the device to precisely control the temporal release profiles of multiple substances.

Animals↗

PubMed2005

In vivo engineering of organs: the bone bioreactor.

Treatment of large defects requires the harvest of fresh living bone from the iliac crest. Harvest of this limited supply of bone is accompanied by extreme pain and morbidity. This has prompted the exploration of other alternatives to generate new bone using traditional principles of tissue engineering, wherein harvested cells are combined with porous scaffolds and stimulated with exogenous mitogens and morphogens in vitro and/or in vivo. We now show that large volumes of bone can be engineered in a predictable manner, without the need for cell transplantation and growth factor administration. The crux of the approach lies in the deliberate creation and manipulation of an artificial space (bioreactor) between the tibia and the periosteum, a mesenchymal layer rich in pluripotent cells, in such a way that the body's healing mechanism is leveraged in the engineering of neotissue. Using the "in vivo bioreactor" in New Zealand White rabbits, we have engineered bone that is biomechanically identical to native bone. The neobone formation followed predominantly an intramembraneous path, with woven bone matrix subsequently maturing into fully mineralized compact bone exhibiting all of the histological markers and mechanical properties of native bone. We harvested the bone after 6 weeks and transplanted it into contralateral tibial defects, resulting in complete integration after 6 weeks with no apparent morbidity at the donor site. Furthermore, in a proof-of-principle study, we have shown that by inhibiting angiogenesis and promoting a more hypoxic environment within the "in vivo bioreactor space," cartilage formation can be exclusively promoted.

Alginates↗

PubMed2005

Combined postmenopausal hormone therapy and cardiovascular disease: toward resolving the discrepancy between observational studies and the Women's Health Initiative clinical trial.

Observational research on postmenopausal hormone therapy suggests a 40-50% reduction in coronary heart disease incidence among women using these preparations. In contrast, the Women's Health Initiative clinical trial of estrogen plus progestin found an elevated incidence over a 5.6-year intervention period through July 7, 2002. Toward explaining this discrepancy, the authors analyzed data from this trial, which included 16,608 postmenopausal women aged 50-79 years, and corresponding data from 53,054 women in the Women's Health Initiative observational study, 33% of whom were estrogen-plus-progestin users at baseline. Estrogen-plus-progestin hazard ratio estimates for coronary heart disease, stroke, and venous thromboembolism in the observational study were 39-48% lower than those in the clinical trial following age adjustment. However, hazard ratios tended to decrease with increasing time from initiation of estrogen-plus-progestin use, and observational study hazard ratio estimates are heavily weighted by longer-term use while clinical trial hazard ratio estimates reflect shorter-term use. Following control for time from estrogen-plus-progestin initiation and confounding, hazard ratio estimates were rather similar for the two cohorts, although there was evidence of some remaining difference for stroke. These analyses have implications for both the design and the analysis of observational studies.

Aged↗

PubMed2005

Engineering vascularized skeletal muscle tissue.

One of the major obstacles in engineering thick, complex tissues such as muscle is the need to vascularize the tissue in vitro. Vascularization in vitro could maintain cell viability during tissue growth, induce structural organization and promote vascularization upon implantation. Here we describe the induction of endothelial vessel networks in engineered skeletal muscle tissue constructs using a three-dimensional multiculture system consisting of myoblasts, embryonic fibroblasts and endothelial cells coseeded on highly porous, biodegradable polymer scaffolds. Analysis of the conditions for induction and stabilization of the vessels in vitro showed that addition of embryonic fibroblasts increased the levels of vascular endothelial growth factor expression in the construct and promoted formation and stabilization of the endothelial vessels. We studied the survival and vascularization of the engineered muscle implants in vivo in three different models. Prevascularization improved the vascularization, blood perfusion and survival of the muscle tissue constructs after transplantation.

Animals↗

PubMed2005

Light-induced shape-memory polymers.

Materials are said to show a shape-memory effect if they can be deformed and fixed into a temporary shape, and recover their original, permanent shape only on exposure to an external stimulus. Shape-memory polymers have received increasing attention because of their scientific and technological significance. In principle, a thermally induced shape-memory effect can be activated by an increase in temperature (also obtained by heating on exposure to an electrical current or light illumination). Several papers have described light-induced changes in the shape of polymers and gels, such as contraction, bending or volume changes. Here we report that polymers containing cinnamic groups can be deformed and fixed into pre-determined shapes--such as (but not exclusively) elongated films and tubes, arches or spirals--by ultraviolet light illumination. These new shapes are stable for long time periods, even when heated to 50 degrees C, and they can recover their original shape at ambient temperatures when exposed to ultraviolet light of a different wavelength. The ability of polymers to form different pre-determined temporary shapes and subsequently recover their original shape at ambient temperatures by remote light activation could lead to a variety of potential medical and other applications.

Journal Article↗

PubMed2005

Risk of cardiovascular disease by hysterectomy status, with and without oophorectomy: the Women's Health Initiative Observational Study.

BACKGROUND: Cardiovascular disease (CVD) is a leading cause of morbidity and mortality in women and may vary by hysterectomy (or oophorectomy) status. This study compared CVD risk factors and rates between postmenopausal women who had and had not undergone hysterectomy, with or without oophorectomy. METHODS AND RESULTS: This analysis was conducted on 89 914 women in the Women's Health Initiative (WHI) Observational Study. Participants reported demographic characteristics, medical history, dietary habits, physical activity, medications, and previous hysterectomy (with or without oophorectomy). Baseline weight, height, waist circumference, and blood pressure were measured. CVD events were ascertained during 5.1 years of mean follow-up and adjudicated with standard criteria. Black, Hispanic, and American Indian women had higher rates of hysterectomy than white women (52.9%, 44.6%, and 49.2% versus 40.0%, respectively), and Asian/Pacific Islander women had lower rates (33.8%). Women with a hysterectomy (regardless of oophorectomy status) had an adverse risk profile at baseline compared with women with no hysterectomy, including a higher proportion of hypertension, diabetes, high cholesterol, obesity, and lower education, income, and physical activity (all P<0.01). Total mortality and fatal and nonfatal CVD were higher among women with a hysterectomy. Hysterectomy (regardless of oophorectomy status) was a significant predictor of CVD (HR: 1.26, P<0.001). After adjustment for demographic variables and CVD risk factors, the effect was reduced and nonsignificant. CONCLUSIONS: Women with a hysterectomy had a worse risk profile and higher prevalence and incidence of CVD in this cohort. Multivariate models suggest that hysterectomy is not the major determinant of this outcome; rather, CVD risk may be due to the more adverse initial risk profile of women who had undergone hysterectomy.

Adult↗

PubMed2005

Characterization of chemisorbed hyaluronic acid directly immobilized on solid substrates.

Hyaluronic acid (HA) has a number of potential biomedical applications in drug delivery and tissue engineering. For these applications, a prerequisite is to understand the characteristic of HA films directly immobilized to solid substrates. Here, we demonstrate that high molecular weight HA can be directly immobilized onto hydrophilic substrates without any chemical manipulation, allowing for the formation of an ultrathin chemisorbed layer. Hyaluronic acid is stabilized on these surfaces through hydrogen bonding between the hydrophilic moieties in HA [such as carboxylic acid (-COOH) or hydroxyl (-OH) groups] with silanol (-SiOH), carboxylic acid or hydroxyl groups on the hydrophilic substrates. Despite the water solubility, the chemisorbed HA layer remained stable on glass or silicon oxide substrates for at least 7 days in phosphate-buffered saline. Furthermore, HA immobilized on silicon and other dioxide surfaces in much higher quantities than other polysaccharides including dextran sulfate, heparin, heparin sulfate, chondroitin sulfate, dermatan sulfate, and alginic acid. This behavior is related to the molecular entanglement and intrinsic stiffness of HA as a result of strong internal and external hydrogen bonding as well as high molecular weight. These results demonstrate that HA can be used to coat surfaces through direct immobilization.

Absorption↗

PubMed2005

Biomaterial microarrays: rapid, microscale screening of polymer-cell interaction.

The identification of biomaterials that induce optimal gene expression patterns and allow for appropriate levels of cellular attachment is of central importance in tissue engineering and cell therapy. Herein, we describe the creation of cell-compatible, biomaterial microarrays, that allow rapid, microscale testing of biomaterial interactions with cells. As proof of principle, we simultaneously characterized over 3456 human mesenchymal stem cell (hMSC)-biomaterial composite interactions, and describe preliminary studies on the utility of these arrays with a neural stem cell line (NSC), and primary articular chondrocytes.

Biocompatible Materials↗

PubMed2005

Retinal progenitor cell xenografts to the pig retina: morphologic integration and cytochemical differentiation.

OBJECTIVE: To investigate the survival, integration, and differentiation of mouse retinal progenitor cells after transplantation to the subretinal space of adult pigs. METHODS: Green fluorescent protein-positive (GFP+) murine retinal progenitor cells were transplanted subretinally as single cells, spheres, or biodegradable polymer-progenitor composites into 24 nonimmunosuppressed adult pigs. Of these, 14 pigs received laser lesions (n = 11) or outer retinal scraping injury (n = 3). Recipients were killed at 30 minutes to 5 weeks after grafting. RESULTS: The GFP+ murine retinal progenitor cells survived well for up to 14 days after transplantation to the pig retina. After 5 weeks, fewer GFP+ cells were found. In the pigs that received laser treatment before grafting of cell suspension, GFP+ cells integrated into the retinal pigment epithelium and all layers of the retina. The GFP+ cells exhibited morphologic evidence of differentiation into mature retinal neurons, although evaluation of marker expression found only nestin and glial fibrillary acidic protein colocalization. In noninjured pigs, cells mainly integrated into the retinal pigment epithelium. In pigs that received composites, cells appeared to mature and extended processes through pores in the polymer matrix. CONCLUSIONS: Retinal progenitor cell xenografts survive for a sufficiently long period to integrate into areas of injury and exhibit morphologic differentiation. By 5 weeks, survival diminishes. Biodegradable polymers may be useful for transplanting retinal progenitor cells in a structurally organized manner. Clinical Relevance Central nervous system (CNS) diseases may cause long-term disabilities. Substantial tissue destruction can be sustained by the complex structures of the brain, spinal cord, or retina without loss of life, yet the lack of effective CNS regeneration frequently results in disruption of activities of daily living and marked degradation in quality of life. It has become clear that an enormous potential for repair is present within the mammalian CNS. The challenge is to harness this potential to treat disease. Transplantation of neuronal tissue to the CNS represents a promising, albeit challenging, approach to the replacement of neurons lost owing to injury or disease.

Absorbable Implants↗

PubMed2005

Exploring polyethylenimine-mediated DNA transfection and the proton sponge hypothesis.

BACKGROUND: The relatively high transfection efficiency of polyethylenimine (PEI) vectors has been hypothesized to be due to their ability to avoid trafficking to degradative lysosomes. According to the proton sponge hypothesis, the buffering capacity of PEI leads to osmotic swelling and rupture of endosomes, resulting in the release of the vector into the cytoplasm. METHODS: The mechanism of PEI-mediated DNA transfer was investigated using quantitative methods to study individual steps in the overall transfection process. In addition to transfection efficiency, the cellular uptake, local pH environment, and stability of vectors were analyzed. N-Quaternized (and therefore non-proton sponge) versions of PEI and specific cell function inhibitors were used to further probe the proton sponge hypothesis. RESULTS: Both N-quaternization and the use of bafilomycin A1 (a vacuolar proton pump inhibitor) reduced the transfection efficiency of PEI by approximately two orders of magnitude. Chloroquine, which buffers lysosomes, enhanced the transfection efficiency of N-quaternized PEIs and polylysine by 2-3-fold. In contrast, chloroquine did not improve the transfection efficiency of PEI. The measured average pH environment of PEI vectors was 6.1, indicating that they successfully avoid trafficking to acidic lysosomes. Significantly lower average pH environments were observed for permethyl-PEI (pH 5.4), perethyl-PEI (pH 5.1), and polylysine (pH 4.6) vectors. Cellular uptake levels of permethyl-PEI and perethyl-PEI vectors were found to be 20 and 90% higher, respectively, than that of parent PEI vectors, indicating that the reduction in transfection activity of the N-quaternized PEIs is due to a barrier downstream of cellular uptake. A polycation/DNA-binding affinity assessment showed that the more charge dense N-quaternized PEIs bind DNA less tightly than PEI, demonstrating that poor vector unpackaging was not responsible for the reduced transfection activity of the N-quaternized PEIs. CONCLUSIONS: The results obtained are consistent with the proton sponge hypothesis and strongly suggest that the transfection activity of PEI vectors is due to their unique ability to avoid acidic lysosomes.

Animals↗

PubMed2005

Role of n-methyl pyrrolidone in the enhancement of aqueous phase transdermal transport.

The role of n-methyl pyrrolidone (NMP) as an enhancer for permeants delivered from an aqueous phase was investigated in the transdermal delivery of the local anesthetics lidocaine free base, lidocaine-hydrochloride (HCl), and prilocaine-HCl. Lidocaine free-base flux increased from H2O/NMP binary systems containing over 50% (v/v) NMP with significant flux enhancement observed above 80% NMP. In this range, drug flux was found to correlate with NMP flux. The addition of oleic acid (1% w/v) further enhanced lidocaine flux sixfold, in these formulations. The H2O/NMP (50% v/v) system enhanced the transport of water-soluble hydrochloride salt derivatives of lidocaine and prilocaine by factors of 4.3 and 2.6, respectively, indicating that NMP was capable of enhancing hydrophilic and hydrophobic drugs from an aqueous phase. These findings were consistent with the model that NMP flux across the stratum corneum improves the transport of formulation solutes.

Administration, Cutaneous↗

PubMed2005

Nonviral delivery of cancer genetic vaccines.

The potential use of genetic vaccines to address numerous diseases including cancer is promising, but currently unrealized. Here, we review advances in the nonviral delivery of antigen-encoded plasmid DNA for the purpose of treating cancer through the human immune system, as this disease has drawn the most attention in this field to date. Brief overviews of dendritic cell immunobiology and the mechanism of immune activation through genetic vaccines set the stage for the desirability of delivery technology. Several promising nonviral delivery techniques are discussed along with a mention of targeting strategies aimed at improving the potency of vaccine formulations. Implications for the future of genetic vaccines are also presented.

Cancer Vaccines↗

PubMed2005

Formulation and surface modification of poly(ester-anhydride) micro- and nanospheres.

This report presents the formulation and surface modification of poly(ester-anhydride) copolymers. Two synthetic schemes were used to generate a variety of copolymers with varying compositions and molecular weights. These polymers were formulated into both micro- and nanospheres using a water-in-oil-in-water double emulsion technique. Microspheres were between 3 and 10 microm in diameter; whereas, nanospheres were between 300 and 500 nm. Finally, both micro- and nanospheres were surface modified using cystamine and quantified, following cystamine reduction, with DTNB (Ellman's reagent). Surface labeling was observed at 0.20-35 micromol/g for microspheres and 0.6-100 micromol/g for nanospheres and followed the expected trend of increased labeling with increased polyanhydride content.

Absorbable Implants↗

PubMed2005

The inflammatory responses to silk films in vitro and in vivo.

Silks have a long history of biomedical use as sutures. Silk can be purified, chemically modified to attach RGD sequences and processed into highly porous scaffolds for tissue engineering. We report biocompatibility studies of silk films (with or without covalently bound RGD) that were seeded with bone-marrow derived mesenchymal stem cells (MSC) and (a) cultured in vitro with human MSC or (b) seeded with autologous rat MSC and implanted in vivo. Controls for in vitro studies included tissue culture plastic (TCP; negative control), TCP with lipopolysaccharide (LPS) in the cell culture medium (positive control), and collagen films; controls for in vivo studies included collagen, PLA and TCP. After 9 h of culture, the expression of the pro-inflammatory Interleukin 1 beta (IL-1beta) and inflammatory cyclooxygenase 2 (COX-2) in human MSC were comparable for silk, collagen and TCP. After 30 and 96 h, gene expression of IL-1beta and COX-2 in MSC returned to the baseline (pre-seeding) levels. These data were corroborated by measuring IL-1beta and prostaglandin E2 levels in culture medium. The rate of cell proliferation was higher on silk films than either on collagen or TCP. In vivo, films made of silk, collagen or PLA were seeded with rat MSCs, implanted intramuscularly in rats and harvested after 6 weeks. Histological and immunohistochemical evaluation of silk explants revealed the presence of circumferentially oriented fibroblasts, few blood vessels, macrophages at the implant-host interface, and the absence of giant cells. Inflammatory tissue reaction was more conspicuous around collagen films and even more around PLA films when compared to silk. These data suggest that (a) purified degradable silk is biocompatible and (b) the in vitro cell culture model (hMSC seeded and cultured on biomaterial films) gave inflammatory responses that were comparable to those observed in vivo.

Animals↗

PubMed2005

In vivo evaluation of tetrahedral amorphous carbon.

The in vivo behavior and tissue reaction to tetrahedral amorphous carbon (ta-C) has been evaluated for periods of up to 6 months in SV129 mice. Two sample types were tested--silicon die coated with ta-C (n = 53) and micromachined particles (n = 40). The coated samples were compared to uncoated silicon die (n = 22). Die samples were implanted subcutaneously, and tissue reaction and capsule formation were evaluated at various time points. Micromachined particles of 1, 3, 10, and 30 microm were injected adjacent to the sciatic nerve, and tissue samples were examined histologically at various time points (4 days-6 months). Tissue reaction to ta-C was mild and was localized to the area of the injection or implantation. Samples with a higher ratio of 3-fold bonding appeared to shed material during the experiments; this was not observed on samples with a higher level of 4-fold bonding, nor on uncoated silicon die. The results strongly suggest that films with greater 4-fold bonding character (more diamond-like) are more resistant to in vivo fragmentation than films with higher 3-fold character (more graphitic).

Animals↗

PubMed2005

Size and temperature effects on poly(lactic-co-glycolic acid) degradation and microreservoir device performance.

The component materials of controlled-release drug delivery systems are often selected based on their degradation rates. The release time of a drug from a system will strongly depend on the degradation rates of the component polymers. We have observed that some poly(lactic-co-glycolic acid) polymers (PLGA) exhibit degradation rates that depend on the size of the polymer object and the temperature of the surrounding environment. In vitro degradation studies of four different PLGA polymers showed that 150 microm thick membranes degraded more rapidly than 50 microm thick membranes, as characterized by gel permeation chromatography and mass loss measurements. Faster degradation was observed at 37 degrees C than 25 degrees C, and when the saline media was not refreshed. A biodegradable polymeric microreservoir device that we have developed relies on the degradation of polymeric membranes to deliver pulses of molecules from reservoirs on the device. Earlier molecular release was seen from devices having thicker PLGA membranes. Comparison of an in vitro release study from these devices with the degradation study suggests that reservoir membranes rupture and drug release occurs when a membrane threshold molecular weight of 5000-15000 is reached.

Absorbable Implants↗

PubMed2005

Fabrication and biocompatibility of polypyrrole implants suitable for neural prosthetics.

Finding a conductive substrate that promotes neural interactions is an essential step for advancing neural interfaces. The biocompatibility and conductive properties of polypyrrole (PPy) make it an attractive substrate for neural scaffolds, electrodes, and devices. Stand-alone polymer implants also provide the additional advantages of flexibility and biodegradability. To examine PPy biocompatibility, dissociated primary cerebral cortical cells were cultured on PPy samples that had been doped with polystyrene-sulfonate (PSS) or sodium dodecylbenzenesulfonate (NaDBS). Various conditions were used for electrodeposition to produce different surface properties. Neural networks grew on all of the PPy surfaces. PPy implants, consisting of the same dopants and conditions, were surgically implanted in the cerebral cortex of the rat. The results were compared to stab wounds and Teflon implants of the same size. Quantification of the intensity and extent of gliosis at 3- and 6-week time points demonstrated that all versions of PPy were at least as biocompatible as Teflon and in fact performed better in most cases. In all of the PPy implant cases, neurons and glial cells enveloped the implant. In several cases, neural tissue was present in the lumen of the implants, allowing contact of the brain parenchyma through the implants.

Animals↗