Patent Application
20070286884
The present invention describes an implantable material comprised of microbial cellulose. The instant implantable material has properties necessary for in vivo applications. For example, the implantable material of the instant invention is a paste that can be molded into a three-dimensional shape and possesses desired conformability characteristics.
The implantable materials of the instant invention are comprised of microbial cellulose. Those methods of preparing microbial cellulose are known to those of ordinary skill and are described, for example, in U.S. Pat. Nos. 5,846,213 and 4,912,049, which are incorporated herein by reference in their entirety. Any cellulose producing organism can be used in producing the raw biosynthetic cellulose material. However, biosynthetic cellulose produced from a static culture of Acetobacter xylinum is preferred.
The microbial cellulose content of the raw material is dependent on the amount of media supplied to the A.x. bacteria. Once the pellicle is harvested, the raw material is physically and chemically processed so as to be a suitable implantable material for medical and surgical uses. For example, the microbial cellulose is first processed and cleaned to remove all non-cellulose material embedded in the cellulose pad and then depyrogenated using chemicals such as sodium hydroxide. After depyrogenation, the cellulose may be cross-linked by irradiation or chemical means if its strength needs to be adjusted. Addition of other agents, such as glycerol, polyethylene glycol, Tween, chitosan, ethyl cellulose, hydroxypropyl cellulose or carboxymethyl cellulose can be used to modify the cellulose surface can also be performed in order to control water absorption and pliability which are desirable properties for implantable materials. The material can remain wet, moist, partially dehydrated, or totally dehydrated by air, heat, lyophilization, freeze-drying or supercritical fluid drying. The material may be further processed by chopping, grinding or milling to a paste consistency. Preferably, the processed microbial cellulose will be further sterilized for applications as medical implantable articles using standard sterilization methods such as gamma irradiation, e-beam irradiation, ethylene oxide or steam sterilization.
In one preferred embodiment, the invention provides a method for preparing an implantable device for medical and surgical applications comprising the steps of providing a microbial cellulose material and incorporating said material into an implantable device for medical and surgical applications. Once produced, the microbial cellulose may be incorporated or fashioned into medical devices by commonly known methods such as molding, cross-linking, chemical surface reaction, dehydrating and/or drying, cutting or punching. Such medical devices include tissue substitutes or scaffolds for repair or reinforcement of damaged hard tissue. For example, the instant microbial cellulose may be used as a scaffold in tissue engineering, substitution and replacement for hard tissue such as bone.
The instant microbial cellulose may be used as a substitute or scaffold in tissue engineering for orthopedic hard tissues such as bone. In this embodiment, the cellulose acts as a scaffold or trellis on which new tissue forms, orients and matures.
In a preferred embodiment, the invention provides a method of bone defect filling, consisting of an implantable composition comprising microbial cellulose and implanting said composition into a subject in need thereof. For example, the instant invention may be easily prepared as a dry or hydrated paste for direct application into bone voids. For this application the material must be conformable so as to fill the site and remain without dislocating.
In a preferred embodiment, the invention provides a method for use as an adjunct to achieve spine fusion. For this application the material must be moldable, easily implanted and remain in situ. Depending on the surgery the material can be implanted alone or as a filler for spine cage implants or cadaveric femoral ring implants.
In a preferred embodiment, the invention provides a bone graft material that is degradable. For this application the cellulose is subjected to oxidation at various levels to render it bioresorbable. Depending on the level of oxidation the material can resorb in weeks to years. This resorption can be tailored to the rate of bone formation so that voids are not created by the material degrading too quickly.
The material can be used for repairing tissue in any bony site in the skeleton including dental and periodontal applications. The microbial cellulose described can be processed using the methods described above to create a paste or gel that can be implanted to fill various sized bony defects.
The instant invention also contemplates an implantable composition comprising microbial cellulose and a medically useful agent. Any number of medically useful agents for tissue repair can be used in the invention by adding the substances to an implantable composition comprising the microbial cellulose carrier, either at any step in the manufacturing process or directly to the final composition. A medically useful agent is one having therapeutic, healing, curative, restorative or medicinal properties. Such medically useful agents include collagen and insoluble collagen derivatives, hydroxyapatite and soluble solids and/or liquids dissolved therein. Also included are amino acids, peptides, vitamins, co-factors for protein synthesis; hormones; endocrine tissue or tissue fragments; synthesizers; enzymes such as collagenase, peptidases, oxidases; cell scaffolds with parenchymal cells; angiogenic drugs and polymeric carriers containing such drugs; collagen lattices; biocompatible surface active agents, antigenic agents; cytoskeletal agents; cartilage fragments, living cells such as chondrocytes, bone marrow cells, mesenchymal stem cells, natural extracts, tissue transplants, bioadhesives, transforming growth factor (TGF-beta) and associated family proteins (bone morphogenetic protein (BMP), growth and differentiation factors (GDF) etc.), fibroblast growth factor (FGF), insulin-like growth factor (IGF-1) and other growth factors; growth hormones such as somatotropin; bone digesters; antitumor agents; fibronectin; cellular attractants and attachment agents; immuno-suppressants; permeation enhancers; and peptides, such as growth releasing factor, P-15 and the like.
The drug can be in its free base or acid form, or in the form of salts, esters, or any other pharmacologically acceptable derivatives, enantomerically pure forms, tautomers or as components of molecular complexes. The amount of drug to be incorporated in the composition varies depending on the particular drug, the desired therapeutic effect, and the time span for which the device is to provide therapy. Generally, for purposes of the invention, the amount of drug in the system can vary from about 0.0001% to as much as 60%.
The active agent may be used to reduce inflammation, increase cell attachment, recruit cells, and/or cause differentiation of the cells to repair the damaged tissue. In addition, implantable materials using microbial cellulose may be applied in a number of other useful areas, including, but not limited to other hard tissue substitutes or scaffolds.
Other objects, features and advantages of the present invention will become apparent from the following examples. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. The invention, thus generally described, will be understood more readily by reference to the following examples, which are provided by way of illustration and are not intended to be limiting of the present invention.
To prepare the microbial cellulose of the invention, Acetobacter xylinum microorganisms are cultured in a bioreactor containing a liquid nutrient medium at 30 degrees Celsius at an initial pH of 3-6. The medium is based on sucrose or other carbohydrates.
The bioreactor is composed of a plastic box fitted with an airtight cover. Dimensions of the bioreactor measured 9 in×13 in. Aeration ports are made in the bioreactor that allows the proper oxygen level to be achieved.
The fermentation process under static conditions is allowed to progress for a period of about 10-14 days, during which the bacteria in the culture medium produce an intact cellulose pellicle. Once the media is expended, the fermentation is stopped and the pellicle removed from the bioreactor.
1. Processing and Depyrogenation Procedures
The excess medium contained in the pellicle is removed by mechanical compression prior to chemical cleaning and subsequent processing of the pellicle. The cellulose pellicle is subjected to a series of chemical wash steps to convert the raw cellulose film into a medical grade and non-pyrogenic implantable material. Processing starts with an 8% sodium hydroxide solution at 70-75 degrees Celsius for 1 hour, followed by a rinse in deionized water and then a soak in 0.25% hydrogen peroxide at 70-75 degrees Celsius for 1 hour.
The resulting films can be tested for pyrogens and mechanical properties. The amount of cellular debris left in the cellulose pad after processing is measured by validated Limulus Amoebocyte Lysate (LAL) testing as outlined by the U.S. Food and Drug Administration (FDA) in 21 CFR10.90. The instant cleaning process outlined above provides a nonpyrogenic cellulose pad (≦0.50 EU/ml). The steps of the LAL test are defined by the test kit manufacturer and can simply be followed to yield the pyrogen level in the cellulose film.
2. Final Product Processing
Once cleaned, the pellicles are mechanically pressed to reduce the water content. The resulting pad is cut to shape and packaged.
Material can be prepared the same as in Example 1, with the added step of soaking in a 1% solution of bovine serum albumin (BSA) for 24 hours. Following saturation in the BSA solution the sample is placed in a 0.9% saline solution of 20× the sample mass. Aliquots are removed from the solution at various time points to determine the BSA release profile. The BSA concentration is assessed via ultraviolet/visible spectrophotometry.
Material can be prepared as per Example 1, however prior to packaging the material can be placed in a Waring Blender with additional water to form a paste. Once processed the material is dehydrated by straining and then packaged. The fiber size can be controlled by blending time to result in materials with different physical properties. Samples containing 5% cellulose were made with processing times of one and five minutes. Stiffness testing was performed with a UNITED Tensile Tester using the circular bend procedure (ASTM Test Method D 4032). Samples were formed into discs with a diameter of between 3 and 4 cm with a thickness of between 5 and 7 mm. Stiffness values for one and five minutes processing were 5.6±1.7 and 2.3±0.7N, respectively, suggesting larger fiber sizes result in a more cohesive material.
Material can be prepared as per Example 3, however, an additional flow enhancer may be added to result in a moldable paste. Tween, PEG, and carboxymethyl cellulose have been added to cellulose paste samples in concentrations ranging from 2.5 to 10% to alter the conformability characteristics of the pastes. Furthermore the addition of a flow enhancer results in reduced fluid and/or active ingredient loss during handling.
A malleable paste containing 2.5% PEG 400, 10% hydroxyapatite (100-200 nm particle size), and approximately 4% cellulose.
500 milligrams of PEG 400 was mixed into 8500 milligrams of a 5% cellulose paste prepared per Example 3. After the PEG and cellulose paste components were well mixed, 1000 milligrams of 100-200 nm hydroxyapatite was mixed into the paste. The resulting paste contained a PEG 400 concentration of 2.5% (w/w), a hydroxyapatite concentration of 10% (w/w), and cellulose concentration of 4% (w/w). This provided a malleable paste with excellent formability properties and moderate water retention properties. A similar paste containing 5% PEG 400, 10% 100-200 nm hydroxyapatite, and 4% cellulose was also prepared. No differences were observed in formability or water retention properties.
A malleable paste containing 5% carboxymethyl cellulose and 5% cellulose.
519 milligrams of carboxymethyl cellulose (molecular weight=250,000) was mixed into 9,574 milligrams of a 5% cellulose paste prepared per Example 3. The components were well mixed and allowed to sit for 72 hours at room temperature. This provided a malleable paste with moderate formability properties and excellent water retention properties.
Material can prepared as in Example 3, however additional calcium salts (e.g. hydroxyapatite, tricalcium phosphate, calcium sulfate) can be added to form a composite paste. Paste samples with 5 and 10% cellulose were made by incorporating 10, 15, 20 and 25% hydroxyapatite (HA). Stiffness values (
BSA release profiles were determined with materials described in Example 3. Samples were loaded by soaking in a 1% BSA solution for 24 hours. After loading the individual samples were packed into a porous nylon sample bag and then placed in an aqueous solution with a volume 20× the mass of the paste sample. Aliquots were removed at various time points and analyzed with UV/Vis spectrophotometry to determine the solution concentration which then could be used to determine the amount of BSA remaining in the paste. Measurements were taken until the solution reached the theoretical equilibrium concentration.
PHMB release profiles were determined with materials described in Example 3 and in addition a 5% cellulose sample with 10% HA was also evaluated. Samples were loaded by soaking in a 1% PHMB solution for 24 hours. After loading the individual samples were packed into a porous nylon sample bag and then placed in an aqueous solution with a volume 20× the mass of the paste sample. Aliquots were removed at various time points and analyzed with UV/Vis to determine the solution concentration which then could be used to determine the amount of PHMB remaining in the paste. Measurements were taken until the solution reached the theoretical equilibrium concentration.
Samples were prepared as described in Example 3 and absorption and donation characteristics of the materials were determined. These samples were tested for absorption from a saturated sponge and donation to a dry surface. For the absorption test, approximately 2.5 g of the paste sample was placed on top of a sponge sitting in a 0.9% saline bath at room temperature. The liquid level was maintained at the level of the sponge. Samples were removed after 24 hr and reweighed to determine the quantity of saline absorbed by the paste, and the absorption was reported as a percentage of the initial weight of the sample.
Material can be prepared as described in Example 3 and dried under ambient conditions at elevated temperatures. After drying the resulting material is hard, brittle, and non-conforming. The stiffness of the air-dried material was determined and a 30-fold increase was observed compared to the stiffness of its non-dried counterpart, as shown in
Material can be prepared as described in Example 3 and dried using a supercritical drying (SCD) process with CO2. The material first undergoes a solvent exchange process with methanol followed by the SCD processing. Following the SCD process the material is soft and spongy with a 3-fold increase in stiffness, with respect to the wet paste, illustrated in
Cellulose pastes comprised of 5% and 10% cellulose were made per Example 3. Samples containing PEG 400, Tween 80, and hydroxyapatite were prepared using the 5% cellulose paste. The pH of each sample was measured and is shown in
Cellulose paste materials with varying degrees of oxidation were produced with materials made per Example 1. The cellulose films were incubated in solutions of varying sodium periodate to cellulose ratios including 0.8:1, 2:1, and 4:1. Incubation was conducted in closed reaction vessels at 30 degrees Celsius within a darkened incubator for approximately 17.5 hrs. Following incubation, the samples were placed in deionized water to extract unreacted sodium periodate. The oxidized cellulose pads were then subjected to a solvent exchange process with methanol, ground in a blender, and dried using the SCD process.
Oxidized cellulose paste samples containing 3% cellulose were made with material as described in Example 3. The cellulose pastes were incubated in solutions of varying sodium periodate to cellulose ratios including 0.8:1, 1.5:1, 2:1, and 4:1. Incubation was conducted in closed reaction vessels at 30 degrees Celsius within a darkened incubator for 16.5 to 17.5 hrs. Following incubation, the samples were placed in deionized water to extract unreacted sodium periodate. The oxidized cellulose pastes were then subjected to a solvent exchange process with methanol followed by SCD processing.
This application claims priority to U.S. Provisional Application No. 60/812,962, filed Jun. 13, 2006, the disclosure of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60812962 | Jun 2006 | US |
