Particulate tissue graft with components of differing density and methods of making and using the same

Abstract

Disclosed are tissue graft compositions made of particles having different densities, methods of making these compositions, and methods of using these compositions for promoting tissue restoration in a patient.

Claims

1. A method for preparing a mixture of bioresorbable particulate matter, comprising: (a) providing a first sheet of bioresorbable material, wherein the first sheet comprises a native extracellular matrix comprising epithelial basement membrane and having a first density and a first rate of resorption; (b) milling said first sheet into a first plurality of bioresorbable particles; (c) providing a second sheet of a bioresorbable material, wherein the second sheet comprises an extracellular matrix having a second density and a second rate of resorption; (d) treating said second sheet to increase or decrease the second density to a third density at least 150% relative to the first density with a third rate of resorption; (d) milling said second sheet into a second plurality of bioresorbable particles; (e) providing a predetermined quantity of said first plurality of bioresorbable particles and a predetermined quantity of said second plurality of bioresorbable particles; and (f) mixing said predetermined quantity of said first plurality of bioresorbable particles with said predetermined quantity of said second plurality of bioresorbable particles.

2. The method of claim 1, wherein treating comprises at least one or more of: compressing, dehydrating, vacuuming, crosslinking, lyophilizing and laminating the second sheet to increase the second density relative to the first density prior to milling the second sheet.

3. The method of claim 1, further comprising treating the first sheet to decrease the first density relative to the second density of the second sheet prior to milling to have a third density wherein treating comprises at least one of: hydrating, freezing in a solution, and expanding pore structure.

4. The method of claim 1, wherein said first sheet of bioresorbable material and said second sheet of bioresorbable material comprises non-cross-linked extracellular matrix.

5. The method of claim 1, further comprising coating the first plurality and second plurality of bioresorbable particles with a biologically active component selected from the group consisting of epidermal growth factor, TGF-alpha, TGF-beta, fibroblast growth factor, platelet derived growth factor, vascular endothelial growth factor, insulin-like growth factor, keratinocyte growth factor, bone morphogenic protein, pharmaceutical agents, DNA vectors and combinations thereof.

6. The method of claim 1, wherein the first rate of resorption is at least 1.5 times that of the second rate of resorption.

7. The method of claim 1, wherein the prepared mixture of bioresorbable particulate matter is used in an implant.

8. The method of claim 1, wherein treating is applied to only a portion of said second sheet to have a third density.

9. The method of claim 1, wherein the first or second sheet is dried, hydrated or frozen prior to milling.

10. The method of claim 1, wherein treating comprises at least one of: hydrating, freezing in a solution, and expanding pore structure of the second sheet prior to comminuting the second sheet to decrease the second density relative to the first density of the first sheet.

11. The method of claim 1, wherein milling further comprises comminuting, grinding, chopping, blending, pulverizing, or mincing.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIGS. 1A-C depict the bioresorption of a prior art ECM such as UBM, UBS, or SIS in a mammalian patient tissue. FIG. 1A depicts the ECM on Day 0 at implantation. FIG. 1B depicts the ECM on Day X after implantation where the ECM has been partially resorbed by the patient tissue. FIG. 1C depicts the ECM on Day Y after implantation where the ECM has been fully resorbed by the patient tissue.

(2) FIGS. 2A-C depict the bioresorption of an exemplary particulate tissue graft made up of two types of particles of different density according to one embodiment of the invention. The open circles () represent a lower density particle and the solid circles (.circle-solid.) represent a higher density particle. FIG. 2A depicts the particles on Day 0 at implantation. FIG. 2B depicts the particles on Day X after implantation where the lower density particles have been partially resorbed by the patient tissue, as evidenced by their decrease in size. FIG. 2C depicts the particles on Day Y after implantation where the lower density particles have been fully resorbed by the patient tissue, and the higher density particles have not yet been fully resorbed by the patient tissue.

(3) FIG. 3 depicts an exemplary composition of ECM particles according to one embodiment of the invention where the particles are of two different densities and the stages of an exemplary method of making the composition according to one embodiment of the invention. Two sheets of ECM are shown, one having density A and the other having density B. The sheets are comminuted and a portion of A particles and B particles are mixed together.

(4) FIG. 4 depicts an exemplary composition of ECM particles according to one embodiment of the invention where the particles are of two different densities and the steps of an exemplary method of making the composition according to one embodiment of the invention. A sheet of ECM is treated so that a portion of the sheet has a density A and a second portion of the sheet has a density B. The sheet is then comminuted to make a composition of particles of two different densities.

(5) FIG. 5 depicts an exemplary composition of ECM particles according to one embodiment of the invention where the particles have a density A, B, or C and the steps of an exemplary method of making the composition according to one embodiment of the invention. A first sheet A of ECM having density A is adhered to a second sheet B of ECM having density B. The sheet is then comminuted to create a composition of particles having density A and particles having density B and composite particles of density C, the composite particles having components of both sheets A and B.

(6) FIG. 6 depicts an exemplary composition of ECM particles according to one embodiment of the invention where the particles have a core particle of the same density, but are coated with different materials having different densities A, B, and C. Accordingly, the coated particles have differing densities A, B, and C.

(7) FIG. 7 depicts an exemplary ECM composition according to one embodiment of the invention where ECM particles having densities A, B, and C are adhered to the surface of a sheet of ECM.

(8) FIG. 8A depicts an exemplary ECM composition according to one embodiment of the invention where ECM particles of a higher density B are compressed together to form a core of particles of density B coated with particles of lower density A or layered with a layer of particles of density A to form a construct in the shape of a mammalian body part, here exemplified as a kidney.

(9) FIG. 8B depicts an exemplary ECM composition according to one embodiment of the invention where ECM particles of a higher density B are compressed together to form a core of particles of density B coated with particles of lower density A or layered with a layer of particles of density A to form a construct in the shape of a mammalian body part, here exemplified as a human nose.

(10) FIG. 9 depicts an exemplary ECM composition according to one embodiment of the invention where a mixture of ECM particles of a higher density B and of a lower density A are compressed together to form a three-dimensional construct in the shape of a mammalian body part, here exemplified as a human nose.

(11) FIG. 10 depicts an exemplary ECM composition according to one embodiment of the invention where a mixture of ECM particles of a higher density B and of a lower density A are compressed together to form a sheet.

(12) FIG. 11 depicts an exemplary multi-laminate ECM composition according to one embodiment of the invention where a mixture of ECM particles of a higher density B and a lower density A are sandwiched between sheets of ECM.

DETAILED DESCRIPTION OF THE INVENTION

(13) Extracellular matrix materials (ECMs) have been used to promote repair and restoration of damaged, deformed, injured, and even missing body tissues in mammals, such as humans. For example, ECMs such as SIS or UBM are implanted at a treatment site to promote repair and restoration of epithelial tissues and connective tissues, including connective tissue layers associated with epithelial tissues. Tissue sites in need of repair or restoration include, but are not limited to tendons, ligaments, breast, bone, kidney, liver, spleen, lymph nodes, urinary bladder, ureter, uterus, blood vessels, intestine, stomach, skin, heart, or a body cavity. Restore, restored, restoring, or restoration means that tissue function and structure is substantially returned to its original condition by the patient's endogenous tissue repair mechanisms in combination with the tissue graft composition.

(14) ECMs such as SIS, UBS, and UBM are rapidly resorbed by host tissues in which the ECM has been implanted. Once the material has been resorbed by the host, the ECM no longer has an influence over tissue restoration and repair at the site of implantation. As ECMs are resorbed, they release bioactive components such as growth factors and other bioactive components naturally present in the native ECM that influence tissue restoration and repair at the site of implantation of the ECM. Once the ECM has been fully resorbed, these growth factors and bioactive components are no longer released by the ECM at the implantation site to influence tissue repair and restoration. In order to moderate the appearance of these factors and components contributed by ECMs during repair of tissues, disclosed herein are compositions of ECM particles that have resorption times that vary from the rapid resorption times of known native ECMs.

(15) In one aspect, the invention relates to tissue graft compositions of ECM particles where the particles vary in density. Density as used herein refers to mass per unit volume of the particle. According to one embodiment of the invention, ECMs are manipulated to alter their densities and are then particularized. The particles, when implanted in a patient, resorb at rates related to their densities, with lower density particles being resorbed by the patient at a faster rate than more dense particles. Lower density particles resorb more quickly than higher density particles, releasing growth factors and other bioactive components quickly, thereby modulating an initial repair response at the tissue site. In contrast, higher density particles resorb slowly, releasing growth factors and other bioactive components more slowly, thereby maintaining the repair response at the site of implantation beyond the initial repair response influenced by the growth factors and bioactive components released by the lower density particles.

(16) According to the invention, having both lower and higher density particles as opposed to just higher density particles alone permits the released growth factors and other bioactive components provided by the lower density particles to modulate an initial rapid repair response at the tissue site due to the rapid resorption of the low density particles. Ideally, such a rapid repair response cannot be provided by the higher density particles alone because their resorption is slower and therefore, the release of bioactive components from the material that encourages tissue repair and restoration is also slower.

(17) Traditional ECMs have a rapid resorption time such that the ECMs may be fully resorbed at the tissue implantation site before the tissue site is completely repaired. Increased density ECM particles or aggregates of ECM particles of varying densities prepared according to the invention have prolonged resorption times and can therefore have an extended influence over tissue repair and restoration at the site of the injury as compared to using traditional ECMs. In other words, tissue graft compositions according to the invention allow the tissue site to repair completely or close to completely before the tissue graft composition is bioresorbed at the tissue site. According to the invention, compositions of ECM particles provide a specific bioresorption profile tailored to the restoration needs of a specific tissue site in a patient.

(18) The invention as described herein has the advantage of reducing the amount of space required to accommodate an ECM tissue graft construct at an implantation site in a patient. For example, by increasing the density of a tissue graft construct according to the invention, more ECM material can be included in a smaller volume. In addition, by increasing the density, the ECM material according to the invention takes longer to be resorbed at the implantation site in comparison to thicker, less dense ECM materials in the patient, thereby exerting a prolonged influence on tissue repair and restoration at the implantation site in the patient.

(19) According to the invention, the rate of resorption of compositions can be measured based on the number of days required for a certain volume of ECM to be resorbed by host tissue at the site of implantation. For example, a composition according to the invention, in one embodiment, includes a lower density particle having a volume of 100 M.sup.2 and a density of X that takes 15 days to be resorbed and a higher density particle having a volume of 100 M.sup.2 and having a density of 2 that takes 45 days to be resorbed. According to the invention, there is not necessarily a direct linear relationship between density and resorption time. As shown by this example, a particle that is twice the density of a first less dense particle may take longer than twice the resorption time of the first less dense particle to be resorbed.

(20) As previously mentioned, ECMs known in the art have a rapid bioresorption rate when implanted at a tissue site in a patient. In contrast to the rapid bioresorption profile demonstrated by known ECMs, compositions according to the invention have varied bioresorption times based on the density of particles in the tissue graft composition. For example, FIGS. 2A-C demonstrate the varied bioresorption profile of a tissue graft composition of particles of differing density according to the invention. FIG. 2A shows ECM particles 16 (of a higher density than the ECM shown in FIGS. 1A-C) and ECM particles 14 (of comparable density to the ECM shown in FIGS. 1A-C) on day 0, the day of implantation at the tissue site. At day X after implantation, as shown in FIG. 2B, the low density particles 14 have been partially resorbed as indicated by their reduced size. The more dense particles 16 have also not yet been fully resorbed. By day Y after implantation, as shown in FIG. 2C, the lower density particles 14 have been fully resorbed, while the more dense particles 16 have not, permitting the more dense particles to continue to promote tissue repair and restoration at the tissue site 10.

(21) In one embodiment of the invention, a tissue graft composition of particles includes particles of a first density and particles of a second, different density. In a further embodiment, the composition also includes particles of a third density; particles of a third and fourth density;

(22) particles of a third, fourth, and fifth density; or particles of a third, fourth, fifth, and sixth density, and so on. For example, in one embodiment, the particle composition has particles of two densities; in another, three densities; in yet another, four densities; in still another, five densities; and in yet another, six densities. According to one embodiment of the invention, the composition has particles of more than six different densities. In yet another embodiment, a particle may be a composite of more than one material, for example, a combination of different ECM types, each ECM type having a different density.

(23) In one embodiment of the invention, the composition has particles of a first density and particles of a second different density where the particles of the second density are more dense than the particles of the first density. In a further embodiment, the particles of the second density are at least about 150% of the density of the particles of the first density (i.e., 1.5 times more dense). This magnitude of difference in density is necessary such that the difference between the rate of bioresorption of the particles of the first density and the rate of bioresorption of the particles of the second density is large enough to allow the tissue being treated to be repaired before the composition is fully resorbed. Particles of a second density having a density less than about 150% of the first density do not have a bioresorption profile significantly different from the bioresorption profile of particles of the first density to allow the tissue site to be repaired before the composition is completely resorbed.

(24) In a further embodiment, the composition further includes particles of a third density. Particles of the third density are more dense than particles of the second density; the particles of the second density are more dense than particles of the first density. In a particular embodiment, the particles of the third density are at least about 150% of the density of the particles of the second density and the particles of the second density are at least about 150% of the density of the particles of the first density (i.e., 1.5 times more dense). Again, this magnitude of difference in density is necessary such that the difference between the rates of bioresorption between the particles of the first density, the second density, and the third density is large enough to allow the tissue being treated to be repaired before the composition is fully resorbed.

(25) In a further embodiment, the particles of a second density that are more dense than particles of a first density have a resorption rate of about 1 time, about 2 times, about 3 times, about 4 times, about 5 times, about 6 times, about 7 times, about 8 times, about 9 times, or about 10 times, for example, the rate of resorption of the particles of a first, less density.

(26) In a further embodiment, the particles of a second density are at least about 2 times, 2.5 times, 3 times, 3.5 times, 4 times, 4.5 times, or 5 times more dense than the density of the particles of a first density. In another embodiment, the particles of a third density are at least about 2 times, 2.5 times, 3 times, 3.5 times, 4 times, 4.5 times, or 5 times more dense than the particles of a second density.

(27) Preferably, the second density particles are at least about 150-500% of the density of the lower first density particles, more preferably about 200-400% of the density of the lower first density particles, and most preferably about 200-300% of the density of the lower first density particles.

(28) The particles according to one embodiment of the invention are extracellular matrix particles (ECM). According to the invention, ECM from which the particles of differing density are made may be derived from native tissues including, but not limited to submucosa, dermis, epithelial basement membrane, aponeurosis, tendon, ligament, smooth and skeletal muscle, and treatment site-specific ECM. The native tissue source may be porcine, bovine, or ovine, for example. The native tissue source may be allogenic, autogenic, or xenogenic to the patient. For example, if the patient is a human, the native tissue source is from another human (allogenic) or from the patient (autogenic). Alternatively, if the patient is a human, the native tissue is from a non-human species (xenogenic).

(29) ECM particles according to the invention have densities ranging from about 150 mg/cm.sup.3 to about 1800 mg/cm.sup.3, more preferably from about 150 mg/cm.sup.3 to about 1200 mg/cm.sup.3, and most preferably from about 150 mg/cm.sup.3 to about 600 mg/cm.sup.3.

(30) In another embodiment of the invention, particles of the invention are composed of, for example, collagen, i.e., type I, II, III and/or IV collagen; a bioresorbable polymer such as poly (L) lactic acid, poly (D) lactic acid, polyglycolic acid, polydioxanone, or tyrosine polycarbonate, and/or hyaluronic acid. Collagen particles according to the invention have densities ranging from about 150 mg/cm.sup.3 to about 1800 mg/cm.sup.3, more preferably from about 150 mg/cm.sup.3 to about 1200 mg/cm.sup.3, and most preferably from about 150 mg/cm.sup.3 to about 600 mg/cm.sup.3. Bioresorbable polymer particles according to the invention have densities ranging from about 150 mg/cm.sup.3 to about 1800 mg/cm.sup.3, more preferably from about 150 mg/cm.sup.3 to about 1200 mg/cm.sup.3, and most preferably from about 150 mg/cm.sup.3 to about 600 mg/cm.sup.3.

(31) According to one embodiment of the invention, bioactive materials such as growth factors, DNA, or other proteins are added to particles of bioresorbable polymers for release as the polymers are resorbed in the patient. In another embodiment, bioactive materials such as growth factors, DNA, or other proteins are added to ECM compositions of the invention for release as the ECM is resorbed in the patient.

(32) In one embodiment according to the invention, all particles of the first density are made from a first starting material and all particles of the second density are made from the same first starting material. For example, in one embodiment, all particles of the first density are made of an ECM material having a first density and all particles of the second density are particles of the same ECM material, but manipulated to have a second, different density. For example, the particles of a first density and the particles of the second density are each UBM particles. In another embodiment, all particles of the first density are a bioresorbable polymer and all particles of the second density are the same bioresorbable polymer. For example, the particles of the first density and the particles of the second density are poly (L) lactic acid.

(33) In another embodiment, all particles of the first density are made from a first material and all particles of the second density are made from a second material. For example, all particles of the first density are made from a first ECM material and all particles of the second density are made from a second ECM material. For example, in one embodiment, the particles of the first density are made of UBM while particles of the second density are made from SIS. In another embodiment, all particles of the first density are made from an ECM material while all particles of the second density are made from a bioresorbable polymer. In another embodiment, the first material is UBM, while the second material is aponeurosis. In another embodiment, the first material is UBM, while the second material is liver basement membrane (LBM). In yet another embodiment, the first material is UBM, while the second material is hyaluronic acid. In an even further embodiment, the first material is UBM, while the second material is poly(lactic-co-glycolicacid) (PLGA)

(34) Particles according to one embodiment of the invention range from about 0.1 M to about 2000 M. More preferably, the particles range in size from about 1 M-about 2000 M, even more preferably from about 1 M-about 1000 M, and most preferably from about 50 M-about 750 M in width, for example, diameter.

(35) According to one embodiment of the invention, the mixture of differing density particles is maintained in a powder form, while in another embodiment, the mixture of particles is maintained in a gel or liquid carrier for injection at a tissue site in a patient. Examples of suitable carriers include but are not limited to hyaluronic acid, gelatin, lecithin, collagen gel, and saline.

(36) In a further embodiment, particles according to the invention contain growth factors and/or other biologically active molecules. For example, particles such as ECM may naturally contain biologically active molecules that are released at the site of implantation as the ECM is bioresorbed by the patient. Alternatively, biologically active molecules may be added to the composition of particles to be delivered to the site of implantation with the particles. In yet another embodiment, particles according to the invention are coated with biologically active molecules. In yet another embodiment, particles of bioresorbable polymers are synthesized and formed into particles in the presence of biologically active molecules so that the biologically active molecules are contained within the bioresorbable polymer particle.

(37) Examples of biologically active molecules include, but are not limited to epidermal growth factor, TGF-alpha, TGF-beta, fibroblast growth factor, platelet derived growth factor, vascular endothelial growth factor, insulin-like growth factor, keratinocyte growth factor, and bone morphogenic protein to name a few, as well as pharmaceutical agents, and DNA vectors.

(38) According to the invention, particles release biologically active molecules in a time dependent fashion. For example, given that lower density particles are bioresorbed by patient tissue at a more rapid rate than higher density particles, biologically active molecules, such as growth factors, are released from lower density particles relatively quickly after implantation as compared to their release from higher density particles. Higher density particles bioresorb more slowly and therefore release biologically active molecules at the tissue site over a longer period of time than the lower density particles, thereby exposing the tissue site to particular biologically active molecules for a longer period of time than if only lower density particles are present.

(39) According to another aspect, the invention relates to methods of making mixtures of particles where a first type of particle has a first density and a second type of particle has a second density.

(40) According to one embodiment of the method of making mixtures of particles of the invention, sheets of ECM are comminuted to create a mixture of particles of differing density. For example, as shown in FIG. 3, a mixture 24 of A particles 14 and B particles 16 where A particles 14 are less dense than B particles 16 is made from two ECM sheets 20, 22 of differing densities. A Sheet 20 is comminuted to form A particles by chopping, cutting, pulverizing, milling, or grinding A Sheet 20 with a suitable device that forms particles such as a blender, a hammer mill, a knife mill, a centrifugal mill, a die and press system, or a roller crusher, for example. B Sheet 22 is comminuted to form B particles 16 by chopping, cutting, crushing, pulverizing, milling, mincing, or grinding B Sheet 22 with a suitable device to form particles 16. A particles 14 and B particles 16 have size ranges and density ranges as described above.

(41) To form the mixture, a predetermined number of A particles 14 is then measured and mixed with a predetermined number of B particles 16 to form a composite mixture 24 of A particles 14 and B particles 16, where the concentration of A particles 14 to B particles 16 is predetermined based on a particular desired resorption profile of the mixture at a tissue site in a patient. For example, for a tissue site that takes a long time to repair, such as a joint, fewer less dense A particles 14 are selected compared to the number of higher density B particles 16 in order to provide a prolonged resorption of ECM at the tissue site. A particles 14 may be selected ata ratio of (A:B) 1:1, 1:2, 1:3, 1:4: 2:3, 3:4, 2:1, 3:1, 3:2, 4:3, or 4:1 B particles 16, for example, depending on the particular tissue site to be treated and the desired resorption profile of the particles for treating that tissue site.

(42) According to a further embodiment of the invention, a mixture of particles includes three types of particles, each type of a differing density. For example, the mixture includes low density particles, intermediate density particles, and higher density particles. For example, according to the invention, a sheet of a third density (C) is comminuted to form particles of a third density C, higher than the density of particles formed from A sheet 20 and B sheet 22. A predetermined number of particles of the third density are measured out and added to the mixture of A particles 14 and B particles 16. Particles of a third density maybe be selected at a ratio of (A:B:C) 1:1:1, 1:1:2, 1:1:3, 1:1:4, 1:2:1, 1:2:2, 1:2:3, 1:2:4, 1:3:1, 1:3:2, 1:3:3, 1:3:4, 1:4:1, 1:4:2, 1:4:3, 1:4:4, 2:1:1, 2:1:2, 2:1:3, 2:1:4, 2:2:1, 2:2:3, 2:3:1, 2:3:2, 2:3:3, 2:3:4, 2:4:1, 2:4:3, 3:1:1, 3:1:2: 3:1:3, 3:1:4, 3:2:1, 3:2:2, 3:2:3, 3:2:4, 3:3:1, 3:3:2, 3:3:4, 3:4:1, 3:4:2, 3:4:3, or 3:4:4, for example, depending on the particular tissue site to be treated and the predetermined resorption profile of the particles for treating the tissue site.

(43) For example, in one embodiment, a composition of the invention has two parts lower density particles to 1 part higher density particles (2:1) and is used for wound treatment, i.e., of the skin and skin adnexa. The higher concentration of lower density particles, when applied to a wound, provides a rapid influx of bioactive components to activate the restoration and repair process as the lower density particles are bioresorbed, followed by a longer term release of bioactive components over time at the implantation site as the higher density particles resorb slowly over time. Additionally, specific bioactive components may be added to the lower or higher density particles to tailor the release of bioactive components to the stage of restoration and repair at the tissue site.

(44) In another embodiment, a composition of the invention has 1 part lower density particles to four parts higher density particles (1:4) or 1 part lowest density particles, 2 parts lower density particles, and 2 parts highest density particles, (1:2:2) and is used for tendon or ligament repair. Restoration and repair of tendons and ligaments takes longer than at body sites that are typically more vascular and more cellular. Accordingly, the 1:4 or 1:2:2 density profile composition provides a longer resorption profile of the composition for restoring or repairing tendons and ligaments as compared to traditional ECMs.

(45) In yet another embodiment, a composition of the invention has 2 parts lower density particles to 3 parts higher density particles to (2:3) and is used for scar revision or plastic surgery applications to promote a rapid early tissue restoration response while also having prolonged bioactive component release at the site to help prevent hypertrophic scar formation over a longer time.

(46) In yet another embodiment, a composition of the invention has one part lower density particles to 2 parts higher density particles (1:2) and is used for applications involving soft tissue augmentation. In a further embodiment, a composition of the invention has one part lower density particles to 3 parts higher density particles (1:3) and is used for ophthalmological applications.

(47) According to the invention, sheets of ECM 20, 22 can be treated to alter their densities prior to being comminuted. The density of a sheet of ECM can be increased by mechanically compressing the material, dehydrating the material, vacuuming the material, crosslinking the material, or lyophilizing the material, for example. The density of a sheet of ECM can be decreased by hydration, expanding the pore structure under vacuum, mechanically expanding the material, or freezing the material in water, for example.

(48) In another embodiment of the method of making a mixture of particles having a first density A and particles having a second density B, particulate matter from a sheet of ECM having areas of density A and density B are removed from the sheet of ECM and mixed together. For example, as shown in FIG. 4, sheet 30 is partially compressed to form a sheet 30 having an area A 31 of a first lower density and area B 32 of a second higher density. The sheet 30 is then milled to produce particles 33 from area A 31 having a lower density A and particles 34 from area B 32 having a higher density B.

(49) According to one embodiment of the invention, the portion of sheet 30 that is compressed relative to the portion that is not compressed or less compressed is determined based on the predetermined bioresorption profile for the particles at a given tissue site in a patient. Accordingly, for example, if a 1:1 ratio of A:B (low density:high density) particles is sought to achieve the desired bioresorption profile, one half of the sheet is compressed to the desired density prior to comminuting the sheet into a mixture of particles of A and B densities. In one embodiment, the size of sheet 30 is selected prior to compression of a portion of the sheet 30 and the whole sheet 30 is comminuted after compression. Alternatively, in another embodiment, a portion of the sheet 30 is compressed after which a portion of sheet 30 is cut away from the whole of sheet 30 with the cut-away portion being comminuted.

(50) In a further embodiment, a portion of the sheet 30 is compressed to have a third density C. The sheet 30 is then milled to produce a mixture of particles having a density A, a density B, and the third density C. The third density may be higher than A or higher than A and B.

(51) Another embodiment of the method of making a mixture of particles having differing densities is shown in FIG. 5. According to one embodiment, a sheet 40 of ECM of a first density A is joined with a sheet 41 of ECM of a second density B to form a composite sheet 45. The composite sheet 45 is then comminuted to create a mixture of particles 46 having different densities. For example, when composite sheet 45 is comminuted, the mixture includes A particles 42 of low density derived from sheet A, B particles 43 of a higher density derived from sheet B, and C particles 44 of an intermediate density formed from the interfacing area 47 of the A sheet 40 and the B sheet 41.

(52) A further embodiment of the method of making a mixture of particles having different densities is shown in FIG. 6. According to this embodiment, particles 65 have a core 61 and a coating 62. The core 61 has a density X and may be coated with any of a variety of coatings 63, 64, 66 having a variety of densities. For example, some particles 65 have cores 61 coated with a first material 66 having a first density to produce particles having a density A, while other particles 65 have cores 61 that are coated with a second material having a second density 63 to produce particles having a density B, while yet other particles 65 have cores 61 coated with a third material having a third density 64 to produce particles having a third density C. Coating the core 61 can be performed, for example, by precipitating the coating onto a particle from a solution, by a roll coater, or by a granulator. Materials for coatings may include, for example, an ECM, a gel made from an ECM such as UBM gel or SIS gel, PLGA, hyaluronic acid, or collagen, for example. The core 61 of particles 65 may include, for example, ECMs as described herein.

(53) In another aspect of the invention, mixtures of comminuted particles of differing density are coated on devices such as synthetic or natural wound repair matrices for implantation into a patient. For example, as shown in FIG. 7, a mixture of A lower density particles 52 and B higher density particles 53 is applied to a sheet 50 of ECM. The particles 52, 53 are adhered to the sheet by compression or use of an adhesive, for example. Accordingly, when the sheet 50 is implanted at a tissue site in a patient, the particles 52, 53 provide more surface area to contact with the tissue site in the patient. The A particles 52 and B particles 53 are more dense in one embodiment than the ECM sheet 50 so that after the sheet 50 has been fully resorbed, the tissue continues to repair in the presence of particles 52 and 53. The remaining particles 52, 53 allow the tissue to repair before the particles 52, 53 of the composition are completely resorbed.

(54) In a further embodiment, mixtures of comminuted particles of differing density coated on a device such as a synthetic or natural wound repair matrix is implanted in a patient for use as an adhesion barrier. For example, as shown in FIG. 7, a matrix in sheet form 50, for example, is coated on one side with particles 52, 53 of differing density according to the invention and is implanted in a patient. Any adhesions forming at the implant site adhere to the particulate side of the matrix and as the particles 52, 53 are resorbed, the adhesions no longer remain attached to the matrix, but to the particles and the adhesions are subsequently resorbed. In an alternative embodiment, particles of differing density according to the invention are sandwiched between a first matrix sheet of a first heavier density and a second matrix sheet of a lighter density to form an implant. The implant is implanted at a site in the patient with the more dense matrix sheet implanted against the tissue requiring repair and the less dense sheet facing outward into the body of the patient. As adhesions form on the outward facing less dense sheet, the less dense sheet is resorbed allowing any adhesions to become unattached from the more dense portion of the implant, the adhesions then being subsequently resorbed by the patient along with the less dense sheet of matrix.

(55) In one embodiment, the mixture of particles 52, 53 is adhered to one surface of the sheet 50, while in another embodiment the mixture of particles 52, 53 is adhered to all surfaces of the sheet 50. For example, the sheet 50 bioresorbs in the patient within 60 days, the lower density A particles bioresorb in 7 days, and the higher density B particles bioresorb in the patient within 30 days.

(56) In a further embodiment, a mixture of comminuted particles according to the invention is coated on a surgical medical device prior to the devices' implantation at a tissue site in a patient. For example, a mixture of lower density A particles and higher density B particles are coated on a medical implant such as, but not limited to, a stent, a cardiac occluder, suture material, or a mesh. The particles are adhered to the device by, for example, a chemical bond, a peptide linker, a bioresorbable adhesive, such as cyanoacrylate or fibrin glue, or by mechanical means such as lyophilization of the particles on the device or compressing the particles onto the device. Once the device is implanted at the tissue site in the patient, the lower density A particles bioresorb quickly providing an initial restoration response at the tissue site in the patient, while the higher density B particles are not fully bioresorbed until the tissue site is completely repaired. For example, the lower density A particles are bioresorbed by the patient within, for example, 7 days, while the higher density B particles are bioresorbed by the patient within, for example, 30 days.

(57) In another aspect, the invention includes aggregates of particles of differing density which form an implant with portions of differing densities for implantation at a tissue site in a patient. According to the invention, as shown in FIGS. 8A and 8B, high density particles B are compressed to form a shape replicating a body part or portion of a body part in a mammal, for example, a human, and form the core 71a, b of an implant 70, 72. Lower density particles A are then compressed or coated around the 71a, b core of B particles to form the peripheral layer of the implant 70, 72. As shown in FIG. 8A, the A and B particles form an ovoid shape implant 70 to be used in replacement or repair, for example, of a kidney or liver. As shown in FIG. 8B, the peripheral A and core B particles form roughly a triangular shape implant 72 to be used in replacement or repair, for example, of a nose.

(58) The implant 70, 72 is implanted into a patient at a site requiring restoration or repair, for example. Upon implantation, the peripheral A particles having a lower density provide an immediate repair and restoration response at the tissue site in the patient. After the peripheral A particles rapidly bioresorb, the higher density core 71a, b B particles continue to provide a repair and restoration response at the tissue site in the patient over a longer period of time than the peripheral A particles alone. The core 71a, b B particles are typically not fully resorbed by the patient until the tissue site is repaired. For example, the peripheral A particles of lower density are bioresorbed by the patient within, for example, 14 days, while the core B particles of higher density are bioresorbed by the patient within, for example, 60 days.

(59) In a further embodiment, as shown in FIG. 9, an implant 74 can be formed by compressing a mixture of lower density A particles 76 with higher density B particles 78. The lower density particles 76 resorb at a faster rate which create channels in the implant and provide space for body tissue to grow into the implant, while the higher density particles resorb more slowly, providing structural support for the construct as tissue restoration and repair occurs and releasing bioactive components over time as the higher density particles resorb. The implant 74 can be compressed into any number of three dimensional shapes such as a kidney, spleen, nose, ear, liver, bone, or other body organ.

(60) In another embodiment of the invention, a three dimensional construct of compressed particles of varying density is made to mimic natural biologic tissue densities. For example, in one embodiment, a construct is made with denser particulate matter on one side, of a sheet for example, and more dense particulate matter is placed on the other side. Such a construct can be used at a bone-cartilage interface where the more dense portion is inserted into the bone defect while the less dense portion is positioned in the cartilage defect. The slower resorbing, higher density particulate remodels according to the slower rate of bone generation resulting in more dense tissue restoration consistent with bone, while the less dense portion remodels more quickly and creates a less dense tissue restoration consistent with cartilage. The invention also contemplates three dimensional constructs of compressed particulate having different density regions that are designed, for example, to mimic the native density variations of a spinal disc or the muscle-tendon interface.

(61) In a further embodiment, as shown in FIG. 10, a sheet 80 is formed by compressing a mixture of lower density A particles 81 with higher density B particles 82. The sheet 80 can be formed, for example, by hydrating the particles and compressing the particles together by application of a mechanical force or vacuum to form a cake or wafer-like construct. The sheet 80 can be used to treat topical wounds, for example, of the skin, but can also be used to treat other areas of the body such as a hernia, intestine, bowel, blood vessels, the heart, or any other organ, by cutting the sheet to size and/or bending or wrapping the sheet to fit the implant location. The sheet 80, with its differing density particles, will break down on implantation as a result of bioresorption of the lower density particles, providing greater surface area for tissue restoration and repair at the site of implantation. The sheet form 82 has the benefit of being easy to apply to the implant site, but also has the ability to provide a prolonged restoration response at the implant site as the particles 81, 82 are resorbed at different rates, in contrast to sheets made of only one density of material.

(62) In a further embodiment, as shown in FIG. 11, mixtures of particles 91 of differing densities, i.e., A particles of a first density 92 and B particles of a second density 93, are sandwiched between sheets 94 of an ECM. For example, a powder form mixture of particles of a first density A 92 and a second density B 93 is sandwiched between sheets 94 of UBM or SIS to form a multi-laminate construct 95 of loose particles 91 and sheets 94. Any number of alternating layers of sheets 94 and the mixture 91 of particles 92, 93 can be constructed according to the invention. For example, in one embodiment, a first sheet 94A is covered with the particle mixture 91; the particle mixture 91 is covered with a second sheet 94B; the second sheet 94B is covered with the particle mixture 91; and a third sheet 94C applied to form a sheet-particle-sheet-particle-sheet multi-laminate construct 95. The sheets of multi-laminate construct 95, when implanted at a site in a patient, provide a microarchitecture for tissue restoration at the site while the particles 92, 93 breaking down at varying rates provide a continuous supply of bioactive factors to assist in restoration at the implant site.

(63) In a further embodiment, particles of a lower density A and particles of a higher density B can be combined with a bone substitute material such as beta-tricalcium phosphate (TCP), hydroxyapatite, sintered bovine bone, sintered porcine bone, demineralized allograft bone, or mineralized allograft bone, for example. In an alternative embodiment, the bone particles and the differing density ECM particles of the invention can be mixed together without an adhesive, binder, or any other additive. The particles are coated on and/or injected into the bone substitute material. When the bone substitute material is implanted at the tissue site in the patient, the bone substitute material provides a rigid form to stabilize the defect while the lower density particles promote rapid tissue repair at the site. Over time, the higher density particles sustain tissue repair at the site of implantation as they are bioresorbed at a slower rate than the lower density particles. The higher density particles are typically not fully bioresorbed until the tissue site is repaired.

(64) According to another embodiment of the invention, a mixture of particles having particles of a lower density A and particles of a higher density B are provided in powder form. A powder has particles of 1-1000 m. The powder can be sprinkled topically on a tissue site in a patient, for example, a cut, scrape, incision, or puncture of the skin, or while a tissue site is exposed during surgery, the powder form can be sprinkled at the tissue site. In another embodiment, the differing density particles are administered via inhalation, for example, through use of an inhaler such as those used to administered asthma treatments.

(65) In an alternative embodiment, a mixture of particles of a lower density A and particles of a higher density B are maintained in a liquid, gel or paste medium. The liquid or gel is injected via a syringe at a tissue site in a patient. Alternatively, the liquid or gel is maintained within a tube and is squeezed out of the tube and applied to the tissue site. In a further embodiment, the powder, liquid, gel, or paste composition includes particles of a third density.

(66) In another aspect, the invention encompasses methods for modulating the rate of tissue restoration at a tissue site in a patient in need of repair or restoration. The method requires administering to a site in a patient a mixture that includes at least two types of particles, each type having a different density, one type of particle being more dense than the other type of particle. The differing density particles are selected to form the mixture based on their respective bioresorption profiles, i.e., the higher density particles are selected because they take longer to be resorbed at the tissue site than the particles of a lower density and the lower density particles are selected because they will be resorbed quickly at the tissue site. According to the invention, a physician particularly selects the mixture to have particles with particular rates of bioresorption tailored for the particular tissue site to be repaired or restored. The mixture is administered to the patient topically, via injection to the tissue site, or it may be delivered to the tissue site surgically, for example.

(67) Tissue sites treatable with compositions according to the invention include, but are not limited to head and neck structures, joints including synovium, joint capsule, labrum, intraarticular ligaments and cartilage, intervertebral discs, pelvic floor, tendon, bone, ligament, kidney, spleen, liver, muscle tissue, urinary bladder, ureter, uterus, intestine, pancreas, blood vessel, skin and skin adnexa, heart including intracardiac structures, nose, ear, breast, or a body cavity.

EXAMPLES

(68) 1. Tissue Restoration of a Chronic Skin Ulcer

(69) A mixture of ECM particles is made by taking 60 mg of particles of UBM of size 50-1000 m and having a density of approximately 0.05 g/cm.sup.3 and 20 mg of particles of UBM of size 50-500 m and having a density of approximately 0.25 g/cm.sup.3, for a 3:1 ratio of particles based on weight. The mixture is applied topically to a chronic skin ulcer. Within 10 days, the 0.05 g/cm.sup.3 particles are resorbed and tissue restoration is observed at the wound site. Within 30 days, the 0.25 g/cm.sup.3 particles are resorbed and tissue at the wound site is fully restored.

(70) 2. Tissue Restoration of a Kidney

(71) A mixture of ECM particles is prepared for implantation in a kidney at a site from which a tumor has been excised. A mixture of ECM particles is made by taking 200 mg of particles of UBM of size 500-1000 m and having a density of approximately 0.10 g/cm.sup.3 and 1000 mg of particles of UBM of size 50-500 m and having a density of approximately 0.35 g/cm.sup.3, for a 1:5 ratio of particles based on weight. The mixture is surgically implanted in the patient via a syringe. Within 20 days, the 0.10 g/cm.sup.3 particles have resorbed and tissue restoration is observed at the wound site via magnetic resonance imaging. Within 60 days, the 0.35 g/cm.sup.3 particles are resorbed and tissue at the wound site is fully restored as viewed via magnetic resonance imaging.