MICROSPONGE DELIVERY SYSTEM (MDS): A UNIQUE TECHNOLOGY FOR DELIVERY OF ACTIVE INGREDIENTS

MICROSPONGE DELIVERY SYSTEM (MDS): A UNIQUE TECHNOLOGY FOR DELIVERY OF ACTIVE INGREDIENTS

Saurabh Kumar*, L.K. Tyagi, and Dashrath Singh

Department of Pharmacy, Institute of Bio-medical Education and Research, Mangalayatan University, Aligarh, Uttar Pradesh, India

ABSTRACT

In pharmaceutical industry, various controlled released dosage forms like solid formulation, semi solid formulation and topical preparation have more importance due to efficacy and patient compliance. Topical preparations have some disadvantages like unpleasant odour, greasiness and skin irritation and fail to reach the systemic circulation in sufficient amounts in few cases. This problem is overcome by microsponge delivery system. Microsponges are tiny sponge like spherical and highly porous micro-sized particles with a unique ability for entrapping actives. They offers programmable release active drug into the skin in order to reduce systemic exposure and minimize local cutaneous reactions to active. These MDS’s are closely related to microspheres, and used in the sun screens, creams, ointments, over- the-counter (OTC)  skin care preparations, recently used in oral drug as well as biopharmaceuticals (peptides, proteins and DNA-based therapeutics) drug delivery. The present review introduces microsponge technology along with its synthesis, characterization, programmable parameters and release mechanism of MDS.

Quasi-Emulsion Solvent Diffusion Method

INTRODUCTION: The microsponge technology was developed by Won in 1987 and the original patents were assigned to Advanced Polymer Systems, Inc ¹. Several predictable and reliable systems were developed for systemic delivered through skin under the title of transdermal delivery system (TDS). It has improved the efficacy and safety of many drugs that may be better administered via skin. But transdermal delivery system is unrealistic for delivery of materials whose final target is skin itself ².

No efficient vehicles have been developed for controlled and localized delivery of drugs into the stratum corneum and underlying skin layers and not beyond the epidermis ³. Conventional formulations of topical drugs are intended to work on the outer layers of the skin.

Typically, such products release their active ingredients upon application, producing a highly concentrated layer of active ingredient that is rapidly absorbed ⁴. Furthermore, the significance of topical drugs suffers from various problems like greasiness, stickiness associated with the ointments and so on, that often result in lack of patient compliance.

These vehicles necessitate a high concentration of active agents for effective therapy because of their low efficiency of delivery system, resulting into irritation and allergic reactions in significant users. Other drawbacks of topical formulations are uncontrolled evaporation of active ingredient, unpleasant odour. The fundamental appeal of the microsponge technology stems from these difficulties experienced with conventional formulations in releasing active ingredients over an extended period of time.

Conventional dermatological products typically provide active ingredients in relatively high concentrations but with a short duration of action. This may lead to a cycle of short term over medication followed by long term under medication. In contrast, microsponge technology allows an even and sustained rate of release, reducing irritation while maintaining efficacy.

Microsponges are patented delivery systems composed of porous microspheres. They are tiny sponge like spherical particles that consist of a myriad of interconnecting voids within a non collapsible structure with a large porous surface. The size of the microsponges ranges from 5 - 300μm (Figure 1) in diameter and a typical 25μm sphere can have up to 250000 pores and an internal pore structure equivalent to 10ft in length, providing a total pore volume of about 1ml/g. These microsponges have the capacity to entrap a wide range of active ingredients such as emollients, fragrances, essential oils and anti-infective, etc. are used as a topical carrier system⁵.

FIG. 1: POROUS NATURE OF A MICROSPONGE

Further, these porous microspheres with active ingredients can be incorporated in to formulations such as creams, lotions and powders. Release of drug into the skin is initiated by a variety of triggers, including rubbing and higher than ambient skin temperature ⁶. Their high degree of cross-linking results in particles that are insoluble, inert and of sufficient strength to stand up to the high shear commonly used in manufacturing of creams, lotions, and powders. Their characteristic feature is the capacity to adsorb or “load” a high degree of active materials into the particle and on to its surface.

Its large capacity for entrapment of actives, up to three times its weight, differentiates microsponge products from other types of dermatological delivery systems. The active payload is protected in the formulation by the microsponge particle; it is delivered to skin through controlled diffusion. This sustained release of actives to skin over time is an extremely valuable tool to extend the efficacy and lessen the irritation.

Advantages of Microsponge Delivery System ^{5, 7}:

• Microsponges can absorb oil up to 6 times its weight without drying.
• It provides continuous action up to 12 hours i.e. extended release.
• Improved product elegancy.
• Lessen the irritation and better tolerance leads to improved patient compliance.
• It can also improve efficacy in treatment.
• They have better thermal, physical and chemical stability.
• These are non-irritating, non-mutagenic, non-allergenic and non-toxic.
• MDS allows the incorporation of immiscible products.
• They have superior formulation flexibility.
• In contrast to other technologies like microencapsulation and liposomes, MDS has wide range of chemical stability, higher payload and are easy to formulate.
• Liquids can be converted in to powders improving material processing.
• It has flexibility to develop novel product forms.
• MDS can improve bioavailability of the drugs.

Characteristics of Microsponges ⁸:

• Microsponge formulations are stable over range of pH 1 to 11;
• Microsponge formulations are stable at the temperature up to 130^oC;
• Microsponge formulations are compatible with most vehicles and ingredients;
• Microsponge formulations are self sterilizing as their average pore size is 0.25μm where bacteria cannot penetrate;
• Microsponge formulations have higher payload (50 to 60%), still free flowing and can be cost effective.

Characteristics of materials that are entrapped in Microsponges: Most liquid or soluble ingredients can be entrapped in the particles ⁹. Actives that can be entrapped in microsponges must meet following requirements,

• It should be either fully miscible in monomer or capable of being made miscible by addition of small amount of a water immiscible solvent.
• It should be water immiscible or at most only slightly soluble.
• It should be inert to monomers.
• It should be stable in contact with polymerization catalyst and conditions of polymerization.

Drugs explored in Microsponge Delivery System ^9-17:

• Ibuprofen
• Fluconazole
• Benzyl peroxide
• Ketoprofen
• Paracetamol
• Dicyclomine
• Flurbiprofen
• Ketoconazole
• Retinol

Formulation Aids: Various polymers like Eudragit RS100, Dimethacrylate, Ethyl Cellulose, Polystyrene and PHEMA can form a microsponge ‘cage’. In addition to actives; some microsponges contain plasticizers like Triethylcitrate (TEC) that help to stabilize their structure ^18-21.

Preparation of Microsponges: Drug loading in microsponges drug delivery system done in two ways, one step process or by two step process as discussed in liquid-liquid suspension polymerization and quasi emulsion solvent diffusion techniques which are based on physicochemical properties of drug to be loaded. If the drug is typically an inert non-polar material, will create the porous structure it is called porogen. Porogen drug, which neither hinders the polymerization nor become activated by it and stable to free radicals is entrapped with one step process.

Liquid-Liquid Suspension Polymerization: The  porous  microspheres  are  prepared  by  suspension  polymerization  method  in  liquid-liquid  systems ²². In  their  preparation,  the  monomers  are  first  dissolved  along  with  active ingredients  in  a  suitable  solvent  solution  of  monomer  and then  dispersed  in  the aqueous  phase,  which  consist  of  additives  (surfactant,  suspending  agents,  etc.).  The polymerization  is  then  initiated  by  adding  catalyst  or  by  increasing  temperature  or irradiation  (Figure  2).

FIG. 2: REACTION VESSEL FOR MICROSPONGE PREPARATION BY LIQUID-LIQUID SUSPENSION METHOD

The polymerization process continues the formation of a reservoir type of system with spherical structure. After the polymerization process the solvent is removed leaving the spherical structured porous microspheres, i.e., microsponges. The various steps involved in the preparation of microsponges are summarized in scheme 1 as follows ¹²:

SCHEME 1:  STEPS IN THE PREPARATION OF MICROSPONGES

Quasi-emulsion Solvent Diffusion: All microsponges were prepared by a quasi-emulsion solvent diffusion method (Figure 3) using an external phase of containing distilled water and polyvinyl alcohol (PVA) 72 000. The internal phase consisted of Drug, ethyl alcohol, polymer and TEC, which was added at an amount of 20% of the polymer in order to facilitate the plasticity. At first, the internal phase was prepared at 60°C and added to the external phase at room temperature. After emulsification, the mixture was continuously stirred for 2 hours. Then the mixture was filtered to separate the microsponges. The product was washed and dried by vacuum oven at 40°C for 24 hours ¹⁹.

FIG. 3: PREPARATION OF MICROSPONGES BY QUASI-EMULSION SOLVENT DIFFUSION METHOD

Evaluation Parameters of Microsponges:

• Particle size (Microscopy)
• Morphology and Surface topography
• Characterization of pore structure
• Loading efficiency and production yield
• Characterization of pore structure
• Compatibility studies
• Resiliency
• Drug release study

Physical Characterization of Microsponges:

Particle Size Determination ²¹: Particle  size  analysis  of   loaded  and  unloaded  microsponges  can  be  performed by  laser light diffractometry or any other suitable method. The values can be expressed for all formulations as mean particle size range. Cumulative percentage drug release from microsponges of different particle size will be plotted against time to study effect of particle size on drug release.  Particles  larger  than 30µm  can  impart  gritty  feeling  and  hence  particles  of sizes between 10 and  25µm  are  preferred  to  use  in  final  topical  formulation.

Morphology and Surface Topography of Microsponges ²³: For  morphology  and  surface  topography,  prepared  microsponges  can  be  coated  with  gold-palladium  under  an  argon  atmosphere  at   room  temperature  and  then  the  surface morphology  of  the  microsponges  can  be  studied  by  scanning  electron  microscopy  (SEM). SEM  of  a  fractured   microsponge  particle  can  also  be  taken  to  illustrate  its  ultra  structure.

Determination of Loading Efficiency and Production Yield ²⁴: The loading efficiency (%) of the microsponges can be calculated according to the following equation:

Actual Drug Content in microsponges

Loading Efficiency=                                                                          X 100

Theoretical Drug Content

The production yield of the microparticles can be determined by calculating accurately the initial weight of the raw materials and the last weight of the microsponge obtained.

Practical Mass of Microsponges

Production Yield=                                                                      X 100

Theoretical Mass (Polymer + Drug)
Determination of   True Density ²⁵: The true density of microparticles is measured using an ultra-pycnometer under helium gas and is calculated from a mean of repeated determinations.

Characterization of Pore Structure ²⁶: Porosity parameters of microsponges such as intrusion extrusion isotherms, pore size distribution, total pore surface area, average pore diameters, shape and morphology of the pores, bulk and apparent density can be determined by using mercury intrusion porosimetry. Incremental intrusion volumes can be plotted against pore diameters that represented pore size distributions. The pore diameter of microsponges can be calculated by using Washburn equation:

Here; D is the pore diameter (μm),  is the surface tension of mercury (485 dyn cm−1), θ is the contact angle (130^o), and P is the pressure (psia).

Total pore area (A_tot) was calculated by using equation,

Here, P is the pressure (psia), V is the intrusion volume (ml g−1), Vtot is the total specific intrusion volume (ml g−1).  The average pore diameter (Dm) was calculated by using equation:

Envelope (bulk) density (ρse) of the microsponges was calculated by using equation:

Here, Ws is the weight of the microsponge sample (g), Vp is the empty penetrometer (ml), VHgis the volume of mercury (ml). Absolute (skeletal) density (ρsa) of microsponges was calculated by using equation:

Here, Vse is the volume of the penetrometer minus the volume of the mercury (ml). Finally, the percent porosity of the sample was found from equation,

Pore morphology can be characterized from the intrusion–extrusion profiles of mercury in the microsponges as described by Orr.

Compatibility Studies: Compatibility of drug with reaction adjuncts can be studied by thin layer chromatography (TLC) and Fourier Transform Infra-red spectroscopy (FT-IR) ²⁷. Effect of polymerization on crystallinity of the drug can be studied by powder X-ray diffraction (XRD) and Differential Scanning Colorimetry (DSC) ²⁸. For  DSC approximately 5mg samples can be accurately weighed into aluminium pans and sealed and can be run at a heating rate of 15^oC/min over a temperature range 25–430^oC in atmosphere of nitrogen ^29-30.

Polymer/Monomer Composition: Factors such as microsponge size, drug loading, and polymer composition govern the drug release from microsponges. Polymer composition of the MDS can affect partition coefficient of the entrapped drug between the vehicle and the microsponge system and hence have direct influence on the release rate of entrapped drug. Release of drug from microsponge systems of different polymer compositions can be studied by plotting cumulative % drug release against time. Release rate and total amount of drug released from the system composed of methyl methacrylate/ ethylene glycol dimethacrylate is slower than styrene/divinyl benzene system. ³

Resiliency: Resiliency (viscoelastic properties) of microsponges can be modified to produce beadlets that is softer or firmer according to the needs of the final formulation. Increased cross-linking tends to slow down the rate of release. Hence resiliency of microsponges will be studied and optimized as per the requirement by considering release as a function of cross-linking with time ³¹.

Dissolution Studies: Dissolution profile of microsponges can be studied by use of dissolution apparatus (USP XXIII) with a modified basket consisted of 5µm stainless steel mesh. Speed of the rotation is 150 rpm. The dissolution medium is selected while considering solubility of actives to ensure sink conditions. Samples from the dissolution medium can be analyzed by suitable analytical method at various intervals ³².

Kinetics of Release: To determine the drug release mechanism and to compare the release profile differences among  microsponges,  the  drug  released  amount  versus  time  was  used. The release data were analyzed with the following mathematical models:

OR

Where, Q is the amount of the released at time (h), n is a diffusion exponent which indicates the release mechanism, and k₁ is a constant characteristic of the drug–polymer interaction. From the slope and intercept of the plot of log Q versus log t, kinetic parameters n and k₁ were calculated.

For comparison purposes, the data was also subjected to Eq., which may be considered a simple, Higuchi type equation;

Above Eq. for release data dependent on the square root of time, would give a straight line release profile, with k₂ presented as a root time dissolution rate constant and C as a constant.

Mechanism of Drug Release: By proper manipulation of the aforementioned programmable parameters, microsponge can be designed to release given amount of active ingredients over time in response to one or more external triggers.

Temperature Change ³³: At room temperature, few entrapped active ingredients can be too viscous to flow suddenly from microsponges onto the skin. With increase in skin temperature, flow rate is also increased and therefore release is also enhanced.

Pressure ³⁴: Rubbing or pressure applied can release the active ingredient from microsponges onto skin.

Solubility ³⁵: Microsponges loaded with water miscible ingredients like antiseptics and antiperspirants will release the ingredient in the presence of water. The release can also be activated by diffusion but taking into consideration, the partition coefficient of the ingredient between the microsponges and the external system.

pH Triggered  Systems ³⁶: Triggering  the  pH-based  release  of   the  active  can  be  achieved  by  modifying  the  coating on  the  microsponge.  This has many applications in drug delivery.

Safety Considerations ^{33, 37-38}: Safety studies of microsponges can be confirmed by:

• Allergenicity in guinea pigs
• Eye irritation studies in rabbits
• Mutagenicity in bacteria
• Oral toxicity studies in rats
• Skin irritation studies in rabbits

Applications of Microsponge Systems: Microsponges are designed to deliver the pharmaceutical active ingredient efficiently at the minimum dose and also to enhance stability, reduce side effects and modify drug release. Microsponges are porous, polymeric microspheres that are used mostly for topical but recently used for oral administration (Table 1).

TABLE 1: APPLICATIONS OF MICROSPONGE SYSTEM

Microsponge for Topical Delivery: Benzoyl peroxide is mainly used in the treatment of mild to moderate acne and athlete’s foot and the most common side effect associated with Benzoyl peroxide is skin irritation and it has been shown that controlled release of Benzoyl peroxide from a delivery system to the skin could lessen the side effect while reducing percutaneous absorption. Topical delivery system with reduced irritancy was successfully developed ³⁹.

Jelvehgari et al., developed Benzoyl peroxide microsponges by using emulsion solvent diffusion method and investigate the parameters affecting the morphology and other characteristics by using scanning electron microscopy (SEM). The morphology and particle size of microsponges were affected by drug: polymer ratio, amount of emulsifier used and stirring rate. The results showed that with increase in the ratio of drug: polymer resulted in a reduction in the rate of release of Benzoyl peroxide from the microsponges. The release data showed that the highest and the lowest release rates were obtained from lotions containing plain Benzoyl peroxide particles and Benzoyl peroxide microsponges with the drug: polymer ratio (13:1) respectively ¹⁹.

Amrutiya et al., developed microsponge based topical delivery system of mupirocin by using emulsion solvent diffusion method for sustained release and enhanced drug deposition in the skin. In-vitro drug release, ex-vivo drug deposition, and in-vivo antibacterial activity of mupirocin loaded formulations were studied. Microsponges were spherical and porous, and there was no interaction between drug and polymer molecules. Emulgels containing microsponges showed desired physical properties. Drug release through cellulose dialysis membrane showed diffusion controlled release pattern and drug deposition studies using rat abdominal skin exhibited significant retention of active in skin from microsponge based formulations by 24 h ⁴⁰.

D’souza et al., developed topical anti-inflammatory gels of fluocinolone acetonide entrapped in eudragit based microsponge delivery system. Fluocinolone acetonide (FA) is a corticosteroid chiefly used in dermatology to lessen skin inflammation and relieve itching. The percutaneous absorption increases risk related with systemic absorption of topically applied formulation.

Thus, the goal of the study was to produce FA entrapped microsponges which were prepared by quasi-emulsion solvent diffusion method in order to control release of drug to the skin which in turn lessens the side effect whereas also reducing percutaneous absorption. FTIR and DSC studies showed that there is no incompatibility between formulation adjuvant and process parameters.

Surface morphology can be done by SEM which showed microporous nature of microsponges. Drug release was also observed controlled with comparative anti-inflammatory activity with the gels that contains free drug ⁴¹.

Grimes et al., developed microsponge based delivery of hydroquinone 4% and retinol 0.15% for the treatment of melasma and post-inflammatory hyper-pigmentation and also to minimize skin irritation. Hydroquinone (HQ) bleaching creams are generally considered as the gold standard for treating hyper-pigmentation. The formulation was evaluated in a 12 week open label study for safety and efficacy. The study included pigmentation intensity, disease severity, lesion area and colorimetry assessments. Adverse events were also recorded. Patients were applied the microsponge formulation entrapped HQ 4% to the full face in morning and evening (twice) daily. After 15 minutes of application of the test product, a broad-spectrum sunscreen was applied once in the morning. Then, patients were evaluated at baseline and at 4, 8, and 12 weeks. The study showed that microentrapped HQ 4% with retinol 0.15% formulation produced improvement at all study end points. The open-label study concluded that microentrapped HQ 4% with retinol 0.15% was safe and effective.¹⁷

Microsponge for Oral Delivery: A Microsponge system offers the potential for active ingredients to remain within a protected environment and provide controlled delivery of oral medication to the lower gastrointestinal (GI) tract, where it will be released upon exposure to specific enzymes in the colon. If this approach is successful then it should open up entirely new opportunities for MDS. It has been shown that microsponge system enhances the solubilization of drugs which are poorly soluble by entrapping these drugs in their pores.

As these pores are very small, the drug is in effect reduced to microscopic particles and drastically increased surface area consequently, increases the rate of solubilization. Additionally, the time it takes the microsponge system to pass through the small and large intestine is considerably increased as a result maximizing the amount of drug that is absorbed.

Jain et al., prepared paracetamol loaded eudragit RS 100 based microsponges by quasi-emulsion solvent diffusion method. The compatibility of the drug with different formulation components was demonstrated. Surface morphology and shape of the microsponges were analyzed using SEM. Compression coating of microsponges with pectin: Hydroxypropylmethyl cellulose (HPMC) mixture followed by tableting was used for colon specific formulations. In-vitro drug release studies were done on all the formulations and the results were evaluated kinetically and statistically. The study concluded that the release data followed Higuchi matrix but diffusion was the main mechanism of drug release from microsponges.  In-vitro studies showed that compression coated colon specific tablet formulations started the release of drug at the 6th hour resultant to the arrival time to proximal colon. ¹³

Jain et al., developed dicyclomine loaded eudragit RS100 based microsponge for colonic delivery by using quasi-emulsion solvent diffusion method. Differential Scanning Calorimetry and Fourier Transform Infra-Red was done to study the compatibility of the drug with various formulation components. SEM was used for demonstration of surface morphology and shape of the microsponges. The formulations were subjected to in vitro release studies, and the results were evaluated kinetically and statistically. Kinetic studies showed that the Higuchi matrix controlled diffusion was the main mechanism of drug release. With an initial burst effect, the drug release was biphasic with 16 - 30 % of the drug was released in the 1^st hour. Cumulative release for the microsponges over 8 hours was ranged from 59 - 86 %. This study concluded an approach for the alteration of microsponges of dicyclomine for prolonged drug release. The distinctive compressibility of microsponges can be applied to get efficient local action as microsponges may be taken up by macrophages which are present in colon ¹⁴.

Orlu et al., prepared a novel colon specific drug delivery system containing flurbiprofen microsponges. Flurbiprofen loaded Eudragit RS 100 based microsponge were prepared by quasi-emulsion solvent diffusion method. Additionally, Flurbiprofen was entrapped into a commercial Microsponge® 5640 system using entrapment method.

Then, the effects of drug: polymer ratio, amount of inner phase solvent, stirring speed and time and stirrer type on the physical characteristics of microsponges was examined and investigate the surface morphology, thermal behaviour, particle size and pore structure of microsponges.

The Compression Coating with Pectin: HPMC mixture followed by tableting was used for colon specific delivery of microsponge formulations. In vitro studies exhibited that compression coated colon specific tablet formulations started to release the drug at the 8^th hrs corresponding to the proximal colon arrival time due to the addition of enzyme. ¹⁵

Gonul et al., developed microsponge of ketoprofen by quasi-emulsion solvent diffusion method with Eudragit RS 100 and tablets of microsponges were prepared by direct compression method and studied the effects of pressure and direct compression on tableting of microsponges. In order to determine the optimum pressure value for the compression of the tablets, different pressure values were applied to the tablet powder mass. Results of the study indicated that microsponge compressibility was much better over the physical mixture of the drug and polymer and due to the plastic deformation of sponge-like structure; microsponges can produce mechanically strong tablets ⁴².

Microsponges for Biopharmaceuticals Delivery: The microsponge delivery system (MDS) is employed for both in the delivery of biopharmaceuticals as well as in tissue engineering. Dai 2010 et al., developed 3D scaffolds hybrid structures that have advantages of natural type I collagen and synthetic PLGA knitted mesh. The collagen microsponges facilitated cell seeding and tissue formation and mechanically strong PLGA mesh served as a skeleton. The scaffolds were divided into three groups:

1. Thin: collagen microsponge formed in interstices of PLGA mesh;
2. Semi: collagen microsponge formed on one side of PLGA mesh;
3. Sandwich: collagen sponge formed on both sides of PLGA mesh.

In the scaffolds Bovine chondrocytes were cultured and transplanted subcutaneously into nude mice for 2, 4, and 8 weeks. All transplants showed natural chondrocyte morphology, homogeneous cell distribution, and abundant cartilaginous ECM deposition. Production of GAGs per DNA and the expression of type II collagen and aggrecan mRNA were much higher in the Semi and Sandwich groups than in the Thin group. Young's modulus showed 54.8 49.3% mechanical strength of the engineered cartilage and in stiffness 68.8 62.7%, respectively, in Semi and Sandwich when compared to native articular cartilage. These scaffolds could be used for the tissue engineering of articular cartilage with adjustable thickness ⁴³.

Iwai et al., developed a biodegradable graft material containing collagen microsponge that would allow the regeneration of autologous vessel tissue in order to avoid these problems. Poly (lactic-co-glycolic acid) has been used as a biodegradable scaffold which was compounded with collagen microsponge to form a vascular patch material. The poly (lactic-co-glycolic acid) collagen patches with or without autologous vessel cellularization were used to patch the canine pulmonary artery trunk. Biochemical and histologic assessments were performed 2^nd and 6^th months after the implantation.

Resulting, there was no thrombus formation in either group but the poly (lactic-co-glycolic acid) scaffold was approximately completely absorbed in both groups. Histologic results showed the formation of an endothelial cell monolayer, a parallel alignment of smooth muscle cells, and reconstructed vessel wall with elastin and collagen fibers. The cellular and extra-cellular components in the patch had enlarged to levels analogous to those in native tissue at 6 months. This patch also shows promise as a bioengineered material for promoting in-situ cellularization and the regeneration of autologous tissue in cardiovascular surgery. ⁴⁴

Tateishi et al., has also been studied developed biodegradable porous scaffolds for tissue engineering. 3D biodegradable porous scaffolds play a vital role in tissue engineering. A novel method were used for preparing porous scaffolds which consists of synthetic biodegradable polymers and developed by combining porogen leaching and freeze-drying techniques utilizing pre-prepared ice particulates as the porogen material. Biodegradable hybrid porous sponges of synthetic polymer and collagen have been prepared by hybridizing synthetic polymer sponges with collagen microsponges. The collagen microsponges were produced in the pores of synthetic polymer sponges. Hybrid sponges of synthetic polymer, collagen and inorganic hydroxyapatite were prepared by depositing hydroxyapatite particulates on the surfaces of the collagen microsponges in the synthetic polymer-collagen sponges. The synthetic polymer sponge were used as a mechanical skeleton to aid the formation of these hybrid sponges into desired shapes and contributed good mechanical strength and handling whereas the collagen and hydroxyapatite are used to promote cell interaction and facilitate cell seeding ⁴⁵.

Patents Information: In 1^st September 1987, Won R (Palo Alto, CA) of Advanced Polymer Systems, Inc. (Redwood City, CA) received (United States Patent 4,690,825) for developing method to deliver an active ingredient by controlled time release using a novel delivery vehicle that can be prepared by a process utilizing the active ingredient as a porogen ¹. On 8^th September 1992, Won R (Palo Alto, CA) of Advanced Polymer Systems, In (Redwood City, CA) received (United States Patent 5,145,675) for developing a two-step method for the preparation of controlled release formulations ⁴⁵. Advanced Polymer Systems, Inc. and subsidiaries is using its patented microsponge^(R) delivery systems and related proprietary technologies to increase the safety, aesthetic quality and effectiveness of topical prescription, over-the-counter ("OTC") and personal care products like Vitamin- A, tretinoin and 5-fluorouracil etc. As on 23^th July 2006, the Company has a total of 10 issued U.S. patents and an additional 92 issued foreign patents. 21 patent applications are pending worldwide.

Dean JR  et al., received US patent no. 4863856 for the development of weighted collagen microsponges having a highly cross-linked collagen matrix that is suitable for use in culturing organisms in motive reactor systems. The microsponges have an open to the surface pore structure, pore volumes and pore sizes suitable for immobilizing a range of bioactive materials ⁴⁶.

Marketed Formulations: MDS is best for skin and personal care products. They can take up large amounts of excess of skin oil while retaining an elegant feel on the surface of skin. This technology is presently employed in approximately number of products (Table 2) sold by leading cosmetic and toiletry companies worldwide.

Among these products include moisturizers, skin cleansers, deodorants, oil control lotions, conditioners, razors, lipstick, powders, makeup and eye shadows which offer various advantages including improved physical and chemical stability, greater available concentrations, reduced skin irritation and sensitization and controlled release of the active ingredients and unique tactile qualities ^47-50.

TABLE 2: LIST OF MARKETED PRODUCTS USING MICROSPONGE DRUG DELIVERY SYSTEM

 CONCLUSION: The microsponge drug delivery technology is widely applicable to the dermatological drug delivery products. The microsponge delivery technology of controlled release system in which active pharmaceutical ingredients are loaded in the microporous beads and initiates reduction in side effects with improved therapeutic efficacy. The microsponge drug delivery system has properties like improved stability and enhanced flexibility in formulation.

MDS is originally developed for topical delivery of drugs like anti-acne, anti-inflammatory, anti-fungal, anti-dandruffs, antipruritics, rubefacients etc. but MDS also expands its application in oral drug delivery and in bone and tissue engineering. Hence, the microsponge drug delivery system focus as an important tool for future inventions in controlled drug delivery system.

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