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Impactfactor.orgAvailable online on www.ijddt.com International Journal of Drug Delivery Technology 2014; 4(4); 58-66 Microsponges: An Overview *Hamid Hussain, Divya Juyal, Archana Dhyani Himalayan Institute of Pharmacy and Research Rajawali Dehradun, India Available Online: 29th September 2014 ABSTRACT
Microsponge and Nanosponge delivery System was originally developed for topical delivery of drugs can also be used for
controlled oral delivery of drugs using water soluble and bioerodible polymers. Microsponge delivery system (MDS) can
entrap wide range of drugs and then release them onto the skin over a time by difussion mechanism to the skin. It is a
unique technology for the controlled release of topical agents and consists of nano or micro porous beads loaded with active
agent and also use for oral delivery of drugs using bioerodible polymers.
Keywords: Microsponge drelivery system (MDS), Nanosponge. Bioerodible.
response to triggers including rubbing, pH, friction, Microparticles and nanoparticles have been increasingly moisture and ambient skin temperature.
investigated to achieve targeted and sustained release of Advantages of microsponges: drugs  and among this microsponge is one of the recent Microsponges are biologically safe and offer unique and an innovative noval approach to deliver a drug in a advantage of programmable release.
They offer entrapment of numerous ingredients and is They are tiny, sponge like spherical particles that consist believed to contribute elegance and enhanced of a myriad of interconnecting voids within a non- collapsible structure with a large porous surface Have the capacity to adsorb or load a high degree of Microsponge delivery systems (MDS) that can precisely active materials into the particle or unto its surface.
control the release rates or target drugs to a specific body Microsponges are stable over a ph range of 1-11 and site have an enormous impact on the health care system.
upto temperature of 130 ºc The microsponge drug delivery technology is widely They are self sterilizing as average pore size is 0.25 µm applicable to the dermatological drug delivery products.
where bacteria cannot penetrate.
But MDS also expands its application in oral drug delivery, Microsponges are capable of absorbing skin secretions bone and tissue engineering, in Detecting the diseases and so reducing the oiliness of the skin upto 6 times of its in RNAi silencing. New classes of pharmaceuticals, biopharmaceuticals (peptides, proteins and DNA-based With size 10-25 microns in diameter it is capable of therapeutics) are fueling the rapid evolution of drug delivery technology. Thus MDS is a very emerging field which is needed to be explored . Microsponges are
The drug releases in microsponges y the external stimuli porous, polymeric microspheres that are mostly used for like ph, temperature, and rubbing.
prolonged topical Microsponges have several advantages over topical designed to deliver a pharmaceutically active ingredient preparations in being nin-allergic, non-toxic, non-irritant efficiently at minimum dose and also to enhance stability, and non-mutagenic.
reduce side effects, and modify drug release profiles .
it is a polymeric microspheres that acquire the flexibility Microsponges are all ways stable i.e, thermal, physical to entrap a wide variety of active ingredients such as and chemical .
emollients, fragrances, sunscreens, essential oils, anti- These are compatible with the majority of vehicles and infective, anti-fungal and anti-inflammatory agents etc and are used as a topical carrier system. Resembling a true These systems have higher payload up to 50 to 60% .
sponge, each microsphere consists of an innumerable of Advantages of Microsponges over Other Formulations interconnecting voids within a non-collapsible structure Microsponges have several other advantages over other with a large porous surface. It is a unique technology for preparations available in the market. Comparison between the controlled release of topical agents which consists of some of them are given below as such; microporous beads normally 10-25 microns in diameter, Advantages over conventional formulations: Conventional loaded with active ingredients that is subsequently releases formulations of topical drugs are intended to work on the them onto the skin over a time in a controlled manner or in *Author for correspondence: E-mail: Hussainhamid05@gmail.com
Hamid Hussain et al. / Microsponges: An Overview… outer layers of the skin. Such products release their active Fig. 1: View of microsponge Fig. 2: Highly porous nature of a Microsponge ingredients upon application, producing a highly It should exhibit complete miscibility in monomer or concentrated layer of active ingredient that is rapidly have the ability to be miscible using the least amount of adsorbed. When compared to the conventional system.
a water immiscible solvent.
Microsponge system can prevent excessive accumulation Must be inert to monomers and do not increase the of ingredient within the epidermis and the dermis.
viscosity of the preparation during formulation.
Potentially, the MDDS can reduce significantly the It should be water immiscible or almost slightly soluble.
irritation of effective drugs without reducing their efficacy.
The solubility of active ingredients in the vehicle should Advantages over microencapsulations and liposomes: The be minimum; otherwise the microsponge will be MDS has advantages over other technologies like diminished by the vehicle before application.
microencapsulations and liposomes. Microcapsules cannot It should maintain (preserve) the spherical structure of usually control the release rate of actives. Once the wall is ruptured, the actives contained within microcapsules will It should be stable in polymerization conditions.
Only 10 to 12% w/w microsponge can be incorporated Liposomes suffer lower payload, difficult into the vehicle to eliminate cosmetic delinquent.
formulation, limited chemical stability and microbial Payload and polymer design of the microsponges for the instability, while microsponges system in contrast to the active must be adjusted to obtain the desired release rate above system has several advantages like stable over a ph of a given period of time. .
range of 1-11 and upto temperature of 130 ºc, stable Methods of preparation for microsponges: Initially, drug thermally, physicaly and chemically, have higher payload loading in microsponges is mainly take place in two ways up to 50 to 60%, have average pore size is 0.25 µm whwre depending upon the physicochemical properties of drug to bacteria cannot penetrate.
be loaded. If the drug is typically an inert non-polar Advantages over ointments: Ointments are often material which will generate the porous structure then, it is aesthetically unappealing, greasiness; stickiness etc. That known as porogen. A Porogen drug neither hinders the often results into lack of patient compliance. These polymerization process nor become activated by it and also vehicles require high concentrations of active agents for it is stable to free radicals is entrapped with one-step effective therapy because of their low efficiency of delivery system, resulting into irritation and allergic Microsponges are suitably prepared by the following reactions in significant users. Other drawbacks of topical formulations are uncontrolled evaporation of active Liquid-liquid suspension polymerization: The porous ingredient, unpleasant odor and potential incompatibility microspheres are prepared by suspension polymerization of drugs with the vehicles, when microsponge system method in liquid-liquid systems. In their preparation, the maximize amount of time that an active ingredient is monomers are first dissolved along with active ingredients present either on skin surface or within the epidermis, in a suitable solvent solution of monomer and are then while minimizing its transdermal penetration into the dispersed in the aqueous phase, which consist of additives (surfactant, suspend-ing agents, etc. to aid in formation of Characters of Drugs to be entrapped in the Microsponges: suspension). The polymerization is then initiated by adding There are certain requirements that should be fulfilled (or catalyst or by increasing temperature or irradiation. The considered) when active ingredients are entrapped into various steps in the preparation of microsponges are summarized as.
Selection of monomer or combination of mono-mers.
IJDDT, October-December 2014, 4(4), 58-66
Hamid Hussain et al. / Microsponges: An Overview… Fig 3: Instrument set up for suspension polymerization technique Fig 4: Quasi-emulsion solvent diffusion method set up. Formation of chain monomers as polymerization begins.
Binding of bunches to form microsponges.  Formation of ladders as a result of cross linking between Quasi-emulsion solvent diffusion method: (Top down chain monomers.
approach): This is top-down approach starting with Folding of monomer ladder to form spherical particles.
preformed polymer. This process involved formation of Agglomeration of microspheres, which give rise to quasi-emulsion of two different phases i.e. internal phase formation of bunches of microspheres.
IJDDT, October-December 2014, 4(4), 58-66 Hamid Hussain et al. / Microsponges: An Overview… and external phase similar to emulsions. The internal phase Determination of true density: The true density of of drug--polymer solution made in a volatile solvent like Microsponges can be measured using an ultra-pycnometer ethanol or acetone or dichloromethane was added to under helium gas and is calculated from a mean of repeated external phase comprising the aqueous polyvinyl alcohol (PVA) solution with vigorous stirring. Triethylcitrate Pore structure: Porosity parameters of microsponges are (TEC), which was added at an adequate amount in order to essential in monitoring the intensity and the duration of facilitate plasticity. Stirring lead to the formation of active ingredient effect. Average pore diameters, shape discrete emulsion globules called quasi-emulsion globules.
and morphology of the pores can be determined by using Solvent was then extracted out from these globules to form mercury intrusion porosimetry technique. The effect of pore diameter and volume on the rate of drug release from Following sufficient stirring, the mixture was then filtered microsponges can also be studied using the same to separate the microsponges. The microsponges were then technique. .
dried in an air heated oven. Conceptually, the finely Compatibility studies: The drug-excipient compatibility dispersed droplets of the polymeric solution of the studies are carried out in order to ensure that there is no drug(dispersed phase) get solidified in aqueous phase via inadvertent reaction between the two when formulated into counter diffusion of organic solvent and water out of and a dosage form. These studies are commonly carried out by into the droplets. The diffused aqueous phase within the recording the differential scanning calorimetry (DSC) of droplets decreased the drug and polymer solubility both the chemicals viz., API and excipient individually and resulting in the co-precipitation of both the components also together and checking for any addition or deletion of and continued diffusion of the organic phase results in any peaks or troughs. For DSC approximately 5 mg further solidification, producing matrix-type porous samples can be accurately weighed into aluminium pans and sealed and can be run at a heating rate of 15oC/min suspension polymerization method, this method offered over a temperature range 25–430oC in atmosphere of the advantage of less exposure of the drug to the ambient nitrogen. [18,19].
conditions, low solvent residues in the product because the spectroscopy can also reveal the solvent get extracted out due to its solubility in aqueous incompatibilities media or due to its volatile nature. [11,12,13] Compatibility of drug with reaction adjuncts can also be Evaluation Parameters studied by thin layer chromatography (TLC) and FT-IR Particle size and size distribution: Particle size and size Effect of polymerization on crystallinity of the drug distribution are evaluated using either an optical can be studied by powder X-ray diffraction (XRD) and microscope or an electron microscope. This is an Differential Scanning Colorimetry (DSC). .
extremely crucial step, as the size of the particles greatly Polymer/ Monomer composition: Factors such as particle affects the texture of the formulation and its stability. Free- size, drug loading, and polymer composition govern the flowing powders with fine aesthetic attributes are possible drug release from Microsponges. Polymer composition of to obtain by controlling the size of particles during the Microsponges Drug Delivery system can affect polymerization. Particle size analysis of loaded and partition coefficient of the entrapped drug between the unloaded Microsponges can be performed by laser light vehicle and the Microsponges system and hence have diffractometry or any other suitable method. The values direct influence on the release rate of entrapped drug.
(d50) can be expressed for all formulations as mean size Release of drug from Microsponge systems of different range. Cumulative percentage drug release from polymer compositions can be studied by plotting Microsponges of different particle size will be plotted cumulative % drug release against time. Release rate and against time to study effect of particle size on drug release.
total amount of drug released from the system composed of methyl methacrylate/ ethylene glycol dimethacrylate is Morphology and Surface topography of SPM: For slower than styrene/divinyl benzene system. Selection of morphology and surface topography, various techniques monomer is dictated both by characteristics of active have been used like photon correlation spectroscopy ingredient ultimately to be entrapped and by the vehicle (PCS), Scanning electron microscopy (SEM), transmission into which it will be dispersed. Polymers with varying electron microscopy (TEM) etc. SEM is used widely for electrical charges or degrees of hydrophobicity or which prepared Microsponges are coated with gold– lipophilicity may be prepared to provide flexibility in the palladium under an argon atmosphere at room temperature release of active ingredients. Various monomer and then the surface morphology of the Microsponges is combinations will be screened for their suitability with the studied. .
drugs by studying their drug release profile. .
Determination of loading efficiency and production yield: Resiliency (viscoelastic properties) of The loading efficiency (%) of the Microsponges can be Microsponges can be modified to produce beadlets that is calculated according to the following equation: softer or firmer according to the needs of the final The production yield of the microparticles can be formulation. Increased cross-linking tends to slow down determined by calculating accurately the initial weight of the rate of release. Hence resiliency of Microsponges is the raw materials and the last weight of the SPM obtained.
studied and optimized as per the requirement by considering release as a function of crosslinking with time IJDDT, October-December 2014, 4(4), 58-66 Hamid Hussain et al. / Microsponges: An Overview… Table 1: Marketed formulations of microsponges[38,39]Product name Dermik Laboratories, Inc.
Carac Cream contains 0.5% fluorouracil; with 0.35% being Berwyn , PA 19312 USA incorporated into a patented porous microsphere consisted ofmethyl methacrylate / glycol dimethacrylate cross-polymer anddimethicone. Carac is a once-a-day topical prescription productfor the treatment of actinic keratosis (AK) that is characterizedby common pre-cancerous skin condition caused byoverexposure to the sun.
Salicylic Peel 20 Retin-A-Micro contains 0.1% and 0.04% tretinoin entrapped into a patented porous microsphere consisted of methyl methacrylate/glycol dimethacrylate cross-polymer to enable inclusion of theactive ingredient, tretinoin, in an aqueous gel. This formulationis used for the topical treatment of acne vulgaris.
Salicylic acid 20%, microsponge technology has excellentexfoliation and used for stimulation of the skin for more resistantskin types or for faster results. It will considerably improvepigmentation, fine lines and acne concerns. Salicylic acid moveseasily through the pores, clearing them out while reducinginflammation. This treatment effectively combats acne leavingan amazingly smooth and clear complexion.
Lightweight cream with a retinol (Vitamin A) in MDS, dual- system delivers both immediate and time released wrinkle- fighting action. Clearly diminishes appearance of fine lines, wrinkles & skin discolorations associated with aging.
Retin-A-Micro contains 0.1% and 0.04% tretinoin entrapped into a patented porous microsphere consisted of methyl methacrylate/glycol dimethacrylate cross-polymer to enableinclusion of the active ingredient, tretinoin, in an aqueous gel.
This formulation is used for the topical treatment of acnevulgaris.
The MicroPeel ® Plus procedure stimulates cell turnover through the application of salicylic acid in the form of microcrystals usingMicrosponge® technology. These microcrystals target the exactareas on the skin that need improvement. The MicroPeel Plusaggressively outperforms other superficial chemical peels byfreeing the skin of all dead cells while doing no damage to theskin.
A night time treatment cream with Microsponge technology using a stabilized formula of pure retinol, Vitamin A. Continued use of Retinol 15 will result in the visible diminishment of finelines and wrinkles, a noticeable improvement in the skindiscolorations due to aging, and enhanced skin smoothness.
SDR Pharmaceuticals, Inc., Lactrex™ 12% Moisturizing Cream contains 12% lactic acid as Andover , NJ , U.S.A. 07821 the neutral ammonium salt, ammonium lactate. Microsponge® technology has been included for easy application and longlasting moisturization. Lactrex™ also contains water andglycerin, a natural humectant to soften and help moisturize drys,flaky, cracked skin.
The Microsponge® system uses microscopic reservoirs thatentrap hydroquinone and retinol.
The microsponges release these ingredients into the skingradually throughout the day. This provides the skin withcontinuous exposure to hydroquinone and retinol over time,which may minimize skin irritation. EpiQuin Micro is aprescription moisturizing fading cream that reducesthe impact of these conditions known as melasma, postinflammatory hyper pigmentation or solar lentigines. Also helpin Age spots, Sun spots and Facial discoloration.
IJDDT, October-December 2014, 4(4), 58-66 Hamid Hussain et al. / Microsponges: An Overview… Table 1: Marketed formulations of microsponges[38,39]Product name Oil free matte block This invisible oil-free sunscreen shields the skin from damaging UV sun rays while controlling oil production, giving you ahealthy matte finish. Formulated with microsponge technology,Oil free matter block absorbs oil and preventing shine withoutany powdery residue.
Sportscream RS and Embil Pharmaceutical Co.
Topical analgesic-anti-inflammatory and counterirritant actives in a microsponge® deliverysystem (MDS) for the management of musculoskeletalconditions.
Oil Control Lotion Fountain Cosmetics A feature-light lotion with technically advanced microspongesthat absorb oil on the skin's surface during the day, for a mattefinish. Eliminate shine for hours with this feature-weightlotion, formulated with oil-absorbing Microsponge technology.
The naturally- antibiotic Skin Response Complex soothesinflammation and tightness to promote healing. Acne-Prone,oily skin conditions.
Kinetics of release: To determine the drug release intervals and analyzed using suitable method of assay mechanism and to compare the release profile differences (Embil and Nacht, 1996; Jelvehgari et al., 2006). To among microsponges, the drug released amount versus determine the drug release kinetics and investigate its time was used. The release data were analysed with the mechanism from microsponges, the release data are fitted following mathematical models: to different kinetic models. The kinetic models used are; Q= k1tn or log Q= log k1 + n log t ………………Equation first order, zero order, Higuchi and Korsmeyer- Peppas models (Higuchi, 1963; Wagner, 1969; Korsmeyer et al., Where Q is the amount of the released at time (h), n is a 1983; Peppas, 1985). The goodness of fit was evaluated diffusion exponent which indicates the release mechanism, using the determination coefficient (R2) values.  and k1 is a constant characteristic of the drug–polymer Safety Considerations interaction. From the slope and intercept of the plot of log Safety studies of microsponges can be confirmed by;
Q versus log t, kinetic parameters n and k1 were calculated Allergenicity in guinea pigs For comparison purposes, the data was also subjected to Eye irritation studies in rabbits Equation (2), which may be considered a simple, Higuchi Mutagenicity in bacteria type equation.
Oral toxicity studies in rats.
Q = k2t0.5 +C ………………… Equation (2) Skin irritation studies in rabbits. [25,26,27]
Equation (2), for release data dependent on the square root Applications of Microsponge Systems: Microsponge of time, would give a straight line delivery systems are used to enhance the safety, release profile, with k2 presented as a root time dissolution effectiveness and aesthetic quality of topical prescription, rate constant and C as a constant. .
over-the-counter and personal care products. Products under development or in the market place utilize the In vitro release studies, release kinetics and mechanism:
Topical Microsponge systems in three primary ways: In vitro release studies can be performed using United As reservoirs releasing active ingredients over an States Pharmacopeial (USP) dissolution apparatus extended period of time, equipped with a modified basket consisted of 5 μm As receptacles for absorbing undesirable substances, stainless steel mesh at 37°C. The release medium is such as excess skin oils, or selected according to the type of formulation that is, topical As closed containers holding ingredients away from the or oral, while considering solubility of active ingredients skin for superficial action.
to ensure sink conditions. Sample aliquots are withdrawn Releasing of active ingredients from conventional topical from the medium and analyzed by suitable analytical formulations over an extended period of time is quite method at regular intervals of time. The drug release from topical preparations (for example, creams, lotions and Cosmetics and skin care preparations are intended to work emulgels) containing microsponges can be carried out only on the outer layers of the skin. The typical active using Franz diffusion cells. Dialysis membrane is fitted ingredient in conventional products is present in a into place between the two chambers of the cell. A relatively high concentration and, when applied to the skin, predetermined amount of formulation is may be rapidly absorbed. The common result is over- mounted on the donor side of Franz cell. The receptor medication, followed by a period of under-medication until medium is continuously stirred at and thermostated with a the next application. Rashes and more serious side effects circulating jacket. Samples are withdrawn at different time can occur when the active ingredients rapidly penetrate below the skin's surface. Microsponge technology is IJDDT, October-December 2014, 4(4), 58-66 Hamid Hussain et al. / Microsponges: An Overview… designed to allow a prolonged rate of release of the active microsponges were prepared by the direct compression ingredients, thereby offering potential reduction in the side method. Results indicated that compressibility was much effects while maintaining the therapeutic efficacy. .
improved in the physical mixture of the drug and polymer; Microsponge for topical delivery: The Microsponge
due to the plastic deformation of the sponge-like systems are based on microscopic, polymer-based microsponge structure, producing mechanically strong microspheres that can bind, suspend or entrap a wide tablets. Colon-specific, controlled delivery of flurbiprofen variety of substances and then be incorporated into a was conducted by using a commercial Microsponge® formulated product, such as a gel, cream, liquid or powder.
5640 system. In vitro studies exhibited that compression- A single Microsponge is as tiny as a particle of talcum coated colon-specific tablet formulations started to release powder, measuring less than one-thousandth of an inch in the drug at the eighth hour, corresponding to the proximal diameter. Like a true sponge, each microsphere consists of colon arrival time, due to addition of the enzyme, a myriad of interconnecting voids within a non-collapsible following a modified release pattern, while the drug structure that can accept a wide variety of substances. The release from the colon-specific formulations prepared by outer surface is typically porous, allowing the controlled pore plugging the microsponges showed an increase at the flow of substances into and out of the sphere.
eighth hour, which was the point of time when the enzyme Several primary characteristics, or parameters, of the addition was made. [33,34].
Microsponge system can be defined during the production Microsponge for Bone and Tissue Engineering: Bone- phase to obtain spheres that are tailored to specific product substitute compounds were obtained by mixing pre applications and vehicle compatibility. Microsponge polymerized powders of polymethylmethacrylate and systems are made of biologically inert polymers. Extensive liquid methylmethacrylate monomer with two aqueous safety studies have demonstrated that the polymers are dispersions of tricalcium phosphate grains and calcium non-irritating, nonmutagenic, non-allergenic, non-toxic deficient hydroxyapatite powders. The final composites and non-biodegradable. As a result, the human body appeared to be porous and acted as microsponges. Basic cannot convert them into other substances or break them fibroblast growth factor (bFGF) incorporated in a collagen down. Although they are microscopic in size, these sponge sheetwas sustained released in the mouse sub-cutis systems are too large to pass through the stratum corneum according to the biodegradation of the sponge matrix, and when incorporated into topical products . Benzoyl exhibited local angiogenic activity in a dose-dependent peroxide (BPO) is commonly used in topical formulations manner. The injection of collagen microsponges for the treatment of acne,with skin irritation as a common incorporating bFGF induced a significant increase in the side effect. It has been shown that controlled release of blood flow, in the murine ischemic hind limb, which could BPOfrom a delivery system to the skin could reduce the never have been attained by the bolus injection of bFGF.
side effect while reducing percutaneous absorption.
These results suggest the significance and therapeutic Therefore, microsponge delivery of Benzoyl peroxide was utility of the type I collagen as a reservoir of bFGF. [35,36] developed using an emulsion solvent diffusion method by Future Prospects: Microsponge drug delivery system holds adding an organic internal phase containing benzoyl a promising opportunity in various pharmaceutical peroxide, ethyl cellulose and dichloromethane into a applications in the upcoming future as it has unique stirred aqueous phase containing polyvinyl alcohol and by properties like enhanced produc suspension polymerization of styrene and divinyl benzene elegancy, extended release, improved drug release profile, .The prepared microsponges were dispersed in gel base reduced irritation, improved physical, chemical and and microsponge gels are evaluated for anti-bacterial and thermal stability which makes it flexible to develop novel skin irritancy. The entrapped system released the drug at product forms. The real challenge in future is the slower rate than the system containing free BPO. Topical development of the delivery system for the oral peptide delivery system with reduced irritancy was successfully delivery by varying ratio of polymers. The use of bioerodible and biodegradable polymers for the drug Microsponge for oral delivery: In oral applications, the
delivery is enabling it for the safe delivery of the active microsponge system has been shown to increase the rate of material. As these porous systems have also been studied solubilization of poorly water-soluble drugs by entrapping for the drug delivery through pulmonary route which such drugs in the microsponge system's pores. As these shows that these system can show effective drug release pores are very small, the drug is in effect reduced to even in the scarce of the dissolution fluid thus colon is an microscopic particles and the significant increase in the effective site for targeting for drug release. These carriers surface area thus greatly increases the rate of also require to be developed for alternative drug solubilization. Controlled oral delivery of ibuprofen administration routes like parenteral and pulmonary route.
microsponges is achieved with an acrylic These particles can also be used as the cell culture media polymer, Eudragit RS, by changing their intraparticle thus can also be employed for stem cell culture and cellular chlorpheniramine regenaration in the body. Due to their elegance, these microsponges, is prepared by the dry impact blending carrier systems have also found their application in method, for oral drug delivery. Controlled oral delivery of cosmetics. These developments enabled researchers to Ketoprofen prepared by quasi-emulsion solvent diffusion utilize them variably. These novelties in formulation also method with Eudragit RS 100 and afterwards tablets of open new ways for drug delivery.  IJDDT, October-December 2014, 4(4), 58-66 Hamid Hussain et al. / Microsponges: An Overview… characterization and release studies. International MDS has become highly competitive and rapidly evolving Journal of Pharmaceutics (2006)124-132.
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Social Movements and Social Policy: The Bolivian Renta Dignidad Political Science Department, University of North Carolina at Chapel Hill Sara Niedzwiecki* Political Science Department, University of New Mexico, Albuquerque firstname.lastname@example.org (856) 725-3672 * We would like to thank Evelyne Huber, John Stephens, and Camila Arza for comments on previous versions of this paper.