Microneedles in Smart Drug Delivery

Muhammad Bilal,1,2,* Shahid Mehmood,3 Ali Raza,4 Uzma Hayat,4

Tahir Rasheed,5 and Hafiz M.N. Iqbal6

1Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City,

Vietnam.2Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam.3Key Laboratory of Protein and Peptide Pharmaceuticals, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China.4School of Biomedical Engineering, Shanghai Jiao Tong University, Shanghai, China.5School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, China.6Tecnologico de Monterrey, School of Engineering and Sciences, Monterrey, Mexico.

Significance: In biomedical setup, at large, and drug delivery, in particular,transdermal patches, hypodermal needles, and/or dermatological creams withthe topical appliance are among the most widely practiced routes for transder-mal drug delivery. Owing to the stratum corneum layer of the skin, traditionaldrug delivery methods are inefficient, and the effect of the administered thera-peutic cues is limited.Recent Advances: The current advancement at the microlevel and nanolevel hasrevolutionized the drug delivery sector. Particularly, various types of micro-needles (MNs) are becoming popular for drug delivery applications because ofsafety, patient compliance, and smart action.Critical Issues: Herein, we reviewed state-of-the-art MNs as a smart and so-phisticated drug delivery approach. Following a brief introduction, the drugdelivery mechanism of MNs is discussed. Different types of MNs, that is, solid,hollow, coated, dissolving, and hydrogel forming, are discussed with suitableexamples. The latter half of the work is focused on the applied perspective andclinical translation of MNs. Furthermore, a detailed overview of clinical appli-cations and future perspectives is also included in this review.Future Directions: Regardless of ongoing technological and clinical advance-ment, the focus should be diverted to enhance the efficacy and strength of MNs.Besides, the possible immune response or interference should also be avoidedfor successful clinical translation of MNs as an efficient drug delivery system.

Keywords: microneedles, drug delivery system, fabrication strategies, influ-encing factors, microneedles types

SCOPE AND SIGNIFICANCE

The most important perspectiveof microneedles (MNs) is to increasethe invasion of a drug to the targetsite. Ultrafine and microstructure ofMNs consist of several hundred-micrometer squares like honeybeecomb that has multiple advantages,including decreasing the relateddetrimental properties and carryingmaximum concentration of drugs.

Along with all these benefits, MNscan also delimit the nonspecific tar-get delivery and restrain the drugs tolocal areas or tissues.

TRANSLATIONAL RELEVANCE

This study provides convincingimportance for further translationalresearch to aid the home health careservices, where injectable medicinesare mostly administered by commu-

Muhammad Bilal, PhD

Submitted for publication November 20,

2019. Accepted in revised form March 29, 2020.

*Correspondence: Department for Management

of Science and Technology Development, Ton Duc

Thang University, Ho Chi Minh City, Vietnam

(e-mail: muhammad.bilal@tdtu.edu.vn).

j 1ADVANCES IN WOUND CARE, VOLUME 00, NUMBER 00Copyright ª 2020 by Mary Ann Liebert, Inc. DOI: 10.1089/wound.2019.1122

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

nity nurses or by trained patients themselves. Suchclinical translation will also facilitate the elderly,ensuring the age-appropriate drug delivery platformat their doorstep.

CLINICAL RELEVANCE

The successful deployment of MNs, regardlessof types and materials, in clinical settings, such ashighly effective and targeted drug or vaccine de-livery, is remarkable. So far, MNs have gainedsubstantial application in the diagnosis, painlessmonitoring of diseases, and biochemical analysisto extract the sample. For instance, MN patchesare used to draw the interstitial fluid from the skinepidermis to monitoring the diseases in a mini-mally invasive way. With the aid of MNs, thisprocess can be a painless and most convenientexperience for the patients.

BACKGROUND/INTRODUCTION

Among different drug delivery routes, oral ad-ministration is promising, primarily due to patientcompliance and safety. However, it is not suitable forall kinds of drugs due to the complex biological en-vironment and internal conditions with pH vari-ability and presence of enzymes.1,2 Therefore,parenteral routes are used for such drugs. Trans-dermal drug delivery is also considered an alterna-tive for the oral route in terms of patient compliance,and also offered the opportunity for controlled andtargeted drug release. However, some drugs cannotpass through skin barrier for their absorption andaction.3 Different strategies have been reported toenhance the penetrability of these drugs, such aschemical/lipid enhancers, ionophoresis, electropora-tion, sonophoresis, and photoacoustic effect. How-ever, these techniques are based on the creation ofsmaller pores in the skin for penetration of small andmacromolecular drugs.4 These approaches, exceptpenetration enhancer addition, are considered effec-tive, but affect patient compliance. Another approachis to develop microsized drug-loaded needles (MNs),which can create nonclinical significant damage toskin for enhancing drug penetration.5

MN concept was generated in the 1970s, andsignificant progress has been reported in the lastdecade, particularly for drug delivery application.6

This technique is particularly important for mac-romolecular drugs such as protein-/peptide-/nucleicacid-based therapeutic agents, which cannot passthrough intact skin owing to higher molecularweight.7 Apart from skin, MNs have also been re-ported for drug delivery through oral mucosa,8

vaginal,9 anal,10 intestinal,11 and cornea.12 In ad-

dition to enhancing drug absorption, controlleddrug release can also be achieved through the se-lection of appropriate material and design. In thisreview, we have discussed the overall concept ofMNs with emphasis on future prospective.

DISCUSSION OF FINDINGSAND RELEVANT LITERATUREMicroneedles—A Smart Approach

MNs are becoming a smart approach with timefrom the conventional transdermal approach.These are preferred over hypodermal needlesbecause of the painless nature of MNs. It is alsobecause they can pass stratum corneum withtolerable pain.13 MNs also exhibit higher bio-availability compared to that of transdermal/topical preparations.14 MN patches can be self-administered with patient compliance and safe-ty.15 The faster drug of action is also attained withMNs due to direct release for absorption in thesystemic circulation.16 Furthermore, stimuli-responsive MNs offer a smart approach for the on-demand release of drugs. Yu et al. developed MNsloaded with insulin as hypoxia-sensitive vesiclesthat disassembled in the presence of local hypoxiainduced by high glucose levels.17

DRUG DELIVERY MECHANISM OF MNS

Drug delivery through MNs is primarily basedon damaging the skin barrier and then release ofdrugs in the upper dermis layer for systemic ab-sorption.18 After crossing the skin barrier, the re-lease of the drug into the body is dependent on thetype of MNs, which can be classified as dissolution-based and diffusion-based drug delivery (Fig. 1).Nonbiodegradable solid MNs usually deliver thedrug through diffusion, while coated and biode-gradable MNs exhibit dissolution-based drug re-lease.19,20 Solid nondegradable MNs are used tocreate microchannels into the skin, followed by theapplication of drug formulation, which crosseschannels through passive diffusion. For instance,Wei-Ze et al.21 used super-short silicone solid MNsfor the creation of micropores in skin to enhancegalanthamine delivery.21 With advancement, bio-degradable and dissolving MNs have gathered at-tention of researchers owing to benefits over solidMNs. Nguyen et al.22 reported poly (vinyl alcohol)-based solid dissolving MNs for the rapid delivery ofdoxorubicin (DOX) after disrupting stratum cor-neum. Furthermore, they found that modificationin DOX distribution in needles can alter drug re-lease profile.22 Another approach is to coat theMNs with a drug-containing solution that will be

2 BILAL ET AL.

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

released into the body by dissolution.23 In recentdecades, hydrogel-forming MNs also become pop-ular, which tend to swell upon insertion into theskin and allowed diffusion of the drug from theattached reservoir.24

FABRICATION AND DESIGNING STRATEGIES

With the advancement in technology, severaltechniques have been evolved to fabricate MNsfrom simple techniques such as micromoldingto three-dimensional (3D) printing. Some of thecommonly reported techniques are summarized inTable 1. While designing MNs through any suit-able method, the size and shape of MNs should beconsidered, as these parameters can influence drugdelivery. Figure 2 illustrates various designs ofMNs with respect to shape and tip.28 The sharp-ness and diameter of MNs could affect dye distri-bution with better results using sharp needles.28,29

One of the benefits of MNs is pain reduction com-pared to hypodermic needles (26G). However, thesize and number of MNs in a patch can influencethe pain sensation in patients. Gill et al. reportedthat a decrease in length and number of MNs coulddecrease the pain.13

VARIETY IN MNS

There are different types of MNs reported for drugdelivery applications, such as solid MNs, hollowMNs, dissolving MNs, coated MNs, and hydrogel-forming MNs. These differ in their design, material,and mechanism of drug release.

Solid MNsSolid MNs are usually applied for the creation of

conduit channels in skin for subsequent delivery ofdrug/vaccine.30 Created microsized conduit chan-nels allow drug diffusion through the skin. Theyhave a disadvantage that created channels shouldbe closed to protect the transport of unwanted ma-terial/pathogens.31 Different materials have beenused to prepare solid MNs such as silicone, metals(stainless steel and titanium), polymers, and ce-ramics. For instance, Narayanan and Raghavanprepared sharp silicon MNs using a wet etchingprocess with 52 times higher mechanical strengthcompared to that of skin for smooth insertion. Theaverage height, base width, and tip diameter ofneedles were 158, 110.5, and 0.4 lm.32 In anotherreport, Nguyen et al. used stainless steel MN rollersand array for the enhancement of penetration ofcaptopril and metoprolol. Array showed about eight

Figure 1. Schematic illustration of drug release from different types of MNs. (1) Stratum corneum, (2) epidermis, and (3) dermis. MNs, microneedles.

Table 1. Fabrication methods of different types of microneedles

Type of Microneedles Methods of Fabrication References

Solid microneedles Dry etching, wet etching, 3D printing, laser ablation, micromolding, magnetorheological drawing lithography,and electroplating

14

Hollow microneedles Wet chemical etching, deep reactive ion etching, laser micromachining, isotropic etching, digital lightprocessing-based projection stereolithography, micropipette pulling, and sacrificial micromolding

25

Coated microneedles Dipping coating, rolling coating, and spray coating after making solid microneedles 23

Dissolving microneedles Negative moldings, two-step process was used 26

Hydrogel forming microneedles Photolithographic process and micromolding process 27

3D, three dimensional.

AN INSIGHT INTO MICRONEEDLES 3

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

times increase in transdermal reflux for captopril,while about five times for metoprolol.22 Li et al. usedpoly(lactic acid) (PLA) to fabricate solid MNs todeliver insulin with three different sizes (Fig. 3a–c).They found that 600-lm-sized MNs have moresuccessful insertion compared to that of size 700and 800 lm (Fig. 3d). Furthermore, adequatecontrol blood glucose level was observed in Balb/cmice diabetic model (Fig. 3e).33 Furthermore,Bystrova and Luttge reported solid ceramic MNsbased on alumina. They used a cost-effective poly-dimethylsiloxane (PDMS) micromolding method forthe fabrication of ceramic solid MNs.29

Hollow MNsHollow MNs have an empty core with a pore at

the tip to deliver fast drug delivery. This type ofMNs is usually used to deliver high molecularweight bioactive molecules.25 These kinds of MNscan accommodate a higher dose of drugs in hollowspace.15 After the application of hollow MNs, thedrug delivery rate can be controlled by varyingpressure from slow delivery to fast delivery.34 Inaddition to drug solution application, hollow MNswere also reported to facilitate transdermal deliv-

ery of nanoparticles. Du et al., reported dermaldelivery of ovalbumin [with or without pily(I:C)]-loaded poly(lactic-co-glycolic acid) (PLGA) nano-particles, liposomes, mesoporous silica nanoparticles(MSNs), and gelatin nanoparticles (GNPs) throughin-house made hollow MNs. PLGA nanoparticlesand cationic liposomes were found to have enhanceddelivery of model antigen compared to that of MSNsand GNPs evaluated by immune response.35 In an-other report, Monkare et al. compared the delivery ofPLGA nanoparticles loaded with ovalbumin andpoly(I:C) using hollow and dissolving MNs. Theyfound hollow MNs had superior ability to delivermodel antigen to induce immune response.36 Over-all, hollow MNs have benefits of fast delivery ofloaded substance and the ability to enhance trans-dermal delivery of nanocarriers.

Dissolving MNsDissolving MNs are composed of biodegradable

and biocompatible materials that tend to degradeand dissolve in body fluid, leading to the release ofloaded cargo. Usually, they are fabricated using themicromolding technique, the fabrication process ofdissolving gelatin/carboxymethyl cellulose poly-

Figure 2. Illustration of various designs of solid MNs with respect to shape and tips. (a) Cylindrical; (b) tapered tip; (c) canonical; (d) square base; (e)

pentagonal-base canonical tip; (f) side-open single lumen; (g) double lumen; (h) side-open double lumen. Reprinted from Ashraf et al.28 with permission underthe terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0). Copyright (2011) the authors; licenseeMolecular Diversity Preservation International, Basel, Switzerland.

4 BILAL ET AL.

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

meric MN patches is shown in Fig. 4.26 Drug re-lease is mainly controlled through the dissolutionrate of materials (Fig. 5).26 These kinds of MNshave the drawback of dose limitation compared tosolid, hollow, and hydrogel-forming MNs.20 Zhao

et al. reported hyaluronic acid-based fast-dissolvingMNs loaded with 5-aminolevulinic acid (5-ALA; aprecursor of proporphyrin IX) for photodynamictherapy using micromolding. They found a fastin vitro release (*100% in 60 min) of 5-ALA using

Figure 3. (a) Shapes of PLA MNs with 600, 700, and 800 lm sizes. (b) Percentage of successful insertion of different sized MNs. (c) The blood glucose levelof mice treated with insulin with MN pretreatment without MN. Subcutaneous insulin injection and nontreated. PLA, poly(lactic acid). (d) Relationship betweenpercentage of successful insertions and the number of insertions with MNs with different heights. The microscopic images of porcine skin treated with theMNs at 1st, 10th and 20th insertion are provided on the left, top and right of the graph respectively. Each data point represents the average of 5 experiments.Standard deviation bars are shown. and (e) Blood glucose levels as a percentage of the initial value in mice after subcutaneous hypodermic insertions ofinsulin (;), transdermal insulin delivery using MNs (-), transdermal insulin delivery without MNs pretreatment (�) and time control (:). Each data pointrepresents the average of 5 experiments. Standard deviation bars are shown. Reprinted from Li et al.33 with permission under the terms and conditions of aCreative Commons Attribution-Non Commercial 3.0 Unported Licence. An open-access article published by the Royal Society of Chemistry.

Figure 4. (A) Schematics of the fabrication process of dissolving gelatin/CMC MN patches. The drug was localized in the tip of the needles. (B) Gross view ofthe gelatin/CMC MN patch. Scale bar = 5 mm. (C) Average dimensions of geometrical parameters of gelatin/CMC MN patches. Reprinted from Chen et al.26 withpermission under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0). Copyright (2018)the authors; Licensee MDPI, Basel, Switzerland.

AN INSIGHT INTO MICRONEEDLES 5

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

Franz diffusion cell. Furthermore, in vivo antitumorefficiency was also confirmed using tumor-bearingBALB/c nude mice. Mice group treated with 5-ALA-loaded MNs with light exposure showed a reductionin tumor volume compared to 5-ALA injection, onlylight and no treatment group.37 Recently, Li et al.prepared separable dissolving MNs made up of bio-degradable polymers (PLGA and PLA) for sustaineddelivery of levonorgestrel (LNG). Upon application,the needles were implanted into skin and slowlysustained.38 In another report, Bhatnagar et al. re-ported PLA and PVP composite dissolvable MNs fordelivery of DOX and docetaxel (DTX). MNs weredissolved fast and showed a fast release of drugs inthe skin. However, the slow permeation of drugsthrough skin was observed. In vivo antitumor effi-cacy was tested on 4T1 bearing BALB/c nude mice,which showed control over tumor volume usingDOX-/DTX-loaded MNs after intratumor applica-tion of MNs.39

Coated/hybrid MNsCoated MNs have a coating of drug solution on

needles with soluble material. They tend to providefast drug release by the fast dissolution of coatinglayer of needles. The higher amount of drug loadedcan be controlled by the thickness of the coatinglayer. Jain et al. used dip-coating method to fabricate

5-ALA solution-coated two-dimensional stainlesssteel MNs for skin tumor treatment.40 Furthermore,they optimize coating parameters in terms of coatingand concentration of the drug. In vivo antitumorefficacy of 5-ALA-coated MNs was evaluated usingA-20 tumor-bearing balb/c mice, which showed asignificant reduction in tumor volume after treat-ment with MNs and light exposure for photodynamictherapy. The dip-coating approach cannot preciselycontrol drug loading. For precise control, Chen et al.devised an adjustable apparatus for dipping of nee-dles into drug solution.41 Coated MNs can be used todeliver peptides whose efficiency can be affected bythe hydrophobicity of peptide, needle morphology,and excipients.42 These types of needles were alsoused for local anesthetic delivery.43

Hydrogel-forming MNsThese are the new type of MNs with a swellable

material composition, which can absorb and swellinterstitial fluid to form 3D network.15 This swellednetwork behaves as a hydrogel conduit for the de-livery of drugs from the attached reservoir in acontrolled manner.44 This type of MNs leaves nomaterial residue upon removal.45 Larger doses canbe administered at a controlled rate using hydrogel-forming needles.24 Different drugs from smallhydrophilic molecules (theophylline) to high molec-

Figure 5. (A) Stereomicroscopy images and (B, C) scanning electron microscopy images of gelatin/CMC MNs before insertion. (D–F) The MNs 10 min afterinsertion. (G–I) The MNs 30 min after insertion. Scale bar = 500 lm. Reprinted from Chen et al.26 with permission under the terms and conditions of the CreativeCommons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0). Copyright (2018) the authors; Licensee MDPI, Basel, Switzerland.

6 BILAL ET AL.

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

ular weight molecules (insulin) have been repor-ted to deliver through hydrogel-forming needles.For instance, Donnelly et al. reported poly (methylvinyl ether-co-maleic anhydride) (PMVE/MA)-based hydrogel-forming MNs and evaluated theirtransdermal delivery efficiency using six moleculeswith different molecular weights (*171 to*67,000Da).45 In another report, Migdadi et al. preparedhydrogel-forming MNs using poly (ethylene glycol)cross-linked poly (methyl vinyl ether-co-maleicacid) with metformin HCl-loaded reservoir. Sus-tained release of metformin with Tmax of 24 h wasachieved using MNs, ensuring transdermal deliv-ery of drug.24 Besides, on-demand drug release canalso be achieved using hydrogel-forming MNs bystimulus control. Hardy et al. prepared hydrogel-forming MNs using light-responsive materials, thatis, 2-hydroxyethyl methacrylate (HEMA) and ethyl-ene glycol di-methacrylate (EGDMA), for on-demanddelivery of ibuprofen.46

COMPATIBILITY—PATIENT COMPLIANCEAND SAFETY

MNs offer a minimum invasive approach withless pain drug delivery, which leads to better patientcompliance compared to conventional invasiveneedle-based drug delivery.13 MNs can help in de-creasing needle phobia, which promotes health suchas a successful immunization program because ofhigher patient acceptability. In addition to compli-ance, MNs help in decreasing costs in terms of ad-ministration, packing, transportation, and disposalcost.47 MNs’ perception and acceptability in pediat-ric population for immunization were also reportedto be positive.48 Besides being painless, the ease ofadministration, and the cost-effectiveness, MNs alsopromote patient compliance by smart delivery of thedrug. For instance, Chen et al., report enzyme-freeglucose-sensitive MNs for on-demand release of in-sulin with 2-month stability in aqueous medium.41

APPLIED PERSPECTIVE AND CLINICALTRANSLATION OF MNS

MNs have a wide range of theranostic applicationsin the field of molecular medicine, cell biology, humanbiology, biomedical engineering, and genetic engi-neering. Potential applications of MNs are shown inFig. 6. MNs are used for the local drug administra-tion and development of personal medication on thebasis of introducing an RNA medicine (small non-coding RNA, small interfering RNA [siRNA], micro-RNA [miRNA], and antisense RNA),49 DNA(recombinant deoxyribonucleic acid),50 peptides (cy-closporine and desmopressin),51 and proteins (FC

proteins, hormones, interleukins, anticoagulants,antibodies, and enzymes).52,53 These molecules playan imperative role in personalized medicine. Still, thelimitations with these molecules as drugs are targetdelivery and to maximize the drug concentrationreached to the affected site. For this reason, MNs canbe used for target drug delivery and also to break thebiological barriers like cellular impermeability andblood-brain barrier (BBB).54

Localized, controlled, and painless delivery ofdrugs are the main advantages of MNs, which canpromote reduction in nonspecific effects as well aspatient compliance. This can make an MN a po-tential drug delivery system in neural activitieswhere the biggest challenges to cross the BBB andreach a significant amount of drug to active site.55

MN shaft length at the microscopic scale is con-trolled by advance microfabrication technologythat shows a considerable advantage. Due to theultrafine microarchitecture of MN, it can be quicklyinjected into stratum corneum (outermost kerati-nized layer with very low permeability) of skin.Most of the nerve terminal branch point is presentunder the depth of about 100 lm of this layer. So,local delivery at this site will help to neutralize thetoxicity, replenish the pain, and local cellular in-jury. On the other hand, skin is considered thelargest organ of the body. Previous research re-vealed that cutaneous uptake of oxygen in all agegroups is 0.529 – 0.265 mL O2 m-2$min-1.56 For thispurpose, there is a large number of blood capillaries

Figure 6. Potential applications of MNs. Each application strongly de-pends on the MN type and materials used for fabrication.

AN INSIGHT INTO MICRONEEDLES 7

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

run under the skin, especially highly saturated instratum corneum. Due to this, drugs administeredunder the skin rapidly join the blood flow and resultin fast treatment. MNs have substantial applicationin the diagnosis, painless monitoring of diseases,and biochemical analysis to extract the sample. Forinstance, MN patches are used to draw the inter-stitial fluid from the skin epidermis to monitor thediseases in a minimally invasive way. Similarly,blood glucose monitoring is a routine test for dia-betic patients and the injection of insulin (forinsulin-dependent diabetes or type 1 diabetes). Withthe aid of MNs, this process can be a painless andmost convenient experience for the patients.57,58

MNs have a broad range and multidisciplinaryapplications. These are also useful in the designingof sensors and electronics fields. They have beenused in purpose-based modification in scanningtunneling and atomic force microscopes. Otherapplications are to construct electrospray emittingnozzles, microdialysis use for low molecular weightcompounds, and printer head nozzles.34

APPLICATIONS OF MNS THROUGHDIFFERENT DRUG DELIVERY ROUTESTranscutaneous or transdermal drug delivery

The transdermal drug administration route iswidely used for injecting drugs. This method is typ-ically applied through topic creams, hypodermalneedles, and patches. Topical creams and hypoder-mal needles are very common in practice from thelast few decades. Hypodermis needles are not alwaysacceptable as when everyone looked back to theirchildhood. Hypodermic needles were biased as a gi-ant monster. On the other hand, topical creamsshowed less bioavailability due to the biodegradationin the body and crossed the strong skin barrier. Skinconsists of three primary layers, peripheral layer iscalled stratum corneum, middle layer epidermis,and core layer is dermis. The stratum corneum is thefirst and most considerable barrier in drug deliverydue to the physiochemical properties, for example,less permeability. It shows significant permeabilityfor low molecular weight drugs (Heparin) and lipo-philic drugs (diclofenac epolamine and capsai-cin).31,59 To increase the permeability and bioactivityand overcome the skin barrier, transdermal drugdelivery system, including MNs, nanocarriers, andpatches loaded with drugs, are used.60,61

MN-based vaccine administration. Skin acts as afirst-line defense and plays a vital role in restrainingthe pathogens. For this reason, a wide range ofantigen-presenting cells flow under the skin, espe-cially dermis and epidermis. Thus, researchers have

preferred intradermal over intramuscular vaccineadministration. With the fast-growing and rapidadvancement in the field of nanotechnology, nano-materials, especially MNs, are considered promisingvehicles in vaccinology.62,63

Currently, MN-based vaccination has gained tre-mendous attraction of biomedical and pharmaceuti-cal researchers because conventional methodsrequired professional training of phlebotomy. Also,careful handling needs to be safe disposal (incinera-tion), avoiding significant injury, and changes to thetransmission of blood-borne infection. Therefore, theintradermal route for vaccine administration is amore handy and safe direction compared to the con-ventional way, although intradermal vaccinationshave some limitations, for example, microdermabra-sion, tape stripping, and ballistic shown low pene-tration. The MNs are fascinating in the case of self-administration vaccination. There are different typesof vaccines that will be administered by MNs.

MN patches for DNA vaccination. A DNA vac-cine is a third-generation vaccination. Nowadays,health care researchers focus on endogenous anti-gen production by introducing engineered DNA(usually come from a plasmid, small circular bac-terial DNA) into the host body. As a result, thehumoral immune system of the host is stimulatedagainst the specific antigen. DNA vaccines at-tained significant attention of medical professionalover conventional vaccination method due to thefollowing reasons: (1) safety profile (no whole bac-terial cell or attenuated virus), (2) host body showsless immunogenic resistance, (3) stability, and (4)large-scale production at commercial scale.64,65

Kim et al. conducted research on the influenzavirus in which only 3 lg of hemagglutinin (HA)DNA was sufficient to stimulate the immune re-sponse administered by MN patches.66 Later on,HA DNA combined with attenuated influenzavaccine is injected by as MN patch for cross-protection.67,68 Another comparative study wasconducted by Fernando’s research team, in whichdesigned MN patches were loaded with NP gene ofA/WSN/33 influenza virus and conventional DNAvaccine administration. The study concluded thatnanopatch-based introduced DNA vaccine has sig-nificantly induced conserved CD8+ T cell epitopeimmunity against influenza virus.69 To enhance theamount of DNA in MN patches, pH-responsivepolyelectrolyte multilayer assembly technique wasintroduced by Kim et al., which facilitates the releaseand carries the maximum amount of vaccine.34

MN patch approach had also been used on dogsto induce immunity against the rabies virus. Arya

8 BILAL ET AL.

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

and her collaborators designed MN patches thatwere used to deliver the DNA vaccine. MNs loadedwith vaccines were dissolved under the skin after afew minutes of injection. These dissolved MNs withDNA vaccines were found as a very safe and fea-sible method for vagrant and domestic dogs.70

Previous studies reported another approach: dry-coated DNA MN patches had been practiced forWest Nile virus to modulate immunity in mice.71

The efficacy of dry-coated DNA vaccine against thehepatitis C virus was also studied, which showedthe activation of immunity by stimulation of virus-specific cytotoxic T lymphocytes.72 Microscale sili-con projections, also known as a microenhancerarray, were designed to enhance the efficiency ofdry coating loaded with DNA vaccine in vivo ex-perimentation for Hepatitis surface antigen.73

For further analysis after in vivo confirmationstudies of cutaneous DNA vaccine management onan animal model, an ex vivo system of human skinhas been done for immunological modulatory over72 h and expression level confirmation.74 Anothercomparative study between conventional and MNpatches was conducted by Kask et al. on low dose ofprotein gD2 of vaginal herpes simplex virus thatwas incorporated in a plasmid. Results confirmedthat the low-dose vaccine in MN patches providesa more protective effect on fetal challenges overtraditional methods in mouse models.75 Simi-larly, another approach was applied for immuni-zation against anthrax in mice. DNA plasmidsolution with anthrax protective antigen wasmounted on positively charged PLGA nanomaterialand that immunized solution applied on mice skinthan pretreated with MN derma rollers.76 Finally,transcutaneous or intradermal vaccination hasshown significant and comparable results.65

MN patches for peptide or subunit vaccina-tion. MN patches have also been used for skin ortransdermal injecting of peptide or subunit vac-cine. Skin is considered a more appropriate routefor a vaccine to grasp the cells of epidermis anddermis, especially dendritic cells (DCs) and Lang-erhans cells (LCs)75 because these cells are theprimary site for modulation of humoral as well ascellular immunity.77 Boks et al. developed nano-porous ceramics microneedle (npMN) arrays loa-ded with OVA257–264 peptides (8-residue peptide)along with agonistic anti-CD40 antibodies as anadjuvant. Ex vivo experimental study on humanskin revealed that npMN array is a promising wayto deliver vaccines as well as in vivo induction ofCD8+ effector T cell responses.78 Primarily, MNpatches incorporated with subunit for influenza

vaccine stimulate the antibody production.79,80

In vivo experimental results confirmed that MN-based transdermal delivery of only 1 lg imiquimod-adjuvanted and HA vaccine against influenza wasshown to be more significant and improved modu-lation of immune response by inhibition of proin-flammatory cytokines as well as reduce the viraltiter in lungs compared to adjuvant TLR (Toll-likereceptor) ligands alone.80 Kim et al. reported co-delivery of M2e virus-like particles (VLPs) with in-fluenza split vaccine to the skin using MNs (Fig. 7).79

Studies conducted on young and elderly healthyvolunteers revealed that there are no significantand noticeable side effect observed during experi-mentation.81,82 In addition, MN patches were alsoapplied to rescue the other diseases by injectingantigens through the transdermal route. MN pat-ches have been used for ovarian cancer vaccinationthrough the intradermal route along with oral ad-ministration. It provokes the systemic, mucosal,and T cell immune response to conquer the tumorgrowth in the cancer microenvironment of mousemodel.82 To stimulate the IgE-mediated immuneresponse against the allergen, Shakya and Gill’sproject introduced the allergen through the skinby MN patches. IgE plays an essential role in themanifestation of many allergic diseases. MNpatch-mediated exposure of allergen significantlyreduces the IgE attachment and the mast cell ac-tivation.83 MN patch BCG (bacillus Calmette-Guerin) and tetanus toxoid vaccines were used tostimulate the humoral immune system againsttuberculosis and tetanus, respectively. A studydemonstrated that MN patches encapsulated withtetanus toxoids stimulate the humoral immuneresponse in pregnant mice, and also reported thehigh antibody titer at prenatal stage as well as inthe postnatal stage up to 12 weeks.84,85 An ex-periment conducted on guinea pigs BCG-coatedMN patch used for intradermal immunization.Results clearly showed that MN patch coated withvaccine vigorously increased the cellular immu-nity in both spleen and lungs.86 MN patch medi-ated combinatorial vaccination of plague,botulism, anthrax, and staphylococcus antigenic-ity, significant protection was observed.87–90

MN patches for virus-like particle delivery. VLPsor viral particles are similar to viruses that consistof self-assembled noninfectious particles, usuallystructural proteins, with higher molecular weightthan peptides and subunit antigen. For this reason,VLPs can be covered with MN patches to prolongthe magnitude of humoral immune response.91–93

Kang’s and his team introduced the VLPs by

AN INSIGHT INTO MICRONEEDLES 9

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

the intradermal route to immunize the mice.The results showed that antigen encapsulatedin patches had adverse effects on immune effi-ciency. Furthermore, these coated patches ex-hibited a protective role to boost up the immuneresponse by increasing the level of IgG and IgA inmice alveolar fluid and lungs.94 Similarly, influ-enza patches encapsulated with VLP trigger theLCs and generate the immune response.95,96 Theseresults proposed that MN patches can offer a ben-eficial platform to successfully deliver VLP vac-cines. Similarly, intradermal immunization by MNpatches impregnated with VLP can play a pro-phylactic role in human cervical cancer comparedto intramuscular immunization.97 Another studystated that DNA- and HPV16 VLP coated MN pat-ches produced vigorously neutralizing the antibodyresponse and deliberated protection over HPV16challenge.98 The results showed that lyophilizedHPV VLP-coated patches are thermostable and morepractical approaches for vaccine administration.

MN patches for whole virus delivery. Attenuatedviruses are merely used for MN patch-based im-munization, but live viruses can be used in thismethod. For the protective immune modulationwith inactivated viruses required time-dependentdose or introduce along with certain potent adju-vants. A large number of studies have been re-vealed that attenuated viruses encapsulated in MNpatches showed a clear dose-sparing benefit overthe traditional counterpart, and MN patch-baseddelivery exponentially increases the efficacy ofvaccine.99–101 These dose-sparing properties of MNpatches were testified with inactivated rotavirusvaccine along with influenza virus in mice.100 MNpatch encapsulated with the inactivated virus canhave distinctive features other than inducingthe humoral response, including high thermosta-bility, produce high magnitude cellular responseby activating the antigen-producing memory Bcells.102,103 Similarly, cytokines play an integralpart in immunity. MN patch loaded with in-

Figure 7. Co-delivery of M2e virus-like particles with influenza split vaccine to the skin using MNs. Reprinted from Kim et al.79 with permission under theterms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0). Copyright (2019) the authors; LicenseeMDPI, Basel, Switzerland.

10 BILAL ET AL.

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

activated influenza vaccine involved in the pro-duction of anti-inflammatory cytokines that takepart in the long-lasting protection and cellularresponse.99 Inactivated influenza vaccine deliv-ery with this method heightened the stability indissolving MN patches.104

A comparative study was conducted on rhesusmacaques in which live measles virus was encap-sulated into MN patch applied on the skin with asubcutaneous injection to generate a humoral im-mune response. It was demonstrated that there wasno significant difference in the magnitude of hu-moral immunity. Besides this, vaccine delivered byMN patch exhibits more thermostability comparedto conventional lyophilized vaccine.105 Various re-search groups administered adenoviruses with MNpatch. As a result, intradermal vaccine delivery oflive virus did not affect the efficacy of cellular im-munity and increases the production of memory Bcell or long-lasting immune response.106–108 Finally,these data showed that the live virus vector could besuccessfully introduced without harming the effec-tiveness of the immune system.

MN patches for drug delivery to other tissuesAfter the successful and momentous transder-

mal MN patch drug delivery, researchers havemoved and target the other body tissues for localdrug delivery.

Oral mucosa. Oral mucosa has a large numberof lymphoid tissues just beneath the epithelial sur-face. For this reason, oral mucosa is one of thepromising sites for vaccine administration. MNpatch coated with antigen injected into the oralmucosa was reported as a successful method toovercome the mucosal barrier. Similarly, the sameexperiment has been done to over-ride the vaginalmucosa. Results showed that MN patch-based vac-cination significantly increases antigen-specific an-tibodies in saliva (IgA) and serum (IgG) compared tointramuscular vaccine administration route.8 Tra-verso et al. design prototype of a device implantedwith MNs to enhance the bioavailability of the oraladministered insulin. This device was placed in in-testinal lumen and insertion of MNs occurred duringthe peristaltic bowel movement, which facilitates thepenetration of insulin by breaking the mucosal bar-rier of gastrointestinal tract.11

Vaginal mucosa. Similar to other target tissuesites for vaccination, it is desirable to break the drugtransport barrier. The vaginal mucosa may also bethe most promising site for vaccine administration.Wang and his collaborators had introduced MNpatch coated with antigen into the vagina, and re-

sults showed robust stimulation of antigen-mediatedantibody (IgG) production both in the serum andreproductive tract. This method of administration ismore efficient as it overcomes poor bioavailability ofbiotherapeutics through the oral route. The biodeg-radation and biological membranes are less perme-able to macromolecules.9,109

Ocular tissue. The bioavailability of oculardrugs (eye drops) is poor because the eye is one of themost sensitive and delicate organs of the body,which is protected with a barrier called corneal ep-ithelium. Jiang et al. reported that pilocarpine andfluorescein coated on MN patches had increased thebioavailability up to twofolds. As a result, that aidedthe long-lasting reconstruction of the pupil, whilecomparing to conventional drug treatment.110,111

Nail. Nails are one of the hardest and mostdifficult parts of the body in terms of drug deliverydue to the presence of a nail bed. A study was con-ducted by Chiu et al. in which the nail bed waspretreated with MN cylindrical roller and then atopical drug was applied. This method increasedthe penetration ability of drugs compared to drugsadministered without MNs because drugs took timeto cross the keratinized architecture of nail.112,113

Anal sphincter. Topical phenylephrine (PE) isused for the treatment of ineffective control overbowel movement (bowel incontinence). Conventionaldelivery of PE dose by gel did not significantly treatfecal incontinence due to insufficient drug concen-tration at the target site. For this purpose, with thehelp of MN patch-coated PE dose increase, the de-livery up to 10-folds, as a result, increases sphincterpressure compared to that of conventional PE gel-based delivery alone.114,115

Dermal papilla. Conventional treatment for alo-pecia is restricted due to the insufficient concentra-tionofdrugsavailable to theaffectedarea.Second, theconsidered amount of dose does not reach because it isunable to cross the corneum stratum. MNs combinedwith hair loss therapeutic drugs, including platelet-rich plasma, minoxidil, and steroids, can significantlyincrease hair growth, although, previously, thistherapy is used for neovascularization, induced col-lagen formation, and increased growth.116,117

Cardiac muscle. Tang et al. designed an MNpatch combined with cardiac stromal cells (MN-CSCs) that was used for rejuvenation of cardiomyo-cytes after the attack of myocardial infarction. In thisstudy, CSCs were introduced into the rat body byintravascular injection as well as direct heart injec-

AN INSIGHT INTO MICRONEEDLES 11

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

tion and epicardial patch transplantation. The resultshowed that MN-CSCs is an innovative and dis-tinctive method to regenerate the cardiomyocyte.118

FUTURE DIRECTIONS

Although MN-based delivery of biologically activemolecules or compounds is an emerging and promis-ing area for prophylactic human diseases, some areasneed to be improved. There is still a need for moreadvanced and precise approaches that have minimalinvasiveness and target specificity, and overcomethe biological barriers with higher drug loading ca-pacity. Two main parameters that need to be con-sidered, while improving MN development, aremechanical strength and immunogenic rejection. Themechanical designing of MNs is the most promisingand fundamental approach to define prophylacticapplications.TheoptimizationofMNs isaverycrucialstep to engineer MNs with precise geometrical andphysical properties. The microscopic miniature de-sign of MNs can be a major area of improvement. Thesaturation or abundance restricted at the microscalelevel can be easily failed, while applying on skin.119

There are two main mechanical events that takea decisive role in MN administration: (1) the frictionforce that plays to neglect the skin friction force toinsert the MNs before puncturing the skin and (2)the friction force must be greater than the skin forceto puncture the skin and perform its function. Astudy was conducted to assess the relationship be-tween geometries and the insertion force of MNs.MN tip radii usually vary between 30 and 80 mmand length around 500 mm. Davis and his team’sneedle applied force (insertion force) was relativelyhigher than the needle tip interfacial area. Differenttypes of MNs have diverse friction force, insertionforce, as well as an interfacial force. For example, inthis experiment, investigators found that solid andthin-walled hollow needles needed approximatelythe same insertion force, that is, range between 0.1and 3 N. The insertion force is directly proportionedto the wall angle, thickness, and tip radius. Ac-cording to the margin, safety criterion (ratio be-tween insertion force and friction force) must behigher, and margin safety is again directly propor-tioned to wall thickness and tip radius.120

These mechanical obstacles seriously restrictthe MN in prophylactic medical applications.Unfortunately, only a few research groups areworking on enhance the mechanical feasibilityand overcome the hindrance during MN admin-istration. Mistilis et al. reported a combinatorialmethod of MN patch encapsulated with an influ-enza virus that can be removed from the cold

chain. They have manufactured the MN to stabilizethe influenza virus that unveiled the noteworthythermostability. This way of manufacturing sup-ports the liquid formulation as well as live and at-tenuated vaccination.121 Some of the groups areworking on dissolved MN patches that are based onorganic gelatin polymers to enhance the activity,including immunogenicity and thermostability.

To specifically target the skin DCs, especially LCs,polymeric dissolve microneedle (DMN) patches en-capsulated with nanomaterial coated with the anti-gen that stimulates the skin LCs explicitly. LCs areimportant immunomodulatory cells that play an im-portant role incellular immunitybyspecifically cross-priming the CD8 T+ cells and CD4 T+ cells. They alsohave priming effects on Th1 and Th17 mediatingimmune response. So these cells are imperativeplayers to modulate the antigen-specific antiviral andantitumor activity. Results showed that activation ofskin LC stimulation mediated by nanoparticle coatedwith immunogenic and transdermal delivery medi-ated by DMNs provides an efficient platform for im-munization strategies.108,122 MNs are usuallyemployed for transdermal route, but various re-searchers also applied through the oral route.109,123

Through the oral administration route, the majorhindrance is strong immunogenic rejection overtakeby IgG and even by mucosal IgA of salivary secre-tions. The same problem is faced while being admin-istered in vaginal and intestinal parts. To combatstrongandrobust immunerejection, including injury,allergic reaction, and irritation, Wang et al. intro-duced a pretreatment method in which nonablativefractional laser treatmentofadministeredanimalhasdone before injecting the MN patches.124

SUMMARY

MNs are the emerging device that possessesdistinctive properties like rapid, painless, and localdelivery compared to existing administration sys-tems for prophylactic disease and biomedical ap-plications. Despite all these facts, MNs mademomentous advancement and transfigured thehealth care field, including diagnosis, therapeutics,and immunization, as well as the world’s mostgrowing field cosmetics. Besides this, there aremany areas of improvements that still need to beaddressed. There is a lot of advancement need todesign and construct the smart and wearabledevices that can be used for therapeutic appli-cations. On the other hand, MN-based wearabledevices need to develop a multifunctional ap-proach in the future. As mentioned in the limi-tation section, MN-based drug administration

12 BILAL ET AL.

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

required many significant modifica-tions and enhancements to make itmore effective, safe, and minimal inva-sive immunogenic or allergic reaction.Silicon chip-based MNs should be de-signed and linked to APP to make it digi-talized and easily accessible. Thecompendium of this review is, MNs are theemerging and most promising area inprophylactic or health care applications.

ACKNOWLEDGMENTSAND FUNDING SOURCES

All listed author(s) gratefully ac-knowledge the literature services pro-vided by their representative departments anduniversities. No external funding was received.

AUTHOR DISCLOSURE AND GHOSTWRITING

No competing financial interests exist. The con-tent of this article was expressly written by theauthor(s) listed. No ghostwriters were used to writethis article.

ABOUT THE AUTHORS

Muhammad Bilal, PhD, accomplished his PhDfrom Shanghai Jiao Tong University with a spe-cialization in Bioengineering. He has publishedmore than 200 scientific contributions in the formof Research, Reviews, Book Chapters, andEditorial-type scientific articles in various areas ofScience & Engineering. He has an H-index = 30,along with more than 2,500 citations. He has guest-edited a couple of special issues and serves as a sci-entific reviewer in numerous peer-reviewed jour-nals. His research interests include environmentalbiotechnology/engineering, nanotechnology, bioca-talysis, enzyme engineering, immobilization, che-mical modifications and industrial applications ofmicrobial enzymes, bioremediation of hazardous andemerging pollutant, liquid, and solid waste man-agement—valorization of agroindustrial wastes andbiomaterials for bioenergy. Shahid Mehmood iscurrently a PhD scholar at the Institute of Bio-physics, University of Chinese Academy of Sciences,Beijing, China. He received his Masters degree fromthe National University of Sciences and Technology,Islamabad. His research focuses on pharmacoki-netics intervention of peptide-based drugs onautoimmune and inflammatory diseases and site-specific protein modifications. Ali Raza is currentlya doctorate student at the School of Biomedical En-gineering, Shanghai Jiao Tong University, China.His research interests are the development of drug

delivery systems and scaffolds for tissue engineer-ing. Uzma Hayat is currently a doctorate student atthe School of Biomedical Engineering, ShanghaiJiao Tong University, China. Her area of interest isthe investigation of biodegradable polymers for bio-medical applications. Tahir Rasheed, PhD, iscurrently a postdoctoral research fellow at theSchool of Chemistry and Chemical Engineering,Shanghai Jiao Tong University, China. He re-ceived his PhD degree in Polymer Chemistryfrom the same university in 2019. His researchinterests focus on multiple disciplines includingcontrollable synthesis, characterization, and self-assembly of alternating copolymers, hyper-bran-ched polymers, with special emphasis on theirpotential applications in the field of sensing andbiosensing, enzyme mimic, catalysis, solid poly-mer electrolytes, and energy storage devices (i.e.,supercapacitor and Li-ion batteries). Hafiz M.N.Iqbal, PhD, is a full-time Professor at the Schoolof Engineering and Sciences, Tecnologico deMonterrey, Mexico. He completed his PhD inBiomedical Sciences with a specialization in Ap-plied Biotechnology and Materials Science atthe University of Westminster, London, UnitedKingdom. Dr. Iqbal’s research group is engaged inBioengineering, Biomedical Engineering, Mate-rials Science, Enzyme Engineering, Bio-catalysis,Bioremediation, Algal Biotechnology, and Ap-plied Biotechnology related research activities.Dr. Iqbal has an H-index = 42, along with morethan 6000 citations. Dr. Iqbal had guest-editedseveral special issues and served as an EditorialBoard member for several peer-reviewed jour-nals. Dr. Iqbal has published more than 250 sci-entific contributions in the form of Research,Reviews, Book Chapters, and Editorials, at severalplatforms in various journals of national/interna-tional repute with high impact factor.

TAKE-HOME MESSAGES

� This work presents state-of-the-art MNs as smart drug delivery platform.Readers can get a balanced overview of numerous types of MNs andtheir revolutionary perspectives in clinical settings.

� Besides the well-established and documented influencing factors, that is,shape, size, geometry, and type of MN, formulation procedures, materialchoice, and end-product viability and stability are also crucial factors thatreaders must take into account.

� Evidently, MN-based unique drug delivery platform has become a robustalternative drug delivery system with added benefits, such as overallefficacy, targeted delivery, painlessness, noninvasiveness, controlledadministration, and so on.

AN INSIGHT INTO MICRONEEDLES 13

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

REFERENCES

1. Sastry SV, Nyshadham JR, Fix JA. Recenttechnological advances in oral drug delivery–areview. Pharm Sci Technol Today 2000;3:138–145.

2. Raza A, Shen N, Li J, Chen Y, Wang J-Y. For-mulation of zein based compression coatedfloating tablets for enhanced gastric retentionand tunable drug release. Eur J Pharm Sci 2019;132:163–173.

3. Tuan-Mahmood TM, McCrudden MT, Torrisi BM,et al. Microneedles for intradermal and trans-dermal drug delivery. Eur J Pharm Sci 2013;50:623–637.

4. Tanner T, Marks R. Delivering drugs by thetransdermal route: review and comment. SkinRes Technol 2008;14:249–260.

5. Prausnitz MR. Microneedles for transdermaldrug delivery. Adv Drug Deliv Rev 2004;56:581–587.

6. Tasca F, Tortolini C, Bollella P, Antiochia R.Microneedles-based electrochemical devices fortransdermal biosensing: a review. Curr OpinElectrochem 2019;16:42–49.

7. Rzhevskiy AS, Singh TRR, Donnelly RF, Anissi-mov YG. Microneedles as the technique of drugdelivery enhancement in diverse organs andtissues. J Control Release 2018;270:184–202.

8. Ma Y, Tao W, Krebs SJ, Sutton WF, HaigwoodNL, Gill HS. Vaccine delivery to the oral cavityusing coated microneedles induces systemic andmucosal immunity. Pharm Res 2014;31:2393–2403.

9. Wang N, Zhen Y, Jin Y, et al. Combining dif-ferent types of multifunctional liposomes loadedwith ammonium bicarbonate to fabricate mi-croneedle arrays as a vaginal mucosal vaccineadjuvant-dual delivery system (VADDS). J Con-trol Release 2017;246:12–29.

10. Baek C, Han M, Min J, Prausnitz MR, Park J-H,Park JH. Local transdermal delivery of phenyl-ephrine to the anal sphincter muscle usingmicroneedles. J Control Release 2011;154:138–147.

11. Traverso G, Schoellhammer CM, Schroeder A,et al. Microneedles for drug delivery via thegastrointestinal tract. J Pharm Sci 2015;104:362–367.

12. Kim YC, Grossniklaus HE, Edelhauser HF,Prausnitz MR. Intrastromal delivery of bev-acizumab using microneedles to treat cornealneovascularization. Invest Ophthalmol Vis Sci2014;55:7376–7386.

13. Gill HS, Denson DD, Burris BA, Prausnitz MR.Effect of microneedle design on pain in humansubjects. Clin J Pain 2008;24:585.

14. Yang H, Wu X, Zhou Z, Chen X, Kong M. En-hanced transdermal lymphatic delivery of doxo-rubicin via hyaluronic acid based transfersomes/microneedle complex for tumor metastasistherapy. Int J Biol Macromol 2019;125:9–16.

15. Waghule T, Singhvi G, Dubey SK, et al. Micro-needles: a smart approach and increasing po-tential for transdermal drug delivery system.Biomed Pharmacother 2019;109:1249–1258.

16. Gupta J, Denson DD, Felner EI, Prausnitz MR.Rapid local anesthesia in human subjects usingminimally invasive microneedles. Clin J Pain2012;28:129.

17. Yu J, Zhang Y, Ye Y, et al. Microneedle-arraypatches loaded with hypoxia-sensitive vesiclesprovide fast glucose-responsive insulin delivery.Proc Natl Acad Sci U S A 2015;112:8260–8265.

18. Larraneta E, Lutton RE, Woolfson AD, DonnellyRF. Microneedle arrays as transdermal and in-tradermal drug delivery systems: materials sci-ence, manufacture and commercial development.Mater Sci Eng R Rep 2016;104:1–32.

19. Lee JW, Park J-H, Prausnitz MR. Dissolving mi-croneedles for transdermal drug delivery. Bio-materials 2008;29:2113–2124.

20. Lee KJ, Jeong SS, Roh DH, Kim DY, Choi HK, LeeEH. A practical guide to the development ofmicroneedle systems–in clinical trials or on themarket. Int J Pharm 2020;573:118778.

21. Wei-Ze L, Mei-Rong H, Jian-Ping Z, et al. Super-short solid silicon microneedles for transdermaldrug delivery applications. Int J Pharm 2010;389:122–129.

22. Nguyen KT, Ita KB, Parikh SJ, Popova IE, BairDA. Transdermal delivery of captopril and me-toprolol tartrate with microneedles. Drug DelivLett 2014;4:236–243.

23. Gill HS, Prausnitz MR. Coated microneedles fortransdermal delivery. J Control Release 2007;117:227–237.

24. Migdadi EM, Courtenay AJ, Tekko IA, et al.Hydrogel-forming microneedles enhance trans-dermal delivery of metformin hydrochloride. JControl Release 2018;285:142–151.

25. Ita K. Ceramic microneedles and hollow micro-needles for transdermal drug delivery: two de-cades of research. J Drug Deliv Sci Technol2018;44:314–322.

26. Chen CH, Shyu VBH, Chen CT. Dissolving mi-croneedle patches for transdermal insulin deliv-ery in diabetic mice: potential for clinicalapplications. Materials 2018;11:1625.

27. Hong X, Wu Z, Chen L, Wu F, Wei L, Yuan W.Hydrogel microneedle arrays for transdermaldrug delivery. Nanomicro Lett 2014;6:191–199.

28. Ashraf MW, Tayyaba S, Afzulpurkar N. Microelectromechanical systems (MEMS) based mi-crofluidic devices for biomedical applications. IntJ Mol Sci 2011;12:3648–3704.

29. Bystrova S, Luttge R. Micromolding for ceramicmicroneedle arrays. Microelectron Eng 2011;88:1681–1684.

30. Gupta J, Gill HS, Andrews SN, Prausnitz MR.Kinetics of skin resealing after insertion of mi-

croneedles in human subjects. J Control Release2011;154:148–155.

31. Ita K. Transdermal delivery of drugs withmicroneedles—potential and challenges. Phar-maceutics 2015;7:90–105.

32. Narayanan SP, Raghavan S. Solid silicon micro-needles for drug delivery applications. Int J AdvManuf Technol 2017;93:407–422.

33. Li QY, Zhang JN, Chen BZ, Wang QL, Guo XD. Asolid polymer microneedle patch pretreatmentenhances the permeation of drug molecules intothe skin. RSC Adv 2017;7:15408–15415.

34. Kim Y-C, Park J-H, Prausnitz MR. Microneedlesfor drug and vaccine delivery. Adv Drug DelivRev 2012;64:1547–1568.

35. Du G, Hathout RM, Nasr M, et al. Intradermalvaccination with hollow microneedles: a com-parative study of various protein antigen andadjuvant encapsulated nanoparticles. J ControlRelease 2017;266:109–118.

36. Monkare J, Pontier M, van Kampen EE, et al.Development of PLGA nanoparticle loaded dis-solving microneedles and comparison with hol-low microneedles in intradermal vaccinedelivery. Eur J Pharm Biopharm 2018;129:111–121.

37. Zhao X, Li X, Zhang P, Du J, Wang Y. Tip-loadedfast-dissolving microneedle patches for photo-dynamic therapy of subcutaneous tumor. JControl Release 2018;286:201–209.

38. Li W, Terry RN, Tang J, Feng MR, SchwendemanSP, Prausnitz MR. Rapidly separable microneedlepatch for the sustained release of a contracep-tive. Nat Biomed Eng 2019;3:220.

39. Bhatnagar S, Bankar NG, Kulkarni MV, Venu-ganti VVK. Dissolvable microneedle patch con-taining doxorubicin and docetaxel is effective in4T1 xenografted breast cancer mouse model. IntJ Pharm 2019;556:263–275.

40. Jain AK, Lee CH, Gill HS. 5-Aminolevulinic acidcoated microneedles for photodynamic therapyof skin tumors. J Control Release 2016;239:72–81.

41. Chen S, Matsumoto H, Moro-oka Y, et al.Microneedle-array patch fabricated with enzyme-free polymeric components capable of on-demandinsulin delivery. Adv Funct Mater 2019;29:1807369.

42. Zhao X, Coulman SA, Hanna SJ, Wong FS,Dayan CM, Birchall JC. Formulation of hydro-phobic peptides for skin delivery via coated mi-croneedles. J Control Release 2017;265:2–13.

43. Baek S-H, Shin J-H, Kim Y-C. Drug-coated mi-croneedles for rapid and painless local anes-thesia. Biomed Microdevices 2017;19:2.

44. Sabri AH, Ogilvie J, Abdulhamid K, et al. Ex-panding the applications of microneedles indermatology. Eur J Pharm Biopharm 2019;140:121–140.

14 BILAL ET AL.

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

45. Donnelly RF, Singh TRR, Garland MJ, et al.Hydrogel-forming microneedle arrays for en-hanced transdermal drug delivery. Adv FunctMater 2012;22:4879–4890.

46. Hardy JG, Larraneta E, Donnelly RF, et al.Hydrogel-forming microneedle arrays made fromlight-responsive materials for on-demand trans-dermal drug delivery. Mol Pharm 2016;13:907–914.

47. Rouphael NG, Paine M, Mosley R, et al. Thesafety, immunogenicity, and acceptability of in-activated influenza vaccine delivered by micro-needle patch (TIV-MNP 2015): a randomised,partly blinded, placebo-controlled, phase 1 trial.Lancet 2017;390:649–658.

48. Marshall S, Sahm LJ, Moore AC. Microneedletechnology for immunisation: perception, ac-ceptability and suitability for paediatric use.Vaccine 2016;34:723–734.

49. Davidson BL, McCray PB, Jr. Current prospectsfor RNA interference-based therapies. Nat RevGenet 2011;12:329.

50. Liu M. DNA vaccines: a review. J Intern Med2003;253:402–410.

51. Morgan D, Diamond DM, Gottschall PE, et al. Abpeptide vaccination prevents memory loss in ananimal model of Alzheimer’s disease. Nature2000;408:982.

52. Neirynck S, Deroo T, Saelens X, VanlandschootP, Jou WM, Fiers W. A universal influenza Avaccine based on the extracellular domain of theM2 protein. Nat Med 1999;5:1157.

53. Subramaniam S, Yugo DM, Heffron CL, et al.Vaccination of sows with a dendritic cell-targeted porcine epidemic diarrhea virus S1protein-based candidate vaccine reduced viralshedding but exacerbated gross pathologicallesions in suckling neonatal piglets. J Gen Virol2018;99:230–239.

54. Xiao S, Yan Y, Zhao J, Zhang Y, Feng N. In-creased microneedle-mediated transdermal de-livery of tetramethylpyrazine to the brain,combined with borneol and iontophoresis, forMCAO prevention. Int J Pharm 2020;575:118962.

55. Kim Y-C, Prausnitz MR. Enabling skin vaccinationusing new delivery technologies. Drug DelivTransl Res 2011;1:7–12.

56. Stucker M, Struk A, Altmeyer P, Herde M,Baumgartl H, Lubbers DW. The cutaneous up-take of atmospheric oxygen contributes signifi-cantly to the oxygen supply of human dermis andepidermis. J Physiol 2002;538:985–994.

57. El-Khordagui L. Microneedles: an emerging ap-proach for active transdermal delivery of insulin.J Bioequiv Availab 2012;4:7.

58. Yang J, Liu X, Fu Y, Song Y. Recent advances ofmicroneedles for biomedical applications: drugdelivery and beyond. Acta Pharm Sin B 2019;9:469–483.

59. Bora P, Kumar L, Bansal AK. Microneedle tech-nology for advanced drug delivery: evolving vis-tas. Curr Res Inf Pharm Sci 2008;9:7–10.

60. Prausnitz MR, Langer R. Transdermal drug de-livery. Nat Biotechnol 2008;26:1261.

61. Prausnitz MR, Mitragotri S, Langer R. Currentstatus and future potential of transdermal drugdelivery. Nat Rev Drug Discov 2004;3:115.

62. Saroja C, Lakshmi P, Bhaskaran S. Recent trendsin vaccine delivery systems: a review. Int JPharm Investig 2011;1:64.

63. Takeuchi A, Nomoto Y, Watanabe M, Kimura S,Morimoto Y, Ueda H. Application of micro-needles to skin induces activation of epidermalLangerhans cells and dermal dendritic cells inmice. Biol Pharm Bull 2016;39:1309–1318.

64. Kutzler MA, Weiner DB. DNA vaccines: ready forprime time? Nat Rev Genet 2008;9:776.

65. Suh H, Shin J, Kim Y-C. Microneedle patches forvaccine delivery. Clin Exp Vaccine Res 2014;3:42–49.

66. Kim YC, Song JM, Lipatov AS, et al. Increasedimmunogenicity of avian influenza DNA vaccinedelivered to the skin using a microneedle patch.Eur J Pharm Biopharm 2012;81:239–247.

67. Coulman SA, Barrow D, Anstey A, et al. Mini-mally invasive cutaneous delivery of macromol-ecules and plasmid DNA via microneedles. CurrDrug Deliv 2006;3:65–75.

68. Ferraro B, Morrow MP, Hutnick NA, Shin TH,Lucke CE, Weiner DB. Clinical applications ofDNA vaccines: current progress. Clin Infect Dis2011;53:296–302.

69. Kang S-M, Song J-M, Kim Y-C. Microneedle andmucosal delivery of influenza vaccines. ExpertRev Vaccines 2012;11:547–560.

70. Arya JM, Dewitt K, Scott-Garrard M, ChiangY-W, Prausnitz MR. Rabies vaccination in dogsusing a dissolving microneedle patch. J ControlRelease 2016;239:19–26.

71. Prow TW, Chen X, Prow NA, et al. Nanopatch-targeted skin vaccination against West Nile vi-rus and chikungunya virus in mice. Small 2010;6:1776–1784.

72. Gill HS, Soderholm J, Prausnitz MR, Sallberg M.Cutaneous vaccination using microneedlescoated with hepatitis C DNA vaccine. Gene Ther2010;17:811.

73. Mikszta JA, Alarcon JB, Brittingham JM, SutterDE, Pettis RJ, Harvey NG. Improved genetic im-munization via micromechanical disruption ofskin-barrier function and targeted epidermaldelivery. Nat Med 2002;8:415.

74. Kim Y-C, Quan F-S, Yoo D-G, Compans RW, KangS-M, Prausnitz MR. Improved influenza vaccina-tion in the skin using vaccine coated micro-needles. Vaccine 2009;27:6932–6938.

75. Kask AS, Chen X, Marshak JO, et al. DNA vaccinedelivery by densely-packed and short micropro-

jection arrays to skin protects against vaginalHSV-2 challenge. Vaccine 2010;28:7483–7491.

76. Kumar A, Wonganan P, Sandoval MA, Li X, ZhuS, Cui Z. Microneedle-mediated transcutaneousimmunization with plasmid DNA coated on cat-ionic PLGA nanoparticles. J Control Release2012;163:230–239.

77. Fehres CM, Garcia-Vallejo JJ, Unger WW, VanKooyk Y. Skin-resident antigen-presenting cells:instruction manual for vaccine development.Front Immunol 2013;4:157.

78. Boks MA, Unger WW, Engels S, Ambrosini M,van Kooyk Y, Luttge R. Controlled release of amodel vaccine by nanoporous ceramic micro-needle arrays. Int J Pharm 2015;491:375–383.

79. Kim MC, Kim KH, Lee JW, et al. Co-delivery of M2evirus-like particles with influenza split vaccine tothe skin using microneedles enhances the efficacyof cross protection. Pharmaceutics 2019;11:188.

80. Weldon WC, Zarnitsyn VG, Esser ES, et al. Effectof adjuvants on responses to skin immunizationby microneedles coated with influenza subunitvaccine. PLoS One 2012;7:e41501.

81. Beran J, Ambrozaitis A, Laiskonis A, et al. In-tradermal influenza vaccination of healthy adultsusing a new microinjection system: a 3-yearrandomised controlled safety and immunoge-nicity trial. BMC Med 2009;7:13.

82. Leroux-Roels I, Vets E, Freese R, et al. Seasonalinfluenza vaccine delivered by intradermal mi-croinjection: a randomised controlled safety andimmunogenicity trial in adults. Vaccine 2008;26:6614–6619.

83. Shakya AK, Gill HS. A comparative study ofmicroneedle-based cutaneous immunization withother conventional routes to assess feasibility ofmicroneedles for allergy immunotherapy. Vac-cine 2015;33:4060–4064.

84. Hiraishi Y, Nandakumar S, Choi SO, et al. Ba-cillus Calmette-Guerin vaccination using a mi-croneedle patch. Vaccine 2011;29:2626–2636.

85. Esser ES, Romanyuk A, Vassilieva EV, et al. Te-tanus vaccination with a dissolving microneedlepatch confers protective immune responses inpregnancy. J Control Release 2016;236:47–56.

86. Mikszta JA, Dekker JP, Harvey NG, et al.Microneedle-based intradermal delivery of theanthrax recombinant protective antigen vaccine.Infect Immun 2006;74:6806–6810.

87. Morefield GL, Tammariello RF, Purcell BK, et al.An alternative approach to combination vac-cines: intradermal administration of isolatedcomponents for control of anthrax, botulism,plague and staphylococcal toxic shock. J ImmuneBased Ther Vaccines 2008;6:5.

88. Seok H, Noh JY, Lee DY, Kim SJ, Song CS, KimYC. Effective humoral immune response from aH1N1 DNA vaccine delivered to the skin by mi-croneedles coated with PLGA-based cationic na-noparticles. J Control Release 2017;265:66–74.

AN INSIGHT INTO MICRONEEDLES 15

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.

89. Ye Y, Wang J, Hu Q, et al. Synergistic transcu-taneous immunotherapy enhances antitumorimmune responses through delivery of check-point inhibitors. ACS Nano 2016;10:8956–8963.

90. Schepens B, Vos PJ, Saelens X, van der MaadenK. Vaccination with influenza hemagglutinin-loaded ceramic nanoporous microneedle arraysinduces protective immune responses. Eur JPharm Biopharm 2019;136:259–266.

91. Song JM, Kim YC, Barlow PG, et al. Improvedprotection against avian influenza H5N1 virus bya single vaccination with virus-like particles inskin using microneedles. Antiviral Res 2010;88:244–247.

92. Quan FS, Kim YC, Song JM, et al. Long-termprotective immunity from an influenza virus-likeparticle vaccine administered with a micro-needle patch. Clin Vaccine Immunol 2013;20:1433–1439.

93. Kim MC, Lee JW, Choi HJ, et al. Microneedlepatch delivery to the skin of virus-like particlescontaining heterologous M2e extracellular do-mains of influenza virus induces broad hetero-subtypic cross-protection. J Control Release2015;210:208–216.

94. Quan F-S, Kim Y-C, Compans RW, Prausnitz MR,Kang S-M. Dose sparing enabled by skin im-munization with influenza virus-like particlevaccine using microneedles. J Control Release2010;147:326–332.

95. Pearton M, Kang SM, Song JM, et al. Influenzavirus-like particles coated onto microneedles canelicit stimulatory effects on Langerhans cells inhuman skin. Vaccine 2010;28:6104–6113.

96. Park J-H, Prausnitz MR. Analysis of mechanicalfailure of polymer microneedles by axial force. JKorean Phys Soc 2010;56:1223.

97. Corbett HJ, Fernando GJ, Chen X, Frazer IH,Kendall MA. Skin vaccination against cervicalcancer associated human papillomavirus with anovel micro-projection array in a mouse model.PLoS One 2010;5:e13460.

98. Kines RC, Cerio RJ, Roberts JN, et al. Humanpapillomavirus capsids preferentially bind and in-fect tumor cells. Int J Cancer 2016;138:901–911.

99. del Pilar Martin M, Weldon WC, Zarnitsyn VG,et al. Local response to microneedle-based in-fluenza immunization in the skin. MBio 2012;3:e00012-12.

100. Moon S, Wang Y, Edens C, Gentsch JR, PrausnitzMR, Jiang B. Dose sparing and enhanced im-munogenicity of inactivated rotavirus vaccineadministered by skin vaccination using a micro-needle patch. Vaccine 2013;31:3396–3402.

101. Sullivan SP, Koutsonanos DG, del Pilar Martin M,et al. Dissolving polymer microneedle patchesfor influenza vaccination. Nat Med 2010;16:915.

102. Carey JB, Pearson FE, Vrdoljak A, et al. Micro-needle array design determines the induction ofprotective memory CD8+ T cell responses in-

duced by a recombinant live malaria vaccine inmice. PLoS One 2011;6:e22442.

103. Vrdoljak A, Allen EA, Ferrara F, Temperton NJ,Crean AM, Moore AC. Induction of broad im-munity by thermostabilised vaccines incorpo-rated in dissolvable microneedles using novelfabrication methods. J Control Release 2016;225:192–204.

104. Dean CH, Alarcon JB, Waterston AM, et al.Cutaneous delivery of a live, attenuated chimericflavivirus vaccines against Japanese encephalitis(ChimeriVaxTM-JE) in non-human primates. HumVaccines 2005;1:106–111.

105. Edens C, Collins ML, Goodson JL, Rota PA,Prausnitz MR. A microneedle patch containingmeasles vaccine is immunogenic in non-humanprimates. Vaccine 2015;33:4712–4718.

106. Carey JB, Vrdoljak A, O’mahony C, Hill AV,Draper SJ, Moore AC. Microneedle-mediatedimmunization of an adenovirus-based malariavaccine enhances antigen-specific antibody im-munity and reduces anti-vector responses com-pared to the intradermal route. Sci Rep 2014;4:6154.

107. Pearson FE, O’Mahony C, Moore AC, Hill AV.Induction of CD8+ T cell responses and protec-tive efficacy following microneedle-mediateddelivery of a live adenovirus-vectored malariavaccine. Vaccine 2015;33:3248–3255.

108. Zaric M, Lyubomska O, Poux C, et al. Dissolvingmicroneedle delivery of nanoparticle-encapsulatedantigen elicits efficient cross-priming and Th1immune responses by murine Langerhans cells. JInvest Dermatol 2015;135:425–434.

109. Zhen Y, Wang N, Gao Z, et al. Multifunctionalliposomes constituting microneedles inducedrobust systemic and mucosal im-munoresponses against the loaded antigensvia oral mucosal vaccination. Vaccine 2015;33:4330–4340.

110. Jiang J, Gill HS, Ghate D, et al. Coated micro-needles for drug delivery to the eye. InvestOphthalmol Vis Sci 2007;48:4038–4043.

111. Than A, Liu C, Chang H, et al. Self-implantabledouble-layered micro-drug-reservoirs for efficientand controlled ocular drug delivery. Nat Commun2018;9:4433.

112. Chiu WS, Belsey NA, Garrett NL, et al. Drugdelivery into microneedle-porated nails fromnanoparticle reservoirs. J Control Release 2015;220:98–106.

113. Flores FC, Chiu WS, Beck RC, da Silva CB,Delgado-Charro MB. Enhancement of tioconazoleungual delivery: combining nanocapsule formu-lation and nail poration approaches. Int J Pharm2018;535:237–244.

114. Sanjay ST, Zhou W, Dou M, et al. Recent ad-vances of controlled drug delivery using micro-fluidic platforms. Adv Drug Deliv Rev 2018;128:3–28.

115. Thomas S, Koop D. Method and apparatus fortreating female urinary incontinence and fecalincontinence. Google Patents, 2019.

116. Dhurat R, Sukesh M, Avhad G, Dandale A, Pal A,Pund P. A randomized evaluator blinded study ofeffect of microneedling in androgenetic alopecia:a pilot study. Int J Trichol 2013;5:6.

117. Fertig R, Gamret A, Cervantes J, Tosti A. Mi-croneedling for the treatment of hair loss? J EurAcad Dermatol Venereol 2018;32:564–569.

118. Tang J, Wang J, Huang K, et al. Cardiac cell–integrated microneedle patch for treating myo-cardial infarction. Sci Adv 2018;4:eaat9365.

119. Li J, Zeng M, Shan H, Tong C. Microneedlepatches as drug and vaccine delivery platform.Curr Med Chem 2017;24:2413–2422.

120. Davis SP, Landis BJ, Adams ZH, Allen MG, PrausnitzMR. Insertion of microneedles into skin: measure-ment and prediction of insertion force and needlefracture force. J Biomech 2004;37:1155–1163.

121. Mistilis MJ, Bommarius AS, Prausnitz MR. De-velopment of a thermostable microneedle patchfor influenza vaccination. J Pharm Sci 2015;104:740–749.

122. Vassilieva EV, Kalluri H, McAllister D, et al. Im-proved immunogenicity of individual influenzavaccine components delivered with a novel dis-solving microneedle patch stable at room tem-perature. Drug Deliv Transl Res 2015;5:360–371.

123. Wang T, Wang N. Preparation of the multifunc-tional liposome-containing microneedle arrays asan oral cavity mucosal vaccine adjuvant-deliverysystem. Methods Mol Biol 2016;1404:651–667.

124. Wang J, Li B, Wu MX. Effective and lesion-freecutaneous influenza vaccination. Proc Natl AcadSci U S A 2015;112:5005–5010.

ABBREVIATIONSAND ACRONYMS

3D ¼ three dimensional5-ALA ¼ 5-aminolevulinic acid

BBB ¼ blood-brain barrierBCG ¼ bacillus Calmette-GuerinCSC ¼ cardiac stromal cellDCs ¼ dendritic cells

DMN ¼ dissolve microneedleDNA ¼ deoxyribonucleic acidDOX ¼ doxorubicinDTX ¼ docetaxel

GNPs ¼ gelatin nanoparticlesHA ¼ hemagglutininLCs ¼ Langerhans cellsMN ¼ microneedle

MSNs ¼ mesoporous silica nanoparticlesnpMN ¼ nanoporous ceramic microneedle

PE ¼ phenylephrinePLA ¼ poly(lactic acid)

PLGA ¼ poly(lactic-co-glycolic acid)VLPs ¼ virus-like particles

16 BILAL ET AL.

Dow

nloa

ded

by "

Inst

itute

of

Bio

phys

ics,

CA

S" f

rom

ww

w.li

eber

tpub

.com

at 0

7/20

/20.

For

per

sona

l use

onl

y.