the clinical efficacy 
 of small - molecule therapeutics is limited 
 by many factors , including poor solubility , inefficient cellular uptake , 
 low bioavailability due to rapid renal clearance , and an inability 
 to target desired locations . moreover , the side effects 
 of cytotoxic agents , such as those used in classical anti - cancer regimens , 
 are often the direct result of the drug s inability to discriminate 
 between healthy and diseased tissue . nanoscale 
 drug delivery vehicles have been under frantic development to address 
 these issues , with the promise that such formulations will offer significant 
 advantages over systemically administered small molecules . as a result , 
 there have been notable successes in the clinical translation of nanoparticle 
 therapeutics , most of which are hypothesized to rely on the enhanced 
 permeation and retention ( epr ) effect 
 as a means to passively accumulate drug - carrying nanomaterial delivery 
 vehicles within diseased tissue . the epr effect is thought 
 to facilitate the accumulation of nanoscale structures in the highly 
 fenestrated vasculature ( 200800 nm pores ) that is characteristic 
 of the rapid angiogenesis seen in cancer , inflammation , and infection . however , given that the epr effect operates via 
 passive accumulation , it offers little control over the timed release 
 of drugs and generally can not be invoked for the treatment of pathologies 
 with normal , or approaching normal , vasculature . furthermore , observations 
 during testing of new , specially designed nanomaterials frequently 
 show behavior that contravenes the commonly held belief that epr is 
 at play in delivery , resulting in materials that lack desirable properties 
 or fault the thesis entirely . efforts to include active accumulation 
 and programmed release properties into nanomaterial designs include 
 displaying targeting moieties , transporting materials with serum 
 proteins , disguising synthetic nanoparticles 
 as red blood cells , using chemical functionalities 
 invoking efficient cellular uptake , labeling 
 particles to enable endosomal release , and preparing 
 nanostructures imbued with the means for timed release of cargo . nature provides inspiration for the creative development of 
 novel 
 drugs and drug delivery platforms . elaborate and efficient viruses 
 have evolved over time , adapting the ability to enter specific cells , 
 disassemble , deliver proteins and nucleic acids , and ultimately replicate 
 themselves to ensure propagation of the process . many of the systems we describe have much in common with the evolved 
 strategies of viruses , albeit to a much simplified and , unfortunately , 
 inefficient degree . at the level of the active small molecule or biomolecule , nature often solves issues of off - target effects by synthesizing these 
 species as inactive or dormant precursors . indeed , many effective 
 small - molecule drugs are delivered in a deactivated form by chemical 
 conjugation of the active core to a cleavable moiety . prodrugs often 
 enable enhanced solubility , membrane permeability and/or environment - specific 
 activation of the parent drug . one example is salicin , a -glucoside 
 that is hydrolyzed by hydrochloric acid in the stomach to yield salicylic 
 acid , the active metabolite of aspirin . similarly , organisms produce many other activatable molecules such 
 as zymogens , deactivated enzymes , that must be activated biochemically 
 ( i.e. , by cleavage of a peptide fragment ) to perform their intended 
 catalytic function . this ensures that the enzyme is only active once 
 it reaches its target destination . for example , pepsin , a highly active 
 proteolytic enzyme that degrades peptides and proteins in the stomach , 
 is synthesized as a zymogen ( pepsinogen ) to ensure that the contents 
 of the cell in which the enzyme is synthesized are not degraded . following secretion from these cells , pepsinogen 
 is activated by the low ph of stomach tissue , where it functions to 
 digest ingested protein . indeed , life depends on our ability to eat , 
 while not being consumed by the molecules and materials that facilitate 
 digestion . hence , biology takes a compartmentalization and selective 
 activation approach to harnessing reactivity for temporal and spatial 
 control of chemical processes . these types of concepts have been borrowed 
 in the attempt to develop synthetic small molecules , macromolecules , 
 and nanomaterials capable of interacting with cellular machinery and 
 with biochemical systems . in recent years , there has been increasing 
 effort in the development 
 of stimuli - responsive nanomaterials with the hope that they will be 
 developed into effective drug delivery or diagnostic vehicles . these 
 synthetic systems utilize an assortment of endogenous or exogenous 
 stimuli to induce a variety of responses that can facilitate targeted 
 drug delivery . most commonly , effective drug delivery is associated 
 with nanomaterial - facilitated accumulation and/or cellular internalization . this perspective on progress and future directions in this area is 
 not meant to be a comprehensive review , nor an exhaustive one . instead , 
 we aim to highlight only certain advances in the field as they relate 
 to stimuli - responsive behavior that is representative of cutting edge 
 attempts to inspire the delivery of therapeutic and diagnostic agents . many existing review articles describing progress in stimuli - responsive 
 materials are organized around the types of stimuli used . here , we first 
 briefly describe and attempt to correlate selected commonly employed 
 stimuli with associated lead references ( table 1 ) . to offer a unique perspective on this topic , this perspective 
 is primarily organized in terms of the type of physical response elicited . it is a challenge in some cases to characterize certain 
 experimental approaches into a single category , therefore , several 
 of our classifications are subjective and may not reflect a complete 
 description of the entire process . throughout , we have aimed to present 
 existing data in a new light and to offer a platform for fresh insight 
 and perspective on the field . given the relative simplicity 
 of a prodrug / zymogen approach , it is not surprising that some of the 
 earliest examples of stimuli - responsive nanomaterials in the literature 
 are those involving a direct cleavage event resulting in the release 
 of therapeutic cargo or the excitation or quenching of a fluorophore 
 for imaging ( figure 1 ) . indeed , the literature 
 is ripe with examples of this process where drugs are covalently attached 
 to an otherwise inert nanomaterial scaffold via a linker that is susceptible 
 to cleavage by an appropriate stimulus . for example , the lysosomal protease cathepsin b has been used extensively 
 to trigger drug release directly inside of cells when a targeting / internalization 
 agent such as folic acid facilitates the internalization of the nanocarrier . two - step approaches , such as 
 polymer - directed enzyme prodrug therapy , are also employed in which 
 a prodrug and its enzyme effector are chemically conjugated to two 
 separate carriers . there are relatively few examples 
 of the two - step strategy , and most purport to rely on the epr effect 
 to localize nanoparticles at tumor tissue . once localized , the protease , 
 which is still active as part of the conjugate , facilitates direct 
 release of the drug from its nanomaterial carrier into tumor tissue , 
 thus minimizing off - target cytotoxicity . cathepsin 
 b conjugate to tumor - bearing mice , which had been pretreated with 
 an hmpa particle linked to doxorubicin via an enzyme - cleavable peptide 
 substrate , resulted in a 3.6-fold increase in the rate of drug release 
 with improved tumor reduction relative to the polymer ( a ) cartoon 
 scheme depicting the direct release of drugs ( red circles ) or activation 
 of diagnostic agents following initiation by a stimulus . ( b ) a literature 
 example of a dendrimer ( called an activatable cell - penetrating dendrimer , 
 acppd ) decorated with activatable cell - penetrating peptides ( acpps ) 
 that also contains encapsulated cy 5 dye for fluorescence imaging 
 or gadolinium cargo for use in mri diagnostics ( yellow circles ) . in 
 this design , enzymes upregulated in cancer cells ( mmps ) facilitate 
 cleavage of the acpp hairpin , exposing a polyarginine cell penetrating 
 peptide motif , which facilitates the entry of the cargo - carrying nanoparticle . 
 prior to cleavage , the acpp forms a hairpin by non - covalent interactions 
 between segments of polyglutamic acid and polyargine ( the cell - penetrating 
 motif ) that flank the recognition sequence of the enzyme . upon cleavage 
 of the peptide hairpin , the polyglutamic acid segment is released , 
 exposing the polyarginine fragment , which can then penetrate cells . 
 ( c ) fluorescence images of mice 48 h post injection of either the 
 cleavable acppd with encapsulated cy 5 , or a non - cleavable acppd ( d - amino acid control ) variant . in these images , there is a substantial 
 increase in florescence at tumors only when the particles with the 
 cleavable acpp are administered , illustrating that this method can 
 be used to target cancer cells and internalize while carrying useful 
 diagnostic or therapeutic cargo . panels b and c are adapted from olson 
 et al . with permission from the national 
 academy of sciences . fluorescence resonance energy transfer ( fret)-based approaches 
 used in diagnostics also invoke a direct cleavage mechanism . in traditional 
 designs , a fluorescent donor and acceptor flank a peptide substrate 
 that is optimized for degradation by the protease of interest . cleavage of this linkage facilitates the physical 
 separation of the donor and acceptor in space , resulting in a decrease 
 in fret . these cleavage events are often mediated by proteases that 
 are dysregulated in a particular pathology , such as cancer , and so the function or abundance of the proteolytic 
 enzyme can be assessed by monitoring changes in fret efficiency . quantum dots ( qds ) are a promising set of materials used in nanomaterial 
 diagnostics , especially those utilizing a sense - and - response switching 
 mechanism . qds are luminescent semiconductor nanocrystals , typically 
 comprised of cdse , phse , or inas cores , that have unique photophysical properties that can address many 
 of the limitations encountered in in vivo imaging 
 by traditional small - molecule fluorophores . for example , the photoemission of a qd can be matched for spectral 
 overlap with a given acceptor by simply tuning the size of the nanocrystal . moreover , qds have been shown to be more resistant 
 to photobleaching than their small - molecule counterparts . given their exceptional promise , qds make intriguing diagnostic 
 agents , especially when encoded to respond to a biological stimulus 
 that is a signature for a specific disease type . described 
 a stimuli - responsive example in which the surface of a qd is modified 
 with peptide sequences that terminate in a fluorescent quencher or 
 acceptor dye . here , an assortment of 
 peptide linkers are employed that are each activated by one of several 
 clinically relevant proteases , including caspase-1 , thrombin , collagenase , 
 and chymotrypsin . in this study , each variation of the assay was shown 
 to be selective for the intended protease . however , it is well documented 
 that qds with cdse semiconductor cores can be toxic to cells resulting , 
 in part , from cd contamination or release . cytotoxicity can be alleviated by modifying the surface of the qd 
 with a shell , such as those comprised of zns . like many small - molecule 
 fluorophores , qds also suffer from  blinking  , an issue 
 which has not yet been solved and may prove problematic for diagnostic 
 applications . upconverting nanoparticles ( ucnps ) could also make exceptional 
 diagnostic tools if rendered responsive to disease - associated stimuli . ucnps function via anti - stokes emission , converting 
 excitation photons of nir light into an emission in the visible spectrum . 
 because they are excited by nir light , they do not exhibit the photodamage 
 to tissue , background fluorescence , or tissue - induced scattering issues 
 that are typical of small - molecule fluorophores that absorb uv light . ucnps are generally comprised of a crystalline host matrix that 
 is doped with a lanthanide ion , which defines the photophysical properties 
 of the material ( i.e. , the excitation and emission wavelengths ) . they 
 are usually synthesized and studied in organic solvents due to poor 
 water solubility or because of disfavorable interactions between water 
 and the chromophore . in recent years , ucnps have been prepared as 
 stable aqueous colloidal dispersions via conjugation to water - solubilizing 
 ligands or shells . moreover , these surfaces are amenable to the conjugation 
 of biomolecules or other stimuli - responsive appendages . to date , there 
 have been several successful attempts to use stimuli to toggle between 
 emissive and  dark  , non - emissive states of uncps . examples include uncps that are quenched by 
 uv light or those that act as ph sensors , but none have been rendered sensitive to specific 
 markers of disease , aside from ph . therefore , uncps have untapped 
 potential for use in advanced diagnostic imaging if strategies are 
 developed to render their photophysical properties sensitive to specific 
 disease biomarkers . the sailor laboratory has demonstrated 
 that these materials exhibit exceptionally long emission lifetimes 
 ( 513 s ) , making possible time - gated imaging of tissue in vivo to minimize background associated with autofluorescence 
 signatures that typically decay in 110 s . moreover , these materials exhibit low toxicity 
 and are hydrolyzed under physiological conditions , making them exceptional 
 candidates for the development of stimuli - responsive in vivo imaging probes . in general , direct release / activation strategies 
 suffer from issues 
 of specificity . for example , linkages susceptible to cleavage at low 
 ph , such as esters , are typically also cleavable by enzymes like esterases , 
 which are abundant in circulation . even peptide sequences programmed 
 to be cleaved by a particular protease with a highly specific cleavage 
 propensity may also be generically cleaved by other proteases that 
 have more permissive active sites . for example , peptide substrates 
 typically used by cathepsin b ( gly - phe - leu - gly ) can also be recognized by a number of other enzymes including 
 pepsin , a key digestive protease . given that the stomach secretes 
 gram quantities of proteases , oral delivery of peptide - based agents 
 is thwarted by proteolytic activation in the stomach , among other 
 potential issues including the low ph environment . indeed peptide - based therapeutics are typically injected 
 at the site of interest due to rapid digestion by proteases that are 
 abundant in circulation . in this regard , 
 materials capable of responding to enzymatic signatures must be designed 
 in such a way that they do not undergo unwanted activation . oral delivery remains a key challenge in this 
 type of approach , and one with significant potential to impact and 
 dictate how materials are delivered for in vivo use . in summary , with any direct sense - and - release strategy , care must 
 be taken when choosing the stimulus . for example , extra - cellularly 
 expressed enzymes may be effective at releasing a therapeutic agent 
 in tumor tissue , but they offer less help in delivering the material 
 to the interior of a cell , where most cytotoxic drugs are active . 
 while it is true that these strategies increase the local concentration 
 of drug at the site of interest , it is possible for these agents to 
 return to the general circulation in the absence of a mechanism for 
 efficient and rapid cellular uptake . therefore , intra - cellularly expressed 
 enzymes like cathepsin b , methionine sulfoxide reductase , glycosidases , 
 or intra - cellular kinases make particularly intriguing stimuli . alternatively , 
 strategies like those developed by the tsien laboratory ( figure 1b , c ) in which a cell - penetrating motif is masked 
 and activated by enzymes overexpressed in certain tumors ( matrix metalloproteinases , 
 mmps ) , are also promising approaches for drug delivery and imaging . expansile particles swell 
 or contract in 
 response to activation by a stimulus . when these nanoparticles swell , 
 they typically become fenestrated or leaky , enabling the release of 
 encapsulated drugs ( figure 2 ) . ph is often used as the stimulus to invoke expansion , as 
 it can alter the protonation state of basic / acidic functionalities 
 such as tertiary amino or carboxyl groups . polymeric micelles , polymerosomes , 
 hydrogels , or other scaffolds loaded with these moieties can act as 
 ph sensors whose hydrophobicity , conformation , or electrostatics are 
 altered based on their protonation state ( see figure 2b  d ) . in one example , polymerosomes 
 composed of poly(l - glutamic acid)-block - poly(l - lysine ) exchange the identity of their hydrophilic corona 
 and hydrophobic core in response to protonation / deprotonation cycles . many other stimuli can also be used to control 
 particle expansion or contraction including temperature or the ionic strength of the 
 solution . ( a ) a general cartoon describing 
 systems that release encapsulated drugs ( red circles ) by expansion 
 into a fenestrated structure upon activation by a stimulus . ( b ) chemical 
 structures of expansile nanoscale particles containing 2,4,6-trimethoxylbenzaldehyde - derived 
 acetals that hydrolyze at ph  5 to yield diol - containing scaffolds , 
 which form micrometer - scale hydrogels . the ph of activation is in 
 line with the ph of cellular lysosomes , and so these materials have 
 been used to release encapsulated drugs inside cellular organelles 
 upon internalization . ( c ) scanning electron microscopy images of the 
 acetal described in b at ph 7.4 , and the diol hydrolysis product at 
 ph 5.0 . note that the diameter of the material expands 350-fold in 
 response to mildly acidic environments . ( d ) experimental data from in vivo studies in which c57bl/6 mice are injected with 
 lewis lung carcinoma cells alongside paclitaxel - loaded expansile ( exp ) 
 and non - expansile particles ( non - exp , which contain related benzaldehyde - derived 
 acetals ) and appropriate controls . note that only mice that received 
 the drug - loaded exapansile particles were free from tumors . panels 
 c and d are adapted from colby et al . with permission from the royal society of chemistry and griset et 
 al . with permission from the american chemical society , respectively . as with any strategy , caution is necessary when designing 
 systems 
 for non - covalent encapsulation , because there is the potential for 
 unintended off - target or burst release upon injection . efficient encapsulation 
 itself can be a challenging task that is sometimes difficult to quantify , 
 and thus more research into encapsulation - and - release efficiencies , 
 such as those reported by adams and co - workers , could help propel these systems further . additionally , 
 for expansile systems , the efficiency of swelling / contraction needs 
 to be robust enough to release cargo in a reproducible , high - fidelity 
 fashion . moreover , thus far , expansion of nanoparticles has been primarily 
 triggered only by bulk environmental properties such as ionic strength , 
 temperature , or ph . however , variations in these properties occur 
 in both healthy and diseased tissue . for example , a lowered ph may 
 very well be a marker of cancerous tissue , but it is also found inside 
 the endosomal / lysosomal compartments of any cell . therefore , it would 
 be intriguing to develop expansile systems for which more specific 
 triggers , such as enzymes or other known biomarkers , may be employed . mechanism in which a nanocarrier is coated with a sterically bulky 
 shell or  gate  that encapsulates drug cargo . upon decaging 
 by removal of the shell or opening of the gate , stimuli - responsive 
 mesoporous silica nanoparticle ( msnp ) systems often employ a gatekeeper 
 approach . msnps are popular drug delivery 
 vehicles because of their low cytotoxicity , large surface area , tailorable 
 pore volumes , and the ease with which their surfaces can be chemically 
 manipulated . in these systems , a sterically bulky gate , such as -cyclodextrin 
 ( -cd , figure 3b , c ) , is installed on the surface of the msnp via a cleavable linkage 
 to block cargo release ( opening of the gate ) until activation by a 
 stimulus . an assortment of gates have been employed including rotaxanes , saccharides , gold 
 nanoparticles , and segments of dna . to further tailor release profiles , the gates 
 are operated by a wide range of endogenous and exogenous stimuli such 
 as ph , redox , uv light , enzymes , and dna recognition . in one intriguing example , double - stranded dna 
 is immobilized via an azobenzene linkage . light - triggered dehybridization / rehybridization of complementary 
 dna leads to channel opening / gating . when dehybridized , the channel 
 opens and releases entrapped doxorubicin . in addition to msnps , a 
 variety of other nanoscale vehicles have been caged by polyethylene 
 glycol ( peg ) shells and other bulky or water - solubilizing appendages 
 to shield covalently or non - covalently linked drugs or targeting agents . ( a ) a cartoon illustration of the gatekeeping strategy in which the 
 pores ( green cylinders ) of mesoporous silica nanoparticles ( msnps , 
 depicted as blue spheres ) are blocked by a gate ( yellow crosses ) . stimulus activation results in opening of the gate and release of 
 encapsulated imaging agents or therapeutics ( red spheres ) . ( b ) a literature example of a 
 ph - activated msnp . in this example , a -cyclodextrin ( -cd ) 
 is used to cap the pores of drug- or fluorophore - carrying msnps . at 
 physiological ph , the -cd encapsulates aromatic amines that 
 are appended to the periphery of the msnp , blocking the nanopore and 
 entrapping cargo . protonation of the amines following a decrease in 
 ph results in the release of the cyclodextrin gate , enabling free 
 diffusion of the pore contents . ( c ) fluorescent images illustrating 
 doxorubicin release from the -cd - gated msnps described in panel 
 b after internalization of the materials into acidified endosomal 
 compartments of kb-13 cells . neutralization of lysosomal ph 
 by the addition of nh4cl results in inhibition of doxorubicin 
 release and toxcity , providing support for the proposed mechanism 
 of activation . panels b and c are adapted from meng et al . with permission from the american chemical society . as with other encapsulation techniques 
 such as the expansile particles 
 described above , undesired leakage of drug could potentially lead 
 to systemic toxicity . additionally , any strategy ultimately relying 
 on a cleavage event will likely suffer from a lack of universal specificity 
 because of the difficulty in preparing a linkage that is recognized 
 by only one stimulus found in vivo . again , here we 
 return to the central , all too common biomarker problem faced by any 
 targeted system from small - molecule drugs to nanoscale carriers . the 
 key and obvious challenge is to solve the problem for responsive systems 
 using truly orthogonal linkers designed to be recognized and cleaved 
 only by a single stimulus . nanoparticles 
 that degrade 
 or otherwise disassemble have been explored as a means to deliver 
 cargo in a spatially and temporally controlled manner . we define disassembly 
 as a process by which a discrete material breaks into pieces via preprogrammed 
 stimuli - triggered events ( figure 4 ) . this is 
 distinct from other processes such as the gatekeeping mechanisms described 
 above where only a shell or portion of a nanoparticle is dispersed 
 in response to the stimulus . there are two main approaches for triggering 
 disassembly : single - event disassembly and multi - step degradation . ( a ) cartoon illustrating 
 a generic disassembly event in which a nanoscale material breaks down 
 into smaller fragments , releasing encapsulated or chemically appended 
 drugs ( blue spheres ) . ( b ) a literature example of a deploymerizaton 
 event that is initiatied under reductive conditions . here , polymersomes 
 assembled from block copolymers composed of a self - immolative poly(benzyl 
 carbamate ) block and a hydrophilic poly(n , n - dimethylacrylamide ) 
 ( pdma ) block . in this report , the self - immolative block was caged 
 with either perylen-3-yl , o - nitrobenzyl , or disulfide 
 moieties ( as depicted ) , which uncage in response to visible light 
 ( 420 nm ) , uv light ( 365 nm ) , or reductive agents , respectively . upon 
 activation , these polymersomes were also loaded with 
 doxorubicin or campothecin , and payload release coincident with depolymerization 
 was observed . ( c ) transmission electron microscopy of the block copolymer 
 before and after treatment with dithiothreitol . panels b and c are adapted from liu et al . with permission from the american chemical society . in a single - event disassembly process , one recognition 
 event initiates 
 a degradation event , leading to the complete disassembly of the nanostructure , 
 either in a one - to - one  fashion or via a cascade mechanism . 
 in a one - to - one model , the stimulus acts stoichiometrically to trigger 
 each cleavage event at each cleavable bond within a micro- or nanoparticle . mechanistically , this is the simplest degradation 
 strategy , and as such , many of the earliest examples of stimuli - responsive 
 nanomaterials dissociate in this manner . the vast majority of these 
 systems are designed to degrade in response to low ph , which may be advantageous for tumor treatment . polymers composed of poly(lactic acid ) ( pla ) or polylactide - co - glycolide ( plga ) are commonly 
 incorporated into degradable nanoparticles , because they contain chemical 
 bonds ( e.g. , esters ) that are susceptible to hydrolysis at acidic 
 ph . heller et al . were among the first to report a ph - sensitive nanoparticle 
 amenable to controlled release , which was designed to contain acid - sensitive 
 maleic anhydrides . since then , there 
 have been extensive reports on nanoparticle systems that fully degrade 
 in response to ph . developed 
 a diblock copolymer system comprised of a hydrophilic peg block , linked 
 through a hydrazone to a hydrophobic poly amino acid ( paa ) , in which 
 drug molecules can be physically entrapped in the core upon nanoparticle 
 formation . this system is stable at 
 ph 7.4 , but rapidly disassembles at ph 6.67.2 , which matches 
 the extra - cellular ph of some tumor tissues . in the cascade 
 model , a stimulus triggers the complete dissolution 
 of the nanoparticle by initiating either a cascade reaction or a rapid 
 depolymerization ( figure 4b , c ) . the advantage of a cascade trigger is the possibility 
 of signal amplification wherein only a single recognition event is 
 needed to initiate the degradation process , ultimately leading to 
 payload release . however , complete disassembly requires a great deal 
 of optimization , including kinetic considerations . for example , insufficient 
 exposure to the stimulus at physiologically relevant concentrations 
 may lead to incomplete degradation , and thus a poor result in vivo . shabat and colleagues developed a dendrimer programmed 
 to degrade in a cascade fashion upon exposure to uv light . in this system , an  adaptor , an o - nitrobenzyl linkage is cleaved , triggering a cascade that 
 results in the degradation of the dendrimer and ultimately the release 
 of multiple cargo units . in this example , disassembly proceeds much 
 slower in the second generation dendrimer than in the first generation 
 variant , likely due to sterics in the larger dendrimer , illustrating 
 the critical need for optimization of these types of systems . more 
 recently , the almutairi group developed a polymeric system more amenable 
 to biomedical applications in vivo as it is degraded 
 in the near - ir , rather than by irradiation with uv light . other stimuli , such as redox , temperature changes , or enzyme triggers , have also been employed to facilitate 
 nanomaterial degradation by either the one - to - one or cascade mechanisms . for example , the sumerlin laboratory has explored the use of block 
 copolymers containing boronic acid moieties whose solubility ( and 
 thus amphiphilicity ) is influenced by either ph or the presence of 
 diol - containing small molecules ( i.e. , sugars ) . remarkably , the disassembly of these materials can be triggered 
 in the presence of glucose at physiological ph , which could be an 
 intriguing approach to the treatment of diabetes . if prepared with 
 a thermoresponsive n - isopropylacrylamide block , the 
 assemblies can be rendered sensitive to three separate stimuli : ph , 
 temperature , and sugar concentration . liu et al . has prepared polymersomes that depolymerize in response 
 to uv light , visible light , or glutathione ( example shown in figure 4b , c ) , depending upon the identify of a small - molecule 
 cap . triggers located in cellular cytosols 
 like glutathionine are particularly intriguing because they ensure 
 that the material remains intact until internalized into a cell . thayumanavan 
 and co - workers have explored protein - binding induced disassembly of 
 dendritic supramolecular assemblies in which a ligand such as biotin 
 is strategically placed on the dendrimer scaffold . binding of a cognate protein like avidin to this ligand 
 then triggers the disassembly of the dendritic complex and coincident 
 payload release . in other work , caruso and co - workers reported an 
 elegant , programmable strategy utilizing a dna - based nanoparticle 
 system that incorporates restriction enzyme cut - sites for simultaneous 
 controlled drug release and nanoparticle dissolution . this stands out as a key example of the exciting trend toward designing far more selective nanoparticle systems wherein 
 a material can be truly encoded with information . these materials 
 open up the possibility that such approaches will be more routinely 
 utilized in future schemes to home and respond to patterns of gene 
 expression and to enzymatic activity in vivo . degradable nanoparticles that undergo multi - step degradation processes 
 can require two or more orthogonal stimuli acting independently to 
 provide more spatiotemporal control of cargo release . generally , one 
 stimulus initiates surface degradation while a separate stimulus triggers 
 bulk degradation and subsequent cargo release . wooley and co - workers 
 recently reported the development of this type of degradable nanoparticle , 
 utilizing redox and hydrolysis through the incorporation of both polycarbonates 
 and disulfides within the same amphiphilic diblock copolymer . developed a system that degrades 
 in response to both light and ph and demonstrated a more selective 
 release of chemotherapeutics in cancer cells in vitro than strategies that utilize only one stimulus . the thayumanavan laboratory has worked to develop composite 
 nanostructures comprised of two separate supramolecular assemblies  a 
 block copolymer micelle core and a nanogel shell  that are disassembled 
 fully in response to decreased ph ( to degrade the core ) and reduction 
 by glutathione ( to disassemble the nanogel ) . this same laboratory has also prepared an amphiphilic micelle - like 
 assembly in which a protein is used to trigger the disassembly of 
 the micelle via a protein  ligand binding event and an enzyme 
 cleaves the otherwise inaccessible hydrophobic segment of the exposed 
 amphiphile , releasing cargo . moreover , 
 cross - linking of this micelle via a uv light - sensitive linkage introduced 
 the necessity of a third stimulus to the system , cleverly creating 
 a triply gated material . these types of degradable systems offer a 
 promising combination of approaches , which may lead to better in vivo specificity . however , given the complexity of the 
 approach , it will perhaps require additional effort and optimization 
 to bring these systems to an in vivo context . assembly is a ubiquitous and 
 necessary process in nature demonstrated elegantly in the ribosomal 
 assembly of proteins from amino acid building blocks , in transcription 
 via the recruitment of transcription factors and rna polymerase , and 
 in the intricate folding of peptides and proteins . mimicking this 
 assembly process in the context of drug delivery remains a challenge , 
 but several success stories ( albeit none at the level of sophistication 
 of natural systems ) serve as inspiration for the future . many groups 
 have reported successful assembly or aggregation of water - soluble 
 unimers into larger structures by ph , thermal , enzymatic , or dna hybridization triggers in vitro , but few strategies have been 
 translated to in vivo applications . herein , we define assembly as a process by which materials 
 are collected into discrete and ordered structures of larger size 
 in response to a stimulus , whereas aggregation involves the assemblage 
 of amorphous structures from initial monomeric units ( figure 5 ) . these are both distinct from the morphology switches 
 described in this perspective in which a discrete , well - defined phase 
 undergoes a change in physical architecture in response to a stimulus 
 to generate a new phase , which does not necessarily correlate with 
 an increase in size . highlighted below are a number of promising drug 
 delivery approaches described in the literature that take advantage 
 of either a nanomaterial assembly event , pretargeting strategy , or 
 thermally controlled aggregation . ( a ) cartoon 
 illustrating a generic 
 assembly event in which an uncaging event occurs ( loss of red blocking 
 unit ) to enable the assembly of many unimers ( blue octagons ) . note 
 that we define an assembly event as a unimer or ill - defined structure 
 assembling into a much larger stucture of higher order . aggregation 
 is a related process in which unimers or disordered phases assemble 
 into larger , amphorphous aggregates . ( b ) chemical structures depicting 
 a literature example of an o - nitrobenzyl  caged  
 amphiphilic peptide that becomes amphiphillic and self - assembles into 
 hydrogel - forming fibers upon removal of the caging group ( i.e. , the 
 red oval in scheme a ) by 350 nm light . ( c ) photograph of the caged 
 amphiphile and the hydrogel formed after removal of the o - nitrobenzyl cage . ( d ) transmission electron microscopy images of 
 the system before and after exposure to uv light . d 
 are adapted from muraoka et al . with 
 permission from john wiley and sons ltd . a common method for in vivo assembly uses 
 a two - step 
 targeting mechanism , in which the nanomaterial scaffold is first passively 
  pretargeted  to the tissue of interest via the epr 
 effect , followed by the systemic administration of a drug that subsequently 
 accumulates on the pretargeted nanomaterial . many groups have used 
 such a pretargeting strategy to deliver radiolabeled antibodies to 
 cancerous tissue , as well as to deliver small - molecule drugs 
 or mri contrast agents via pretargeted gold nanomaterials . this strategy is advantageous because it offers 
 the possibility of decreasing systemic toxicity by first administering 
 a non - toxic material to target the tissue of interest and then subsequently 
 administering the cytotoxic agent . likewise , in in vivo imaging applications , the two - stage approach leads to decreased 
 background signal . these strategies typically use highly selective 
 or bio - orthogonal chemistry to link the two materials together , such 
 as a biotin the assembly of nanofibers 
 can be induced in vivo by hydrogelation or by assembly 
 of amphiphilic materials . pre - hydrogelator  
 peptide . upon cleavage 
 of a phosphate group from the pre - hydrogelator , ulijn and co - workers have employed a similar approach utilizing 
 enzyme - triggered reverse hydrolysis to assemble a hydrogel from hydrogelators 
 that could be used for controlled release of encapsulated drugs . the stupp laboratory has pioneered the 
 use of masked amphiphilic 
 peptides to assemble nanofibers in solution ( de - masking ) via ph , osmolarity changes , uv light , or enzyme triggers ( figure 5b  d ) . the properties 
 of the resulting hydrogel can be tuned by changing the length and 
 identity of the hydrophobic tail or 
 by modifying the hydrophilic peptide to contain cross - linking domains , -sheet - forming domains , or bioresponsive units . covalently 
 linked drugs can be released in a controlled manner via slow hydrolysis 
 of an acid labile linker . furthermore , 
 therapeutic peptides such as a vegf mimic or a cytotoxic peptide have been used 
 as part of the nanofiber . in these cases , other strategies rely on polymeric systems that exhibit pronounced 
 structural differences in response to thermal triggers . thermoresponsive 
 polymer systems exhibit a lower critical solution temperature ( lcst ) , 
 which defines a transition between hydrophilic and soluble polymers 
 to hydrophobic and amorphous aggregated structures in aqueous solution . this transition can be triggered by heat application at a localized 
 area , such as a tumor , to cause specific aggregation at this location . chilkoti and colleagues were the first to demonstrate the in vivo utility of this strategy with elastin - like peptide 
 ( elp ) unimers , which have an lcst between body temperature and 42 
 c , which is within the approved temperature range for clinical 
 hyperthermia . using the local heating 
 strategy , a 2-fold increase in accumulation of elp in tumors was observed 
 by the hyperthermia strategy compared to scenarios in which no heat 
 was applied . elps have also been conjugated to cell - penetrating peptides and shown to increase the intra - cellular delivery 
 of therapeutic peptides and doxorubicinin vivo . in addition to amorphous 
 aggregated structures , elps can also be formulated so that they aggregate 
 into well - ordered micelles upon heating . it has been demonstrated that the micelles of elps labeled with 
 integrin or cd13 receptor - targeted peptides ( arg - gly - asp or asn - gly - arg 
 peptides , respectively ) show a greater uptake in tumors than their 
 unimers alone , possibly due to ligand multivalency . furthermore , elp micelles loaded with doxorubicin aggregate 
 upon heating and have shown an increased 
 accumulation in tumors that were subjected to heating and cooling 
 cycles , compared to those that were either constantly heated or kept 
 at physiological temperature . moreover , elps represent a unique material 
 synthon in that they are genetically encodable , and thus their synthesis 
 can be programmed into a cell via standard genetic engineering approaches . others have demonstrated that assembled 
 materials undergo size 
 transitions at temperatures below their lcst . this was demonstrated 
 with oligoethylene glycol - based dendron assemblies that show an increase 
 in encapsulated guest release with decreasing temperature . these materials serve as stable hosts above 
 their lcst , but lose their ability to house guests at lower temperatures , 
 making possible alternative , temperature - dependent routes to drug 
 delivery . the morphology of discrete biological 
 materials often plays a significant role in facilitating molecular 
 and cellular interactions , dictating tissue function , and guiding 
 biodistribution in vivo . at the micrometer length 
 scale , genetic abnormalities producing 
 malfunctioning hemoglobin result in deformed erythrocytes , which lack 
 the flexible properties of normal red blood cells and can block blood 
 vessels and lead to infarcts . at the 
 nanoscale , bacteriophages bind a host and flex their tail fibers to 
 achieve optimal positioning for a syringe - like injection of genetic 
 material into a bacterium through a rifled helical sheath . other biological processes , such as hiv fusion , 
 are also dependent on complex nanoscale morphological changes . over the past 50 years , scientists have made 
 many advances in understanding nanomaterial assembly events by studying natural and synthetic self - assembling systems through 
 the use of both microscopy and spectroscopy . these advances have paved 
 the way for chemists to synthesize nanomaterials that have unique 
 functions dictated by morphology and composition . we have developed various approaches aimed at manipulating the morphologies 
 of synthetic nanomaterials in situ , in the hope of 
 changing material properties and hence switching functionality . our 
 ability to drive and control these manipulations hinges upon the premise 
 that the morphology of these systems is dictated by kinetic and thermodynamic 
 principles inherent to the building blocks of a material in a particular 
 environment . ultimately , dramatic changes in morphology achieved in vivo could lead to changes in pharmacokinetics , stability , 
 bioavailability , and biodistribution . for example , discher and co - workers 
 have demonstrated that flexible filaments composed of polymer micelle 
 assemblies remain in circulation in rodents up to 10 times longer 
 than spherical assemblies composed of similar chemistries . indeed , the circulation time of soft polymeric 
 fibers is highly dependent on the length of the fiber itself . a stimulus - driven 
 transition between these two morphologies could therefore lead to 
 retention of a drug - loaded nanodelivery vehicle or excretion of a 
 used carrier . the morphological manipulation of nanomaterials 
 can be achieved with a range of stimuli and with predictable consequences 
 depending on the nanomaterial composition . it should be noted that 
 we define a change in morphology as distinct from assembly or disassembly 
 of a material in that a transition from one ordered phase to another 
 must occur ( figure 6 ) . scenarios in which a 
 material retains its phase and assembles into a higher order structure 
 of the same phase or begins as an ill - defined or unstructured phase 
 prior to assembly into a much larger and more ordered material are 
 treated here as assembly events and not as morphology switches . some 
 of the earliest published examples of synthetic materials that change 
 shape in response to a stimulus involve block copolymer nanomaterials 
 comprised of polystyrene and poly(ethylene oxide ) that change from 
 rods to vesicles due to the addition of lithium chloride . shortly after , inorganic materials chemists 
 discovered that gold nanoparticles could be induced to change shape 
 from rods to spheres via a photoannealing process owing to the plasmon 
 resonance of gold nanoparticles . materials 
 decorated with thermoresponsive polymers show dramatic shape changes 
 upon heating , which is caused by a reorganization of solubilizing 
 polymer domains that results primarily from desolvation processes 
 that occur when the materials are heated above their corresponding 
 lcsts . indeed , such effects are witnessed in self - assembled materials 
 composed of dendronized oligoether moieties laterally grafted from 
 aromatic scaffolds . such materials switch 
 from two - dimensional sheets to tubular scrolls due to the dehydration 
 of the oligoether moieties upon heating above the lcst . assemblies 
 of molecules and polymers are held together by a combination of covalent 
 and non - covalent interactions . disrupting non - covalent interactions 
 via the introduction of a small molecule capable of hydrogen bonding work by zhu 
 and co - workers demonstrates that nanostructures based on polyphosphazene - derived 
 block copolymers switch from network aggregates to multilamellar spheres 
 upon the introduction of the small molecule indomethacin ( figure 6b , c ) . materials that respond 
 to various stimuli by changing intra- and 
 inter - molecular packing parameters resulting in transitions between 
 distinct morphologies or phases . ( a ) a cartoon depicting spherical 
 nanoparticles transitioning into cylindrical structures upon the introduction 
 of a stimulus . ( b ) polymers composed of poly(n - isopropylacrylamide ) 
 ( pnipaam ) and ethyltryptophan ( ettrp ) organized along a polyphosphazene 
 backbone assemble into discrete phases in solution . these phases are 
 formed by non - covalent molecular interactions and can be disrupted 
 by the introduction of other molecules , leading to changes in the 
 overall material morphology . upon the introduction of a small molecule 
 ( indomethacin ) that hydrogen - bonds with the polymer , disruption of 
 intra- and inter - polymer interactions results in reorganization of 
 the material and ultimately rearrangement into a different preferred 
 phase . ( c ) transmission electron microscopy images depicting a phase 
 transition from network - like bicontinuous rods to vesicular or multilamellar 
 large compound structures . panels b and c are adapted from zhang et 
 al . with permission from john wiley and 
 sons ltd . because polymeric nanomaterials 
 composed of amphiphilic subunits 
 assemble into a given shape based largely upon the ratio of hydrophobic 
 and hydrophilic domains in each subunit , these parameters can be manipulated in situ to achieve shape modulation . stimuli for such transformations 
 have included light , ph , temperature changes , shear , dna hybridization , and enzymatic manipulation . note , 
 however , that there are few examples of synthetic drug delivery 
 systems whose function is dictated by discrete changes in the morphology 
 of a nanoscale material . presumably , this is due to the fact that 
 tracking the change in morphology of a nanoscale material in vivo is an extremely challenging task due to the dearth 
 of imaging techniques providing adequate resolution and signal - to - noise 
 in this context . indeed , in situ imaging techniques 
 are being rapidly developed with significant progress in recent years . our laboratory and others have worked to contribute to the advancement of in situ imaging techniques using electron microscopy to monitor dynamic 
 structures at high resolution . we look forward to witnessing such 
 advances that enable the preparation and use of more and more complex 
 and dynamic nanomaterials in the years to come . nanoscale drug delivery systems that 
 function or achieve enhanced function via autonomous motion ( figure 7 ) represent an attractive approach to advanced materials 
 that are capable of controllable localization , controlled percolation , 
 or deep tissue penetration not otherwise achievable . the ability to 
 direct motion at the nanoscale remains an elusive challenge for chemists 
 and engineers due to difficulties in the synthesis and positioning 
 of functional motors at this length scale . these materials are 
 often prepared by incorporating motors that use catalysis to generate 
 chemical concentration gradients and propel themselves through solution 
 via self - electrophoresis . ( a ) cartoon depiction of the motion of a 
 nanomaterial generated via the depletion of chemical fuel placed on 
 one side of the object . ( b ) platinum  copper nanorods that catalyze 
 the reduction of iodine to iodide while oxidizing copper at the opposite 
 end of the rod . the reduction that takes place at the platinum end 
 of the nanorod generates a flow of electrons toward the platinum end 
 of the material . this electron flow generates a charge differential , 
 thus inducing fluid movement toward the platinum segment and propelling 
 the nanorod in the opposite direction of the fluid . ( c ) optical microscopy 
 snapshots tracking movement of pt  cu nanorods over time ( images 
 ( g ) and ( h ) depict instances of surface - immobilized nanorods ) . panels 
 b and c are adapted from liu et al . with 
 permission from the american chemical society . nature has evolved complex networks that utilize numerous 
 protein 
 interactions and small - molecule metabolism to drive motion via cooperative 
 changes in conformation or polymerization and depolymerization reactions . canonical motor proteins , including kinesins , myosins , and dyneins , 
 are involved in chemotaxis , trafficking , signal transduction , and 
 locomotion and take advantage of polymeric scaffolds such as actin 
 and tubulin to coordinate their movements . the keys to achieving motion in synthetic nanomaterials thus far 
 are asymmetric placement of an appropriate motor and propulsion using 
 an adequate fuel . the first reported successful attempt at synthesizing 
 a nanoscale object capable of autonomous motion involves the asymmetric 
 positioning of a platinum - based catalyst within a discrete metallic 
 nanomaterial . in this example , the motive 
 force ( on the order of piconewtons ) is generated via the establishment 
 of a chemical concentration gradient produced only at the platinum 
 end of a gold the stimulus , or fuel , is hydrogen peroxide , which 
 is rapidly converted to water and oxygen gas by the platinum surface 
 of the nanorod . other work involving tubular nanorods takes advantage 
 of bubbles formed during electrocatalytic peroxide degradation to 
 propel the nanomaterial . similar achievements 
 have been demonstrated using enzymes as catalysts and their corresponding 
 substrates as fuel . one could imagine 
 taking advantage of this type of system to direct motion of a delivery 
 vehicle through substrate concentration gradients in vivo . indeed , delivery of drug cargo to cultured cells has been achieved 
 with reasonable results . progress toward realization of these goals in an in vivo setting has been demonstrated in a few recent examples and will continue to advance as we gain an 
 understanding of how to control motion in complex environments with 
 fuels limited to endogenous biological components . a different 
 approach to generating motion of nanoscale devices 
 relies on the formation of transient hydrogen bonds propagated by 
 dna hybridization / dehybridization and subsequent polymerization reactions . cranes  have 
 been developed to move cargo taking advantage of brownian motion and 
 the free energy of hybridization / dehybridization between dna strands . the achilles heel of these systems , as functional units in biological 
 settings , will be the enzymatic degradation of the nucleic acid components 
 used to assemble such machines . to achieve the goal of harnessing 
 motion at the nanoscale for the 
 delivery of therapeutic payloads , stable materials must be synthesized 
 that take advantage of abundant biomolecules in a given environment 
 in order to fuel themselves , while simultaneously achieving active 
 steering . at the present time , external steering provided via acoustic 
 waves or magnetism seems to 
 be the simplest way to spatially direct materials . alternatively , 
 motion across very small distances could potentially be achieved using 
 machinery implanted in cellular membranes or at the surfaces of intra - cellular 
 organelles , for example . indeed , there are many issues left to be 
 resolved including the ability to incorporate and release drug payloads 
 at specific locations with temporal control in relevant biological 
 contexts . further , anchoring materials into precise locations in vivo and controlling or monitoring the function of those 
 materials in live animals has not been demonstrated , to our knowledge . there are , nevertheless , intriguing future prospects in which nanoscale 
 platforms could be used to import or transfer therapeutic agents between 
 extra - cellular and intra - cellular locations or between different tissues 
 in a living organism . communication with and control over synthetic 
 nanomotors , once placed in the bloodstream or within the peritoneal 
 cavity , for example , will be the key to developing such technologies 
 further . the holy grail in the 
 field of drug delivery is to develop drug 
 delivery vehicles that can discriminate between healthy and diseased 
 tissue , internalize directly into cells , and facilitate the efficient 
 delivery of cargo . such a  magic bullet    has been an elusive target for decades because 
 there are few truly unique biomarkers or stimuli that are present 
 only in specific cells or tissues . therefore , scientists and engineers 
 are tasked to find new ways to discriminate between cell types and , 
 furthermore , to take advantage of these key differences to actuate 
 a material for drug release . for example , rna expression profiles 
 serve as transient barcodes distinguishing cell identity and state . drug delivery devices capable of harnessing this information would 
 have significant impact on the field as a whole . there are also significant 
 challenges in the delivery of materials to relevant cellular compartments 
 with high efficiency . designing non - toxic materials capable of efficient 
 cellular entry , endosomal escape , and predictable intra - cellular trafficking 
 is a major challenge that will require the engineering of materials 
 capable of responding to various stimuli and orchestrating several 
 specific interactions in a coordinated fashion . it is likely 
 that strategies relying solely on direct cleavage 
 of a covalently linked drug carried on a nanoparticle platform will 
 suffer from a lack of specificity due to cleavage by indiscriminate 
 triggers present throughout complex biological milieu . oftentimes 
 materials that show cleavage selectively in a test tube or in cell 
 cultures are less effective in vivo due to the wide 
 range of biological components that hydrolyze or cleave chemical linkages 
 via redox or enzymatic activity . we anticipate that strategies involving 
 a more complex manipulation of nanocarrier properties , such as shape 
 or architecture , could provide opportunities for developing more selective 
 or efficient materials for drug delivery . moreover , materials that 
 assemble into nanoscale structures directly at the site of interest 
 could avoid the issue of size - dependent reticuloendothelial system 
 uptake that is typical for injected nanomaterials . encoding materials with biological information in the form 
 of peptides , proteins , and nucleic acids is a promising way to integrate 
 such materials with biological environments as noted in some example 
 strategies presented in this perspective . in our own laboratory we 
 have developed a strategy for manipulating the aggregation propensities 
 of spherical micellar nanoparticles by encoding them with peptides 
 that are substrates for proteases that are overexpressed in cancerous 
 tissue ( i.e. , mmps ) . the materials are designed to 
 circulate freely as spherical micelles until they encounter mmps in 
 tumor tissue . cleavage of the hydrophilic peptide portion of the amphiphilic 
 polymer changes the properties of each amphiphile enough to facilitate 
 aggregation of multiple peptide - based micelles to form a microscale , 
 interconnected net ( figure 8) . these aggregates 
 are simply too large to re - enter circulation and remain localized 
 in tumors for days after injection in vivo ( figure 8b ) . we envision that scaffolds of this sort could 
 be loaded with drugs , which could be triggered for release by a separate 
 stimulus or slowly by non - specific hydrolysis once accumulated . this 
 type of strategy encourages tissue - specific accumulation of material 
 and could be further coupled with stimuli - specific drug release in 
 order to achieve the high specificity provided by multiple stimuli . strategy 
 for facilitating direct accumulation of nanomaterial - derived 
 scaffolds in tumor tissue . ( a ) a general scheme depicting the design 
 of the system consisting of a spherical particle composed of polymeric 
 peptide - amphiphiles , where the hydrophilic portion is a substrate 
 for tumor - associated mmps . as nanoscale spherical assemblies , the 
 nanoparticles are free to circulate into and out of tissue until they 
 encounter locations with a large abundance of mmps , indicative of 
 cancer or inflammation . upon cleavage of the peptide , fragmented amphiphiles 
 reassemble into a large , interconnected scaffold that that is too 
 large to re - enter circulation . ( b ) fluorescence data of tumor - burdened 
 mice 1 h or 3 days post intra - tumoral injection of the mmp - responsive 
 nanoparticles labeled with alexafluor-647 . enzyme - responsive particles 
 ( prepared from l - amino acids ) are assembled as designed and 
 are retained in the tumor tissue for at least a week , at which point 
 the animal is sacrificed . non - responsive particles ( comprised of d - amino acids ) are rapidly cleared from the tumor tissue . we 
 envision that responsive systems such as these could be used to trigger 
 accumulation of drug - loaded particles that could be further activated 
 to release drugs in a tissue - specific fashion or passively via long - term 
 degradation . another key concern is the toxicity of the nanomaterial 
 or what 
 remains of the material after it has responded to a stimulus . for 
 example , doxil , a pegylated lyposomal formulation of the anti - cancer 
 drug doxorubicin , frequently causes palmar - plantar erythrodysesthesia , 
 also known as hand - foot syndrome . the 
 successful implementation of any drug delivery agent will clearly 
 require that such concerns be met . caution and foresight must be used 
 whenever preparing materials with toxic metals or chemical scaffolds 
 that are difficult to degrade in biological milieu . finally , 
 we must note that material properties that can potentially 
 be tuned in response to stimuli are not limited to those discussed 
 in this perspective . for example , it can be envisioned that tuning 
 the elastic modulus or the electromagnetic properties of 
 a nanomaterial in response to a given stimulus may be of significant 
 value in terms of developing advanced materials for biomedical applications 
 in the future .
nature 
 employs a variety of tactics to precisely time and execute 
 the processes and mechanics of life , relying on sequential sense and 
 response cascades to transduce signaling events over multiple length 
 and time scales . 
 many of these tactics , such as the activation of 
 a zymogen , involve the direct manipulation of a material by a stimulus . 
 similarly , effective therapeutics and diagnostics require the selective 
 and efficient homing of material to specific tissues and biomolecular 
 targets with appropriate temporal resolution . 
 these systems must also 
 avoid undesirable or toxic side effects and evade unwanted removal 
 by endogenous clearing mechanisms . 
 nanoscale delivery vehicles have 
 been developed to package materials with the hope of delivering them 
 to select locations with rates of accumulation and clearance governed 
 by an interplay between the carrier and its cargo . 
 many modern approaches 
 to drug delivery have taken inspiration from natural activatable materials 
 like zymogens , membrane proteins , and metabolites , whereby stimuli 
 initiate transformations that are required for cargo release , prodrug 
 activation , or selective transport . 
 this perspective describes key 
 advances in the field of stimuli - responsive nanomaterials while highlighting 
 some of the many challenges faced and opportunities for development . 
 major hurdles include the increasing need for powerful new tools and 
 strategies for characterizing the dynamics , morphology , and behavior 
 of advanced delivery systems in situ and the perennial 
 problem of identifying truly specific and useful physical or molecular 
 biomarkers that allow a material to autonomously distinguish diseased 
 from normal tissue .