Review—Reorientation of Polymers in an Applied Electric Field for Electrochemical Sensors

This mini review investigates the relationship and interactions of polymers under an applied electric ﬁ eld (AEF) for sensor applications. Understanding how and why polymers are reoriented and manipulated under an AEF is essential for future growth in polymer-based electrochemical sensors. Examples of polymers that can be manipulated in an AEF for sensor applications are provided. Current methods of monitoring polymer reorientation will be described, but new techniques are needed to characterize polymer response to various AEF stimuli. The unique and reproducible stimuli response of polymers elicited by an AEF has signi ﬁ cant potential for growth in the sensing community. cally, electronic polarization, atomic polarization, dipole relaxation, ionic relaxation, and dielectric relaxation.


Introduction and Backgound
Our goal, within this review, is to demonstrate what polymers are most influenced by an AEF. Local polarization and local effects from AEF are worth considering, as local effects affect measurements in sensor systems. The localized mechanical torque from an AEF is complex, lending to advance textbooks and in-depth review articles. [1][2][3][4][5][6][7][8][9] This is briefly summarized here, when discussing what happens with local charging effects or electric displacement. The capacitive behavior of the polymer becomes dependent on the geometric and spatial positioning of the polymers. [10][11][12][13] Based on our review, there can be inconsistencies in reported measurements of polymers used in the presence of an AEF. In the case of electrochemical sensors, where a localized AEF is present, inconsistencies are a result of limited accounting of the localized energy induced by localized an AEF. This is especially important for researchers using polymers in electrochemical sensors.
Definitions of an applied electric field.-An applied electric field (AEF) is defined as a difference in voltage between two points over a distance; the units of an AEF are [V/cm]. An AEF (E  ) is a directional vector from positive to negative sources, [Eq. 1]; Fig. 1a displays a voltage induced AEF, where Φ is the electrical potential (also often called voltage) [Eq. 2], q is a point charge, k is a Coulomb's constant defined in [Eq. 3], and Δr is the relative distance between charges or reference point. At a point in space, there is an exhibited force, F, on the point-charge q, defined as [Eq. 4].
If the electric field is uniform in one direction across the entire field, a simpler form of an AEF, [Eq. 5], can be thought of as a force per unit charge across a certain distance (d).
Influences of the electric field on local charges.-By putting a polymer anywhere in an AEF, it would have an electrostatic potential energy, (U E ), [Eq. 6] and [Eq. 7] is energy stored in a system of point charges based on Coulomb's constant (k i ) with units of Joules [V·A·s]. The electrostatic potential energy is defined for any position r in the electric field, defined as multiple potentials for a point charge q. [ ] e e e = where a dielectric medium constant, ε i [Eq. 8], is equivalent to a multiplication factor, ε r , of the permittivity of free space constant ε 0 (8.85 × 10 −12 F m −1 ). In the case of water, the multiplication factor is 80 [Eq. 9] and a simple dodecane polymer the multiplication factor is 2 [Eq. 10]. 9 80 9 These equations demonstrate the ability to gauge the extent the AEF is affecting the solution vs the point charges or polymer. For example, based on this estimate and simplification, the water is 40× more polarizable than the dodecane. 9 The polymer can further influence the overall electric potential, which can be estimated using the divergence theorem of an isolated closed system, the volumetric net free charge density (ρ E ) can be estimated as [Eq. 11].
Where D  is the electrical displacement vector associated with the specific material. Therefore, the relationship between this electrical displacement and the electric field is [Eq. 12].
Electrostatic potential energy exists for any particle, and we can predict and observe how that electrostatic energy can be influenced in an applied electric field. An AEF has the ability to manipulate the electrostatic charging of a polymer particle, and therefore, manipulate the orientation. For example, a polymer with an electroactive monomer can oxidize when exposed to an AEF, which will change the polymer to be slightly positive. The positively charged particle will cause the entire molecule to traverse or align with the direction of the electric field as seen in Fig. 1b. 14 Literature describes that AEFs are able to control domain orientation on both straight electric field lines (as illustrated in Fig. 1b), and curved lines as well. This demonstrates that polymer orientation in an AEF is anisotropic. 15,16 Within an AEF, an untethered polarized particle will (a) spin or turn around in solution (3D) or (b) align with their respective polarity on a dry surface (2D). The orientation of polar polymer particles can be controlled by the AEF vector direction. 17 The polymer movement and orientation can also include stretching or tension forces upon the polymer, causing potential conformational changes. 18,19 Influences of electric charge on polymers.-Orientation, and the response of polymers, to an AEF has been investigated extensively; changes in voltage and direction of the AEF produces distinctive polymer responses and orientation. [20][21][22][23][24] For example, increasing voltage has been found to change the extent of polymer orientation because microdomains cause the macropolymer to traverse parallel to the electric field. 25 The ability of the polymer to respond to an AEF is dependent on its innate dielectric and electrochemical properties. 26,27 The free energy and dielectric properties of the system influence the stored energy and capabilities of polymers to initiate movement and orientation. 28,29 When a dielectric polymer is in a system and exposed to an AEF, the difference in localized polarity will induce surface charges on the polymer. 30,31 An equation linking the dielectric constant, ε, to the AEF is the capacitance, C, [Eq. 13]; A is the surface area of the electrode, and the thickness d of the dielectric layer.
Even insulating polymers can also be polarizable in an AEF, and therefore, manipulated in the presence of current and voltage. 15 To understand the dielectric constant further, and begin to hypothesize how different dielectric component relationships may change in relation to each other, the Clausius-Mossotti relation is given in [Eq. 14] and [Eq. 15]; η i is the density in number per cube meter, and α i is the molecular polarizability [C·m 2 /V].
An AEF can create a controlled, specific, and reproducible response in polymers that can cause the polymer to reorient; the reorientation is defined and dependent on the functional groups of the polymer and the strength of the AEF. The localized charging around the polymer is strongly dependent on the present media or solvent. An AEF produces two phenomena: (1) local electric field and (2) polarization of the localize, surrounding media. 35,36 For example, in water, the hydrogens and oxygens have a positive and negative dipole effect respectively that cause a localized rotation of water. The total response of the AEF contains a response of the solvent along with polymer point charges. 37,38 Additionally, the reorientation or displacement of solvent can further influence localized charging of the polymer. 38 A polymer tethered on a surface, as in those that are found in self-assembled monolayers, could also cause a conformational change by moving the charged end away from or towards the surface, Fig. 1c. Further, electrode geometry and set up determines polymer orientation, and the orientation can be customized by changing the electrode, type of polymer, overall charging, and other features.
The conformational change can be thought of as a "smart response" induced an AEF instigated by the electric potential of the surface. 39 The ability to manipulate polymers to this degree leads to applications of self-assembled nanostructures in a predictable and reproducible fashion. 40,41 Charged end-groups, as depicted in Fig. 1b, isn't necessary or sufficient to reorient the polymer. For example, polystyrene-block-poly(methylmethacrylate) copolymer, or p(S-b-MMA), has been demonstrated to move and reorient in an AEF due to the dielectric microdomain differences in an AEF which results in a dipole moment (p  ). 15 Polystyrene block copolymers have been used in sensor applications, including a polystyreneß-poly-2-vinylpyridine (PS-ß-P2VP) di-block copolymer as the material of a photonic sensor. As part of the fundamental properties of multiple mer groups, polymers, PS-ß-P2VP was tunable in an AEF, allowing for optimizable sensing. 42 There are limitations to reorientation through an AEF. A conductive polymer in an AEF will electrically dope. The electrical doping can cause the polymer to traverse, if free in solution, based on the charging and polarity of the AEF. Conductive polymers tethered to a surface can cause swelling or deswelling based on the influence for the polymer to traverse towards or from the surface. Semi-conductive polymers are more complex, as the mechanism depends on the homogeneity of the polymer. If homogeneous, the polymer will respond similar to conductive polymers after the activation threshold is achieved. Homogeneous insulating polymers should similarly be relatively unresponsive in an AEF. Heterogenous polymers with a dipole moment, or two dielectric segments, will cause compressive or tension forces [Eq. 21] based on the dipole moment that is parallel to the AEF normal vector. A torque rotation [Eq. 22] will be caused based on the dipole moment that is not parallel to the AEF normal vector. 9,10 Because this force depends on the dipole moment vector, longer chains typically are unresponsive in an applied electric field, as the dipole moment vector isn't as strong. Therefore, this phenomenon is typically reserved for polymers with a relatively shorter chain length. Further, the observed rotation can be controllable and predictable.
Context dependent surface reorientation.-The orientation of block copolymers is context dependent, or in other words, the same force on the same polymer will cause a different response dependent on the initial conditions of the polymer. 43,44 Interfacial interactions also play an important role in determining orientation behavior of polymers. For di-block copolymers, complete lamellar or parallel microdomain orientation is only possible when all interfacial interactions are balanced. 45 Orienting the microdomains of polymer films depends on factors such as interfacial interactions, differences in dielectric constants of each block, and the intensity of the AEF. Differences in strength of interfacial interactions leads to parallel orientation of these microdomains. 15 In many cases, lamellar polymers are pre-oriented, and actually align themselves in-line with, or parallel to, the electric field lines, as shown in Fig. 1b; however, this is also dependent on the anisotropic polarizability of the monomer. [46][47][48] A larger AEF can result in normal, or perpendicular, orientation. 45,49 Further, the interaction of block-copolymer nanostructures and AEFs can result in spacing of molecules. The direct effects of anisotropic (i.e. aligned in parallel to) AEFs can induce highly accurate and reversible nanostructure tuning of polymer patterns. [50][51][52] Complex nanostructure geometries and patterns can be generated by controlling the AEF direction and strength. Electrochemically induced or controlled block copolymer lithographic techniques can be used to make complex geometric shapes to fabricate novel sensor constructs. [53][54][55] Further, the fabrication technique can also influence sensing capabilities. 56,57 For example, fabricating more organized and reproducible nanowire conductive polymer gas sensors could have significant impact on the ability to selectively and sensitively measure gasses in chemiresistor motifs. [58][59][60] An activation threshold, a minimum AEF magnitude necessary to control the orientation, must be overcome before movement in the polymer is observed. 61,62 The activation rate only takes several milliseconds, but tuning the activation rates can result in greater controlled mechanism of the polymer. 63,64 The activation threshold of a polymer can be approximated by factoring the polymer concentration, polymer composition, inter-polymer interactions, and polymersolvent interactions. [65][66][67][68][69] Studying the activation thresholds and dielectric behaviors of different responsive polymers will lead to better sensor design with this phenomenon in mind. If the activation threshold can be quantified for individual polymer sensor systems, optimization of polymer behavior can be achieved. Full characterization of polymers used in electrochemical sensors allows for more precise control of a polymer-based sensor, and increased standardization of sensors utilizing AEF responsive polymers.
Electroporation.-Electric fields may also be useful in the study and control of electroporation, which is the application of a short pulse of optimal voltage to create temporary pores in a lipid membrane. An example of electroporation is the induction of the delivery of materials into a membrane. Voltage used in electroporation disrupts the lipid bilayer, changing the rate of transport across the membrane. Electroporation has been known to disrupt the permeability of biological membranes by changing localized transport of biomolecules such as cells and small molecules. [70][71][72][73] Electroporation has applications in the fields of genetics, biochemistry, and biosensors. Within the field of sensors, electroporation can be used as a tool to detect and control the transfection of DNA. 70,72 Electroporation of membranes has also been used to investigate calcium and cytokine signaling of cells. 74,75 Electroporation continues to be a growing field with new patents and techniques generated over the past several years especially in areas of sensing with active drug delivery feedback mechanisms in bioprocessing applications. [76][77][78] Monitoring surface morphologies.-Appropriate surface characterization, under an AEF, is needed to monitor and test the orientation of polymer systems in order to fully understand the AEF influence on the system. Currently, the most popular methods to monitor polymer surfaces include dielectric spectroscopy, laser interferometric, smallangle X-ray scattering (SAXS), and microscopy analyses. The difficulty of measuring a surface within an AEF is that the applied potential typically induces a piezoelectric vibrational response. 79,80 The vibrational response is difficult to compensate for, but several researchers have made progress with laser interferometric, Raman spectroscopy, and X-ray diffraction analyses. 81 Dielectric spectroscopy, which is a subcategory of impedance spectroscopy, is an investigative technology to study the electric dipole moment response (p  ) to an applied electric field (E  ). While alternative methodology exists, it is possible to obtain the necessary data as a subset (or more specific operation) of electrochemical impedance spectroscopy (EIS). EIS gives a better understanding of the ionic transport models through dielectric medium. 82 Further, EIS has the capability of determining the type of polymer polarization, specifically, electronic polarization, atomic polarization, dipole relaxation, ionic relaxation, and dielectric relaxation. [83][84][85] The complexed dielectric permittivity constant, , ̲ e can also be determined with EIS. 86 The responses we are most interested in with this review are dipole relaxation (dipoles realigning associated with the AEF) and dielectric relaxation (combination of dipole relaxation with ionic relaxation). 85 An alternative methodology is using a dielectricrheological device that can simultaneously measure dielectric and mechanical functions of polymers. The results of this technique can provide a better understanding of the mechanical strain and torque on the polymer in solution. [87][88][89][90][91] Laser interferometric involves measuring extremely small displacements in refractive index and surface irregularities to create a nanoscale representation. 29,92 Laser interferometric has also been used to measure the thickness response of a surface, which can provide details of polymer collapse or adsorption. 93,94 Double-beam laser interferometric was used to study AEF induced displacements in ferroelectric piezoelectric films. 95 Polarized Raman spectroscopy has been used to monitor carbon nanotube alignment in situ in a polymer matrix, while under an applied electric field. Polarized Raman spectroscopy is an ideal noninvasive and non-destructive technique to look at real-time orientation of carbon nanotubes by taking advantage of advantage of the anisotropy of scattering of the CNTs. 96 Polarized Raman spectroscopy is Most importantly, this technology allows for a real time assessment of a polymer integrated substance monitoring orientation, under an applied electric field. 97,98 Scanning Electron Microscopy with X-ray Diffraction (SEM/ XRD) is a technique that uses an electron beam to provide a detailed, nanoscale image of the object in question. SEM/XRD has been used by polarizing the sample plate to observe and monitor the alignment on an electrospun surface. [99][100][101][102] Using laser interferometrics and SEM/XRD, polymers can be monitored in two-dimensional space within an AEF. But most sensor polymers are tethered to a surface and operate in an aqueous environment within a micro-3D space. Therefore, imaging tools, such as atomic force microscope (AFM) or Small-angle X-ray scattering (SAXS), integrated with an AEF, would provide a more comprehensive ability to observe reorientation in this space in realtime. New technology would be necessary to ensure the successful orientation of electroactive polymers in aqueous media for threedimensional sensing, along with the effectiveness of each orientation technique. By understanding the more efficient methods of polymer orientation, we can further a deeper understanding of these mechanisms. These new monitoring tools will aid in the development of new electrochemical responsive polymers for sensor applications.
Bio-and chemical-sensor applications.-Biosensors are important to understand biological entities, living organisms, enzymes, proteins, and other analytes by detecting the presence or absence of biochemicals. Biosensors have a wide range of applications, including diagnostic, biochemical, environmental, and energy. 103-106 A specific sensor is typically two primary components, a transducer and a recognition element. 107 Electrochemical transducers are becoming increasingly more common to provide a fast and readable output for biomedical and environmental point-of-need sensing. [108][109][110][111][112] Synthetic and natural polymers are becoming increasingly common recognition elements-either comprising of the entire recognition element or just a portion. 107,111 Polymer orientation can be affected by factors such as temperature changes, exposure to light, changes in pH, tensile stress, and an applied electric field (AEF). 21,22,39,113,114 Polymers can be unintentionally manipulated by localized electric fields associated with electrochemical transduction.
Electric fields applied to polymer-coated sensors has the potential to reproducibly stabilize surfaces or control functional behavior. Both polymer brushes and films have been widely used in sensor applications, and the ability to control the orientation of these surfaces could increase stability and reproducibility in sensor-based technologies. 39,[115][116][117] Understanding the effect of AEF on polymers could also control and standardize seemingly "random" behavior of these surfaces. 118,119 Monitoring microscopic surfaces in situ, while possible, is not yet standardized, and therefore, real-time monitoring of polymers under an AEF is difficult to replicate and reproduce. 97

Electro-Responsive Polymers
Block copolymers (BCPs).-Block copolymers (BCPs) have interesting properties when exposed to an AEF; BCPs are linear polymers that have two or more block-like arrangements of monomers with different dielectric properties. BCPs do not have to be polar or electrochemically active to respond to an AEF. 120 The driving force behind BCPs thin film alignment within an AEF is the anisotropic free energy of the BCPs. 121 An AEF is known to have an anisotropic destabilizing effect on polymer microstructures. 46 Specific, BCP orientation can be altered depending on the electrode geometry, allowing for optimizable complex polymer responses. 101,122 Also, the effect of an AEF on the polymer is only observed in polymers smaller than tens of micrometers in length. 29 Orientation of the polymer is necessary for a reproducible sensor response; use of an AEF to orient the BCPs on the surface to monitor chemi-adsorbed molecules could aid in increased sensitivity and reproducibility. [123][124][125][126][127] AEF can create nano-and micro-domain structural changes that can be used for the formation of pores for sensing. [128][129][130] Non-electroactive polymers responding to electric fields imply that non-uniform polymers might have similar behavior in the right circumstances. 131 As previously described, some researchers use an AEF for a stimulus behavior or orientation of polymers for increased response and controlled activity. For example, block copolymers with Poly(pphenylenevinylene) (PPV) oligomers were used as the active material of a gas sensor electronic nose to differentiate between vapors. 132 The sensor provided fast and reproducible result due to dynamic response of polymer chain confirmation changes induced by an AEF. 133 Polymers, as an active sensing material, not only worked, but was advantageous to previous sensors because of their low power consumption, room temperature operation, fast response, and high selectivity. 132,134 However, this practice is not yet widely adapted within the electrochemical sensor community, and therefore, some investigators are unclear about the effect of AEF on polymers in sensors.
Ferroelectric polymers.-Ferroelectric polymers maintain a permanent electric polarization which can be manipulated with an AEF and are Often used with electromechanical or acoustic transducers because of their inherent piezoelectric behavior. 135 While not necessarily reoriented within an AEF, the ferroelectric polymers can change properties in an electric field via ferroelectric, piezoelectric, or pyroelectric behaviors. 136,137 Piezoelectric indicates the ability to change electric charge based on mechanical stress. Pyroelectric indicates the ability to change electric polarization induced heating by temperature changes. Ferroelectric indicates the ability to spontaneously and reversibly polarize the polymer based on external magnetic of electric fields. The net polarization can be modified through a mechanic stress, temperature, or field magnitude respectively, generating higher permittivity of the polymer. 100 Ferroelectric polymers are typically crystalline in structure, and only the crystalline regions of the polymer are involved in the ferroelectric switching. 138 These concepts are typically reversible, and therefore, lends to greater control of the sensor surface.
As an example, a common ferroelectric polymer is a poly (vinylidene fluoridetrifluoroethylene copolymer), or p(VDF-co-TrFE). p(VDF-co-TrFE), is a semi-crystalline ferroelectric copolymer commonly used in organic electronics. p(VDF-co-TrFE) has also been used in sensing applications, being used for its piezoelectric properties to measure cardiac functions in detecting cardiac diseases. [139][140][141] Dipole inversion of the p(VDF-co-TrFE) causes a decrease or extinction of its ferroelectric properties. 140,142 The ferroelectric properties of p(VDF-co-TrFE) depend highly on the thermal processing conditions in which it was created. [143][144][145] The applied external electric field induces irreversibly the orientation of the polar dipole axis along the direction of the electric field. The orientation of the P(VDF-co-TrFE) lamellae on the surface can also direct and influence the polarization abilities of the polymer film. 146,147 If the lamellae are on the face of the film, higher electric field is required to induce polarization. Polymer lamellae on the edge of the film require less voltage in an applied electric field to induce polarization.
Polymers with magnetic field moieties.-Magnetic fields may also be used in the alignment of polymers. For example, when the magnetic field is stronger than the anisotropic magnetostatic energy density of a BCP, the individual BCP grains to rotates and aligns with the magnetic field. 148,149 This creates organized and oriented microdomains, orienting the overall BCP for applications in lithography, soft robotics, and sensors. Biphenyl moieties, attached to the diblock copolymer poly(ethylene oxide-b-methacrylate/LC), or PEO-b-PMA/LC¸causes a positive magnetic susceptibility anisotropy and orientation parallel to the magnetic field. 149 Other polymer paradigms also work in context of magnetoelectric polymers. Hyperbranched polymers (HBP) has been found to be a functional material that can be adapted for optical, electronic, and magnetic applications. 150 Used magnetic fields have been used to manipulate the surface, HBPs have been used in chemisensors for applications in explosive sensors. 151 Another common paradigm are metal or carbon backbones/substrates in which the polymers are grown on-top of. 96,148,[152][153][154][155] These flexible films have interesting polymers with incredible spatial resolution. While not the focus of this paper, these paradigms have had significant impact in optical applications.
Electroactive and conductive polymers.-Electrochemically active polymers (EAPs) have been widely used as actuators in chemical and mechanical stimuli sensing as they can respond innately to an AEF. [156][157][158] While not typically thought to respond in an AEF, EAPs have the ability to swell/deswell due to doping effects, and be manipulated in an AEF. In the field of soft actuators, electroactive polymers are a popular material, as they can exhibit different stresses and strains based on the electric pulses. EAPs can be used in artificial muscle, as they can be electrically stimulated by an AEF to elicit a desired shape change. 159,160 An example of an ionic EAP is a conducting polymer that have been used in many designs and methods for sensors. 160 The ability to be enzymatically modify conductive polymers provides a direct use and advantage in electrochemical biosensing applications. Further, in the recognition paradigm, an AEF can affect binding affinity of charged molecules to a polymer. 161 Electroactive polymers have also been directly used as pads on robotic arms and grippers, in order to actuate EAP linkages to get tactile feedback. 159 The resonant frequency of EAPs can actually be tuned for polymer membranes by applying an AEF. 162 Electroactive polymers can be made into wearable or 3D structures for biosensing applications. [163][164][165][166] While commonly used for various applications, greater response characteristics can be explored by fundamental studies on the electromechanical response under an AEF during electrochemical sensing applications.
Biomolecules can be immobilized onto conducting polymers without any loss of activity, further changing the polymers biocompatibility. Further, it is possible to immobilize biomolecules within the polymer framework, leading to the release or capture of the biomolecule based on a stimulus response, leading to a rapid, selective, and sensitive transfer of materials. 167,168 This is an essential tool for in-vivo biosensing applications, and for the continuous monitoring of drugs, metabolites, and analytes in biological fluid. 167 Stimuli-responsive polymers and self assembled monolayers.-Another species of polymer with a broad future potential for sensing are stimuli responsive polymers, or smart polymers. Stimuli-responsive polymers are polymers that produce a discrete "on-off" response to a small external change. 169 These small external changes are almost negligible when considered for one mer of the polymer, but cumulatively, the polymer response has an almost additive property, and generates a significant response. The ability to generate a scalable onoff response is promising for fields of drug delivery, biomimetic actuators, separation, and sensors. 170 Stimuli-responsive polymers have also been widely used in sensors. Three examples of stimuli-response of smart polymers can be identified as (1) linear free chains in solution collapsing upon external stimuli, (2) cross-linked gel swelling triggered by the environment, and (3) chain absorbed or grafted to a surface where the polymer swells or collapses reversibly on the surface. 170 All three examples can be conjugated with biomolecules and used in electrochemical sensing examples. Stimuli-responsive polymers can also be deposited onto specific electrode areas, allowing for increased tunability and optimization of modified biosensors. 171 Some stimuli-responsive polymers, especially those that have hydrophobic groups, are influenced by a wetting/dewetting folding process. 16,172 Within an AEF, localized wetting/dewetting of the polymer can get influenced and disrupted. 173,174 A change in wetting behavior is likely to shift the stimulus response of the polymer. For example, in a recent protocol paper, pNIPAM has been shown to change the polymer's transition temperature in an applied electric field of 1 V cm −1 . 131 Even though pNIPAM is thought to be a temperature-induced polymer, the thermodynamic transition is ultimately driven by wetting/dewetting of localized electrostatic and hydrophobic groups within the polymer. 16,[174][175][176] Therefore, stimuli-responsive polymers driven by influences of water behavior can be further influenced by an applied electric field. Within a sensor system, a change in stimulus response will influence the detection capabilities, which can be demonstrated by electrically or magnetically induced stimuli-responsive polymers in self-assembled monolayers.
Self-assembled monolayers (SAM), or polymer brushes, are polymers tethered on one end to a surface to create a brush-like effect. 177 Polymer brushes can be used to target specific desired properties such as wettability, friction, conductivity, adhesion, colloidal stability, biocompatibility, or absorptivity of a surface. 16,178 By using well-understood and easily controlled polymers, we can apply their properties to characterize a response to an unknown variable. 161 Self-assembled monolayers are often used to detect anything form metal ions, biomolecules, and microorganisms. One use of SAMs is for electrochemical switching, which enables real time monitoring, or a controllable release method for certain polymers. 179,180 Electrochemical switching of surface tethered polymers can be used to deter the formations of biofilms, a process that causes human infections, pipe clogging, heat transfer reductions, and ship's hull fouling; electrically switching changes the physiochemical properties of a surface from an attractive, negatively charged surfaces to a repellent hydrophobic surface. [181][182][183] Switching can also be context dependent. In the case of a proteins, pH would change the charged state of the C-terminus. pH dependency of a polymer switch operated with an AEF has been demonstrated which leads to real-time sensing of a bacteria biofilm. 39 pH context dependent stimulus response would extend to other materials, such as PMMA. 97,179

Conclusions
The uses of electrochemically controlled polymers have endless applications in the fields of medicine, environmental science, and sensors. From reproducible, non-invasive, wearable, reversible, and point-of-care systems and technologies, the reliability of methods like the ones presented could be extremely high. Though theoretical applications are numerous, not all applications have been thoroughly investigated and developed. While many applications are at the end stages, and being integrated in marketable devices, other applications are still in the basic premise and research stages.
One identified improvement to propel our ability to use electrochemically responsive polymers is to observe the response in realtime, at the time of activation, to holistically analyze the mechanisms and mechanics. Further, because of the activation energy, and differences in localized dielectric potentials, each polymer should be independently tested instead of assuming a specific result. Also, researchers commonly use polymers in the presence of an AEF and do not test or comment on a potential response. We encourage the observation of all polymer surfaces in an AEF, especially when difficult to reproduce results are obtained, to improve the characterization of these systems and future studies. Thorough characterization allows for novel and unique AEF responsive sensors to emerge. Future development of AEF responsive polymers used in sensors leads to increased specificity, sensitivity, and dynamic ranges of sensors in various point-of-care devices.