Tunable drug loading and release from polypeptide multilayer nanofilms.

Polypeptide multilayer nanofilms were prepared using electrostatic layer-by-layer self-assembly nanotechnology. Small charged drug molecules (eg, cefazolin, gentamicin, and methylene blue) were loaded in polypeptide multilayer nanofilms. Their loading and release were found to be pH-dependent and could also be controlled by changing the number of film layers and drug incubation time, and applying heat-treatment after film formation. Antibioticloaded polypeptide multilayer nanofilms showed controllable antibacterial properties against Staphylococcus aureus. The developed biodegradable polypeptide multilayer nanofilms are capable of loading both positively- and negatively-charged drug molecules and promise to serve as drug delivery systems on biomedical devices for preventing biomedical device-associated infection, which is a significant clinical complication for both civilian and military patients.


Introduction
Biomedical devices are indispensable in the care of patients. However, biomedical device-associated infection is a signifi cant clinical complication. In orthopedics, infection prevention is one of the major goals of injury management. Infection rates are 7%-9% for elbow replacements and 1%-2% for hip replacements, 1 and patients with open fractures have a high risk of infection due to bacterial contamination and soft tissue damage. The incidence of Gustilo grade III open fractures may exceed 30% 2,3 and 2%-15% (or higher) of combat-related extremity injuries with developed osteomyelitis. 4 To reduce the risk of biomedical device-associated infection, attention has turned recently to developing drug-containing fi lms on biomedical devices, 5-10 as such fi lms can enhance the device's specifi c functions including fi ghting infection and promoting wound-healing. 5,7,10 For instance, dip coating, 8 spin coating, 9 spray coating, 6 and covalent conjugation of antibiotics 10 have been developed to prepare antimicrobial fi lms on biomedical devices. Much effort has been devoted to controlling drug release via manipulating the dissolution or degradation of the fi lms.
More recently, electrostatic layer-by-layer self-assembly nanotechnology has been developed 11 and used to construct polyelectrolyte micro-or nanocapsules 12,13 and multilayer fi lms 14,15 for drug delivery. Certain drug molecules, such as active proteins, enzymes, nucleic acids, and DNA, have been immobilized into polyelectrolyte multilayer fi lms. The advantages of polyelectrolyte multilayer fi lms as drug delivery systems include: (i) drug molecules can act as either functional drugs or components of the fi lm, and they can also form a stable crosslinking structure with other fi lm component(s) via multivalent interactions (eg, electrostatic or hydrogenbonding interactions), (ii) sustained drug release is possible through controlling the fi lm properties, 16 (iii) polyelectrolyte multilayer fi lms have the potential to protect drug molecules from losing their biological functions, [17][18][19][20][21] and (iv) the fi lm preparation process is simple and can be automated. 11,14-17, The drug release behavior of polyelectrolyte multilayer films depends on the permeability, the disassembly or erosion of the multilayer structure, and other experimental variables. A variety of polyelectrolyte multilayer films has been studied to control drug release via ionic strength, temperature, pH, enzyme, and hydrolytical degradation, 12,13 Hayne and colleagues have bonded a thiol-bearing molecule, 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), into multilayers and 2-nitro-5-thiobenzoate dianions were released from the fi lms by the breakdown of disulfi de bonds between the DTNB and one of the fi lm components. 22 Rubner and colleagues have loaded Ketoprofen or cytochalasin D into polyelectrolyte multilayers and have shown unique zero-order drug release over a period of a few days. 23,24 Caruso and Quinn have developed thermo-responsive multilayers containing poly-(N-isopropylacrylamide-co-acrylic acid) and have achieved sustained drug release. 16 In addition, hydrogels and micelles have been introduced into polyelectrolyte multilayers as "drug containers" to manipulate drug-loading capacity. [25][26][27] However, only a few studies are reported on controlling the loading and release of small drug molecules. 22,[28][29][30] It is still challenging to achieve a controllable release of small charged drug molecules, probably due to the weak interactions between small drug molecules and the fi lm components. All of these have limited the applications of polyelectrolyte multilayer fi lms for controlled drug release, especially on biomedical devices. Introducing "binding-sites" with tunable properties within nanofi lms could be very useful in achieving controllable drug loading and release in polyelectrolyte multilayer fi lms on biomedical devices.
In this work, we prepared polypeptide multilayer nanofi lms using weak polyelectrolytes of poly-L-lysine (PLL) and poly-L-glutamic acid (PLGA), and we studied the loading and release behavior of small charged drug molecules. One advantage of such biodegradable drug release systems is that drug "binding-sites" within the multilayer nanofi lms can be created and tuned simply by immersing the multilayer nanofi lms fabricated at one pH into an aqueous solution of a different pH. For instance, PLL/PLGA multilayer nanofi lms were prepared at pH 4.0, and positivelycharged drugs including gentamicin and methylene blue (MB) were loaded into the multilayer nanofi lms by immersing the nanofi lms in a drug-containing solution of a higher pH (eg, pH 7.0). Negatively-charged drugs such as cefazolin were incorporated into nanofi lms formed at pH 10.0 by incubating the nanofi lms in a cefazolin-containing solution of a lower pH (eg, pH 7.0). We showed that the loading and release of small charged (positively and negatively) drug molecules could be tuned by changing the number of fi lm layers, the pH of the application environment or the pH of drug solutions, and applying post-preparation heat-treatment of the nanofi lms. The driving force of drug loading and release from the multilayer nanofi lms is mainly electrostatic interaction, attraction or repulsion, between the small charged drug molecules and the charged side-chains (binding-sites) of PLL or PLGA. Moreover, we found that polypeptide multilayer nanofi lms loaded with antibiotics presented antibacterial properties against Staphylococcus aureus (S. aureus). Therefore, the developed approach is promising for controlling the loading and release of small charged drug molecules and achieving drug release systems for preventing biomedical device associated infection.

Materials and methods Materials
Poly-L-lysine (M n = 150 kDa), PLGA (M n = 50 kDa), MB, gentamicin, and cefazolin were used (Sigma Aldrich, St. Louis, MO). The structures of these polymers and drugs are shown in Figure 1, where cefazolin, a widely used antibiotic, is a negatively-charged small drug molecule, and gentamicin, another common antibiotic, and MB, a dye indicator, are positively-charged small molecules. Quartz slides were purchased from SPI Supplies, Inc. (West Chester, PA), cut into 25 mm × 10 mm × 1 mm, and cleaned by incubating in a piranha solution (4:1 H 2 SO 4 /H 2 O 2 ) for 2 h at 80 °C followed by rinsing with deionized water. Stainless steel sheets were purchased (Small Parts, Inc., Miramar, FL) and cut into discs (10 mm × 0.25 mm), which were ultrasonicated in a 2% sodium dodecyl sulphate (SDS) solution for 30 min, washed in deionized water, and rinsed with an ethanol-NaOH solution and deionized water.
Buffer solutions in the pH range 4.0-10.0 were used throughout this study. Buffer solutions of pH 7.0-10.0 were prepared using 50 mM glycine-NaOH and buffer solutions of pH 4.0-7.0 were prepared using 10 mM Tris-HCl, 10 mM NaAc, and 130 mM NaCl. Gentamicin (10 mg/mL) and MB (3 mg/mL) solutions were prepared in Tris-HCl buffer solutions of pHs 4.0, 5.0, and 7.0. Cefazolin was dissolved in the glycine-NaOH buffer solutions of pHs 7.0, 8.0, and 9.0 at a concentration of 10 mg/mL. PLL (1 mg/mL) and PLGA (1 mg/mL) solutions were prepared by dissolving PLL and PLGA in the buffer solutions of pHs 4.0, 5.0, 7.0, 9.0, and 10.0.
Tunable drug loading and release from polypeptide multilayer nanofi lms Assembly of polypeptide multilayer nanofi lms Polypeptide multilayer nanofi lms were prepared at pHs 4.0, 5.0, 7.0, 9.0, and 10.0 using a dipping-machine (Riegler and Kirstein GmbH, Berlin, Germany); all the aqueous media used were of the same pH for the same sample. In brief, pre-cleaned quartz slides or stainless steel discs were dipped in a PLL solution for 20 min followed by rinsing with corresponding buffer solution for 3 min and drying with air. The samples were then dipped in a PLGA solution for 20 min, rinsed with buffer solution for 3 min, and dried with air. These two dipping processes, ie, dipping in PLL and PLGA solutions, were referred to as one deposition cycle. By repeating the deposition cycle, polypeptide multilayer nanofi lms, (PLL/ PLGA) n , were prepared where n is the number of deposition cycles or bilayers.
The formation of polypeptide multilayer nanofi lms on quartz slides was examined using UV-vis spectrometry. Images of PLL/PLGA multilayer nanofi lms were obtained using atomic force microscopy (AFM, PicoSPM ® II, Tempe, AZ) operating in a tapping mode with a silicon nitride cantilever tip. The growth of multilayer nanofi lms on stainless steel discs with bilayers was measured using ellipsometry (M-2000, JA Woollam Co., Lincoln, NE).

Stability of polypeptide multilayer nanofi lms
The stability of polypeptide multilayer nanofi lms in aqueous media was tested. In one set of studies, (PLL/PLGA) 20  absorbances in the range of 190-290 nm were recorded using UV-vis spectrometry. All the data were averaged from three measurements. The absorbances of the same polypeptide multilayer nanofi lm were compared between the time points studied.
Antibiotic-and MB-loading in polypeptide multilayer nanofi lms The loading of positively-and negatively-charged small drug molecules and drug models in polypeptide multilayer nanofi lms was studied. The infl uences of number of fi lm layers as well as the drug-loading time and drug solution pH were investigated. Polypeptide multilayer nanofi lms prepared at In general, polypeptide multilayer nanofi lms on quartz slides were incubated in the corresponding drug solutions at ambient temperature. At time periods of 2, 5, 10, 20, 40, and 60 min, the samples were rinsed with deionized water and dried with N 2 gas followed by UV-vis absorbance measurements. The loading of cefazolin, gentamicin, and MB was determined by measuring the absorbance at 270 nm, 270 nm, and 665 nm, respectively, using UV-vis spectrometry. 28,29 In order to obtain the total loading amounts of cefazolin, gentamicin, and MB in polypeptide multilayer nanofi lms, the drug-loaded samples were ultrasonicated in 1 mL PBS for 30 min, and the ultrasonication process was repeated three or more times until no peak absorbance referring to the corresponding drugs on quartz slides could be observed using UV-vis spectrometry. The drug concentration in the PBS was analyzed by recording the peak absorbance of the drugs. Raw data were converted to concentration of drug (C n , μg/mL) referring to the standard curves we obtained (data not shown). The drug released into the PBS solutions (M n , μg/cm 2 ) was calculated from an equation: M n = C n × V/A n , where V is the total volume of the PBS and A n is the surface area of the nanofi lms on substrates. The total drug loaded in polypeptide multilayer nanofi lms was determined as the cumulative amount of drugs released during the ultrasonication processes. It is worth noting that the peak absorbance of gentamicin at 270 nm was observed in gentamicin-loaded polypeptide multilayer nanofi lms but the absorbance of the released gentamicin in the PBS solutions was hard to detect. As a result, the actual amount of gentamicin loaded was not reported in this study.
In vitro drug release from polypeptide multilayer nanofi lms Drug-loaded polypeptide multilayer nanofi lms on quartz slides were incubated in 10 mL PBS of a certain pH (eg, pHs 4.0, 5.0, 7.0, 9.0, or 10.0). 0.6 mL of PBS solution was taken at a certain time period and 0.6 mL of fresh PBS was added to keep the volume of the release medium constant. The sample solutions of cefazolin and MB were analyzed using UV-vis spectrometry by measuring their absorbance at 270 nm and 665 nm, respectively. The total drug release (μg/cm 2 ) was calculated as detailed before.

S. aureus Kirby-Bauer disk diffusion assay
A modifi ed Kirby-Bauer technique was used to assess the antibacterial activity of polypeptide multilayer nanofi lms loaded with antibiotics. 31,32 A clinical isolate S. aureus was grown overnight in Mueller-Hinton broth, and the turbidity was adjusted to 0.5 McFarland. A cotton swab was dipped in the S. aureus suspension and rubbed across the surface of a Mueller-Hinton blood agar plate. Cefazolin-and gentamicinloaded polypeptide multilayer nanofi lms on stainless steel discs were inserted parallel to the agar plate surface. The plates were inverted and incubated at 37 °C without shaking for 24 h before observation. The diameters of the zones of inhibition were measured six times from different directions, and the experiments were repeated at least three times. The average diameters of the zones were calculated.

Growth curve and stability of PLL/PLGA multilayer nanofi lms
The formation of PLL/PLGA multilayer fi lms was examined using UV-vis spectrometry, ellipsometry, and AFM. Figure 2a shows that the absorbance of PLL/PLGA multilayer fi lms on quartz slides increased with increasing number of bilayers. In the pH values studied (ie, pH 4.0, 7.0, and 10.0), the smallest increase in absorbance was observed in the multilayer fi lms constructed at pH 7.0. Figure 2b presents the thickness of polypeptide multilayer fi lms. One can see that the thickness growth of PLL/PLGA multilayer fi lms increased linearly with an increasing number of deposition bilayers. Similar to the UV-vis absorbance data, the least thickness was observed in much smaller than those of the nanofi lms assembled at pHs 4.0 and 10.0. One concern in developing polyelectrolyte multilayer fi lms is their stability. Our stability studies of the PLL/PLGA nanofi lms in aqueous solutions showed no obvious changes in absorbance (data not shown) in the wavelength range of 190-290 nm after more than one week at all the pH values studied (ie, pH 4.0, 7.0, and 9.0). This means that PLL/PLGA multilayer nanofi lms are stable and can tolerate pH shifts in our drug loading and release processes.

Tunability of drug loading in PLL/PLGA multilayer nanofi lms
The infl uence of pH at which the multilayer nanofi lms were prepared and time of incubation on drug loading was studied in (PLL/PLGA) 20  loading of drugs increased with the incubation time and the loading of cefazolin and MB could reach their maximum loading (ie, capacity) within 10 and 20 min, respectively.
(PLL/PLGA) 20 nanofi lms formed at different pHs showed different drug-loading capacities. More drugs were captured in the nanofi lms prepared at a pH away from pH 7.0 than at pH 7.0, and loading was faster in the nanofi lms assembled at pHs 10.0 and 4.0 than at pH 7.0. Cefazolin-loading capacity in the nanofi lms formed at pH 10.0 was ∼330 μg/cm 2 ; it was the highest and it was almost six times that of nanofi lms prepared at pH 7.0 (∼60 μg/cm 2 ). Similarly, the MB-loading capacity in (PLL/PLGA) 20 nanofi lms assembled at pH 4.0 was about ten times that in the nanofi lms prepared at pH 7.0. The infl uence of drug solution pH on drug loading was also investigated. Figure 4 shows that more cefazolin was loaded in (PLL/PLGA) 20 nanofi lms at a lower pH in the range of pH 7.0-10.0, and more MB was loaded at a higher pH in the range of pH 4.0-7.0. Cefazolin-loading capacity at pH 7.0 was about four times that at pH 9.0. Loading of cefazolin at pH 10.0 was not detected and the loading capacity of MB at pH 7.0 was about sixteen times that at pH 4.0. Figure 5a presents the drug-loading capacity versus bilayers of (PLL/PLGA) n nanofi lms. The amounts of drugs, either cefazolin or MB, loaded increased approximately linearly with an increasing number of bilayers.
The infl uence of post-preparation heat-treatment on drug loading is shown in Figure 5b. After heat-treatment at 120 °C for 4 h, both (PLL/PLGA) 10 and (PLL/PLGA) 20 nanofi lms had ∼30% increase in drug loading.
In addition, Figure 6 shows that gentamicin was loaded into PLL/PLGA multilayer nanofi lms within 20 to 40 min, and gentamicin and MB, both positively-charged, had similar drug-loading kinetics in (PLL/PLGA) 20 nanofi lms.
Tunability and kinetics of drug release from PLL/PLGA multilayer nanofi lms Environmental conditions (eg, pH) infl uenced the drug release from polypeptide multilayer nanofilms. We studied the release of cefazolin and MB from (PLL/PLGA) 20 nanofi lms in a wide range of pH values. Figure 7 shows that more cefazolin was released at a higher pH than at a lower pH in the pH range 7.0-10.0; the amount of cefazolin released at pH 10.0, ∼340 μg/cm 2 , was about twice that released at pH 7.0, ∼190 μg/cm 2 . Also, more MB was released at a lower pH than at a higher pH in the pH range 4.0-7.0.
Moreover, the number of fi lm bilayers, the pH at which the nanofi lms were prepared, and post-preparation heat-treatment infl uenced drug release from polypeptide multilayer nanofi lms. The number of fi lm bilayers infl uenced the drug release behavior. Figure 8a shows that there was a burst release in the fi rst few pH of drug solution Tunable drug loading and release from polypeptide multilayer nanofi lms hours (mainly the fi rst hour) in all the cefazolin-loaded samples. Up to 95% cefazolin was released from 2-and 5-bilayer PLL/ PLGA nanofi lms within 48 h, and 85% and 75% were released from the 10-and 20-bilyer PLL/PLGA nanofi lms, respectively. More cefazolin was released from a higher number of bilayers of nanofi lms. For instance, 50, 90, 180, and 250 μg/cm 2 cefazolin were released at 48 h from 2, 5, 10, and 20-bilayer PLL/PLGA nanofi lms, respectively. Similarly, there was a burst release in the fi rst few hours in MB-loaded PLL/PLGA multilayer nanofi lms, and more MB was released with an increasing number of bilayers (Figure 8b). Figure 9 shows that the pH at which PLL/PLGA nanofi lms were prepared also affected drug release. More cefazolin was released from (PLL/PLGA) 20 nanofi lms assembled at a higher pH in the pH range 7.0-10.0, and more MB was released from (PLL/PLGA) 20 nanofi lms deposited at a lower pH in the pH range 4.0-7.0.
In addition, post-preparation heat-treatment infl uenced drug release (Figure 10). A greater amount of cefazolin was released from heat-treated samples than from untreated ones. Meanwhile, the heat-treatment slowed drug release from the PLL/PLGA nanofi lms. Up to 80% of loaded cefazolin was released in the fi rst 24 h from the 10-and 20-bilayer PLL/PLGA nanofi lms without heat-treatment, while approximately 67% and 60% was released from the heat-treated 10-and 20-bilayer PLL/PLGA nanofi lms, respectively (Figure 10 inset).

Discussion
In this study, we developed biodegradable polypeptide multilayer nanofi lms made of two weak polyelectrolytes as potential drug release systems on biomedical devices. Our developed polypeptide multilayer nanofi lms on stainless steel discs and quartz slides possessed the capability to load both negatively-and positively-charged small drug molecules, and the drug loading and release were tunable and pH-dependent.

Mechanism of pH-dependent drug loading and release
Compared to strong polyelectrolytes, such as poly(dially ldimethylammonium chloride) (PDDA) and poly(styrene sulfonate) (PSS), weak polyelectrolytes such as PLL, PLGA, poly(ethyleneimine) (PEI), poly(acrylic acid) (PAA), and poly(allylamine hydrochloride) (PAH), may present various ionization statuses and surface charges as their environmental pH changes. At a pH below or above the isoelectric point (pI), a weak polyelectrolyte is partially charged and may adopt a coiled structure in polyelectrolyte multilayer nanofi lms due to the decrease in charge repulsion among themselves. 13,20 The coil-structured polymer contains uncharged segments that may be converted to be charged ("binding sites") if the environmental pH is switched to a value at which the weak polyelectrolyte becomes more ionized. As a result, binding sites within multilayers made of weak polyelectrolytes can be created for capturing oppositely-charged drug molecules.
In our study, we prepared PLL/PLGA multilayer nanofi lms using electrostatic layer-by-layer self-assembly based on alternative deposition of PLL and PLGA on a substrate. Mechanisms of drug loading and release from the developed PLL/PLGA multilayer nanofi lms are proposed in Figure 12. PLL and PLGA are weak polyelectrolytes; PLGA possesses a net negative charge at a pH higher than pH 2.6, the pI of PLGA, 33,34 and PLL presents a net positive charge at a pH lower than its pI, pH 12.5. 32 Therefore, in the multilayer fi lms formed in an acid solution, eg, pH 4.0, PLGA is partially charged and adopts a coiled structure (Figure 12 top part), and its net charge decreases as pH decreases in the pH range 2.6-7.0. This results in more deposition of PLGA and thicker fi lms at pH 4.0 than at pH 7.0; PLL/PLGA multilayer nanofi lms assembled at pH 4.0 presented a higher absorbance and higher thickness than those deposited at pH 7.0 ( Figure 2). When incubating In the pH range 4.0-7.0, the higher the drug solution pH, the more binding sites are available and the more drugs can be captured. Therefore the capture of drugs is pH-dependent. Meanwhile, the release of drugs from PLL/PLGA multilayer nanofi lms is also pH-dependent. When drug-loaded PLL/PLGA multilayer nanofi lms serve in an environment of pH (eg, pH 4.0) lower than the pH (eg, pH 7.0) at which the drugs are loaded, the net charge of PLGA reverses and PLGA becomes less ionized. This leads to weakening of the interactions between the positively-charged drug molecules and the negatively-charged PLGA molecules. As a result, drugs are released from PLL/PLGA multilayer nanofi lms.
Similarly (Figure 12 bottom part), PLL/PLGA multilayer nanofi lms assembled at a high pH (eg, pH 10.0) contain partiallyionized and coil-structured PLL. This allows the PLL/PLGA multilayer nanofi lms the capability to load negatively-charged drugs at pH 7.0, and drug release can be induced in an environment of a higher pH, eg, pH 8.0, 9.0, or 10.0.

Tunable loading of charged drugs in polypeptide multilayer nanofi lms
The pH at which polypeptide multilayer nanofilms are prepared may infl uence the amount of polymers deposited thereby influencing subsequent drug loading. Figure 3a shows that the drug-loading capacity of (PLL/PLGA) 20 nanofi lms prepared at pH 10.0 was approximately six times that of nanofi lms assembled at pH 7.0. The higher loading of cefazolin in those nanofi lms prepared at pH 10.0 was due to the higher amount of PLL assembled (corresponding to the higher absorbance and increased thickness at pH 10.0 in Figure 2) and the more ionization of PLL as the pH changes from fi lm preparation at pH 10.0 to drug loading at pH 7.0, compared to those nanofi lms prepared at pHs 9.0 and 7.0. As a result, binding sites on PLL molecules for negatively-charged drug molecules were created as the pH shifts from the fi lm preparation pH to the drug-loading pH and more binding sites on PLL would be available at a lower pH in the pH range 7.0-10.0. This is consistent with the increased loading of cefazolin in PLL/PLGA multilayer nanofi lms assembled at pH 10.0 than at pHs 9.0 and 7.0 (Figure 3a). Similarly, (PLL/PLGA) 20 nanofilms assembled at a lower pH in the pH range 4.0-7.0 would have more PLGA deposited, corresponding to higher absorbance and higher thickness at pH 4.0 than at pH 7.0 in Figure 2. This thereby leads to more binding sites on PLGA for positively-charged drug molecules when the drugs are loaded at pH 7.0. As shown in Figure 3b, the (PLL/PLGA) 20 nanofi lms formed at a lower pH in the pH range of 4.0-7.0 showed higher MB loading. Therefore, polypeptide multilayer nanofi lms assembled at different pHs had various capacities for drug loading after fi lm formation.
Meanwhile, the pH at which drugs are loaded could also be used to tune drug loading in polypeptide multilayer nanofi lms. Using the electrostatic layer-by-layer self-assembly technique, we can also control the amount of polymers deposited and tune the subsequent drug loading by manipulating the number of deposition bilayers. Figure 2 shows that the absorbance and thickness, and thereby the amount of polypeptides deposited, increased with increasing deposition bilayers. The increase in polypeptide deposition could lead to an increase in binding sites and drug loading when the pH shifts from the deposition pH to the drug-loading pH. As a result, the amounts of cefazolin and MB loaded in PLL/PLGA multilayer nanofi lms increased approximately linearly with increasing deposition bilayers (Figure 5a).
In addition, heat-treatment after fi lm formation may influence drug loading. We found that heat-treatment after multilayer fi lm formation led to an increase (∼30%, Figure 5b) in cefazolin loading; however, the reason for the increase is unknown and will be studied in the future. In the literature, heat-treatment led to the formation of crosslinkings between polyelectrolyte multilayers due to the formation of amide bonds from carboxylic and amine groups within polyelectrolyte multilayers, 35 and the swelling properties of heat-treated fi lms changed signifi cantly over a wide pH range. 36 Tunable drug release from polypeptide multilayer nanofi lms In previous studies, drug release from polyelectrolyte multilayer fi lms was mainly controlled by manipulating the permeability and degradation of the fi lms. In our study, PLL/ PLGA multilayer nanofi lms were stable in aqueous solutions and could tolerate pH shifts in our drug loading and release processes. The release of drug molecules from polypeptide multilayer nanofi lms is mainly due to the change of interaction between drug molecules and polypeptide nanofi lms, and drug diffusion. By changing the interaction between drug molecules and PLL/PLGA multilayer fi lm components, one can tune drug release. As shown in Figure 12, when there is a pH shift from drug loading to drug release, the interaction between the drug molecules and the corresponding oppositely-charged polypeptides changes and drugs can be released. Cefazolin and MB were loaded at pH 7.0 and more cefazolin was released at pH 10.0 than at a lower pH in the pH range 7.0-10.0. More MB was released at pH 4.0 than at a higher pH in the pH range 4.0-7.0 (Figure 7), due to the change in the interaction between the drug molecules and the corresponding oppositely-charged polypeptides. For instance, as the pH changed from the drug loading (pH 7.0) to a higher drug release pH in the pH range 7.0-10.0, more binding sites of PLL were reversed and became uncharged; therefore, more cefazolin was released from the nanofi lms.
Moreover, the history of polypeptide multilayer nanofi lms had a signifi cant impact on the amount of drugs released. Drug release could be tuned by controlling the number of fi lm layers, the pH at which the nanofi lms were prepared, and post-preparation heat-treatment. It was found that more drugs (cefazolin and MB) were released from PLL/PLGA multilayer nanofi lms with more bilayers (Figure 8). More cefazolin was released from (PLL/PLGA) 20 nanofilms assembled at pH 10.0 than those assembled at a lower pH in the pH range 7.0-10.0, and more MB was released from (PLL/PLGA) 20 nanofi lms assembled at pH 4.0 than those nanofi lms prepared at a higher pH in the pH range 4.0-7.0 ( Figure 9). The higher drug release was probably associated with the corresponding higher drug loading in the PLL/PLGA multilayer nanofi lms under those conditions ( Figure 3). The post-preparation heat-treatment also had some effect on drug release, as it seemed that heat-treatment slowed drug release compared to the control samples (Figure 10 inset). This was likely related to the possible crosslinking formation and the reduced swelling of polyelectrolyte multilayers in aqueous media after heat-treatment. 36 The higher release amounts of cefazolin after the burst release period was because of higher amounts of cefazolin loaded in the nanofi lms after heat-treatment (Figure 5b).

In vitro antibacterial activity against S. aureus
Quantitative assessment of the therapeutic activity of antibiotic-loaded polypeptide multilayer nanofi lms was conducted. The diameter of a zone of inhibition provides a quantitative measure of the amount of in vitro active antibiotic (eg, cefazolin and gentamicin) released and diffused into the agar plates. Figure 11 shows that PLL/PLGA multilayer nanofi lms containing cefazolin and gentamicin presented large zones of inhibition, and those samples without antibiotics had no antibacterial effects. The zone of inhibition became larger with increasing number of bilayers; this is because more drugs (eg, cefazolin) were loaded and subsequently released from the PLL/PLGA multilayer nanofi lms with more bilayers (Figure 5a). The difference in the sizes of zone of inhibition in (PLL/PLGA) 20 nanofi lms assembled at different pHs was also related to the different amounts of antibiotics loaded and released from the PLL/PLGA multilayer nanofi lms. The more antibiotics loaded, the bigger the zone of inhibition. Figure 3 shows that the (PLL/PLGA) 20 nanofi lms formed at different pHs possessed different drug-loading capacities, and more cefazolin was loaded at pH 10.0 than at pHs 9.0 and 7.0. As a result, a larger zone of inhibition was observed in cefazolin loaded (PLL/PLGA) 20 nanofi lms assembled at pH 10.0 than at pHs 9.0 and 7.0.
Our studies showed that stainless steel discs coated with antibiotic-loaded polypeptide multilayer nanofi lms exhibited in vitro antibacterial activity against S. aureus. Stainless steel is one of the commonly used metal implants in orthopedics and S. aureus is the most common source of osteomyelitis and septic arthritis. 37,38 Therefore, the developed antibioticloaded polypeptide multilayer nanofi lms have the potential to prevent orthopaedic device-associated infection, and further studies will be carried out to investigate the effi cacy of such antibiotic-loaded nanofi lms in preventing infection in vivo in an open fracture rat model we recently developed. 39

Conclusions
A multilayer self-assembly technology was applied to construct biodegradable polypeptide multilayer nanofi lms made of PLL and PLGA, which are weak polyelectrolytes that enable the fi ne tuning of drug loading and release after fi lm formation. Our studies showed that the loading kinetics of gentamicin and MB, both positively-charged, were very similar. The loading and release of both negatively-and positively-charged drug molecules (eg, cefazolin, gentamicin, and MB) could be tuned by several variables during and after the fi lm preparation. Such variables include the number of deposition layers, pH of fi lm preparation, and post-preparation heat-treatment. The loading of drugs (eg, cefazolin and MB) increased approximately linearly with an increasing number of layers, and heat-treatment before drug loading enhanced the drug-loading capacity. The pH of fi lm preparation also signifi cantly altered the fi lm formation including surface morphology. In addition, the drug-loading pH and the incubation time in the drug solution could be used to tune the amount of drugs that could be loaded, and the pH of an application environment also had a signifi cant impact on drug release. The developed antibiotic-loaded polypeptide multilayer nanofi lms presented tunable antibacterial properties and potentially have signifi cant applications in medicine, eg, antibacterial drug delivery systems for preventing biomedical device-associated infection.