Mussel adhesive protein as an environmentally-friendly harmless wood furniture adhesive

International Journal of Adhesion & Adhesives 70 (2016) 260–264

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Mussel adhesive protein as an environmentally-friendly harmless wood furniture adhesive Young Hoon Song a,b,1, Jeong Hyun Seo a,b,1, Yoo Seong Choi a,c, Do Hyung Kim a, Bong-Hyuk Choi a, Hyung Joon Cha a,n a

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea c Department of Chemical Engineering, Chungnam National University, Daejon 305-764, Republic of Korea b

art ic l e i nf o

a b s t r a c t

Article history: Accepted 7 July 2016 Available online 14 July 2016

Recent adhesive technologies have focused on the development of high-quality and eco-friendly adhesives. Thus, there is a gradual shift from the currently used chemical-based adhesives toward harmless adhesives with improved quality and performance. Here, we evaluated the potential use of bacteriaproduced recombinant mussel adhesive protein (MAP) as a harmless wood furniture adhesive. We formulated a MAP wood adhesive as an inclusion body type for economical preparation, and we confirmed its harmlessness through the non-detection of volatile organic compounds and heavy metals. The formulated MAP showed sufficiently strong bulk adhesive strength for the dried gluing of wood adherends. We also found that the formulated MAP wood adhesive exhibits robust adhesion in various environmental conditions, including open assembly times, incubation times, temperatures, and humidity levels. In summary, the developed recombinant MAP could be successfully used as a promising environmentally-friendly, harmless wood furniture adhesive. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Mussel adhesive protein Wood furniture adhesive Harmless adhesive Environmentally-friendly adhesive

1. Introduction Chemical-based adhesives have been widely used in most household and industrial fields due to their outstanding adhesive strengths and low production costs [1]. However, they generate volatile organic compounds (VOCs) and contain harmful components such as formaldehyde, phenol, and trichloroethane, which are regarded as endocrine-disrupting chemicals that might cause health hazards such as sick house syndrome and atopic dermatitis [2–7]. Although many efforts have been made to reduce harmful components, such as VOCs and endocrine-disrupting chemicals, chemical-based adhesives have a limitation in terms of toxicity. Thus, current chemical-based harmful adhesives are continuously shifting toward environmentally-friendly adhesives [8,9]. Biological adhesives derived from nature, such as tannin, casein, soybean, lignin, gelatin, and fibrin, have been suggested as alternative adhesives [10–15]. Differently from chemical-based adhesives, biological adhesives are distinctly regarded as environmentallyfriendly and non-toxic adhesives, but limitations relating to

relatively low adhesive properties and high production costs remain [16–18]. Mussel adhesive proteins (MAPs) have attracted interest for their various potential uses as adhesives in cell and tissue engineering, biomedicine, and biotechnology [19–23]. These adhesive biomaterials have been regarded as environmentally friendly due to their non-toxic, biocompatible, and biodegradable properties. However, their applications have been greatly limited due to the difficulty of obtaining sufficient quantities. Previously, a genetically redesigned recombinant hybrid MAP was produced in considerable quantities using a bacterial expression system with significant adhesion ability [24] and was successfully applied for diverse purposes [25–33]. In the present work, we evaluated the potential application of a recombinant MAP as an environmentally-friendly adhesive for wood-based furniture through investigations of its harmlessness and adhesion properties under various conditions.

2. Materials and methods 2.1. Preparation of inclusion body and powder MAPs

n

Corresponding author. Fax: þ82 542795528. E-mail address: [email protected] (H.J. Cha). 1 These authors contributed equally to this work.

http://dx.doi.org/10.1016/j.ijadhadh.2016.07.008 0143-7496/& 2016 Elsevier Ltd. All rights reserved.

Escherichia coli BL21 (DE3) cells expressing recombinant MAP were cultured in 150 L Luria-Bertani (LB) medium with 50 μg/mL

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Fig. 1. (A) Schematic illustration of the measurement of bulk shear strength with wood adherends and (B) bulk adhesive strengths of MAP wood adhesive and MAP powder. Adhesive strengths were measured after 24 h of curing of attached wood adherends at room temperature and 60% relative humidity without an open assembly time. Data are shown as means and standard deviations (N ¼ minimum of 8).

ampicillin (Sigma-Aldrich, St. Louis, MO, USA) in a 300 L fermentor (Fermentec, Cheongju, Korea) at 37 °C and 250 rpm. When a cell density (OD600) reached approximately 0.2–0.5, 1 mM isopropyl-βD-thiogalactopyranoside (IPTG; Sigma-Aldrich) was added to induce the expression of recombinant MAP. After 9 h of incubation, the cells were harvested by centrifugation at 18,000g for 10 min at 4 °C. The harvested cell pellet was resuspended in 5 mL lysis buffer (10 mM Tris–Cl and 100 mM sodium phosphate; pH 8.0) per gram (wet weight) and lysed by a cell disruption system (Constant Systems, Daventry, UK) at 20 kpsi. Cell debris was centrifuged at 18,000g for 20 min at 4 °C to collect the inclusion body. The inclusion body MAP was stored in  80 °C before further analyses. To obtain powder MAP, the inclusion body was washed with TTE buffer (1% TritonX-100, 1 mM EDTA, 0.1 mM PMSF, and 50 mM Tris–HCl; pH 8.0) followed by distilled water (DW) and resuspended in 25% (v/v) acetic acid. The acetate-extracted soluble MAP was dialyzed twice in DW and freeze-dried to obtain powder. Powder MAP was also stored in  80 °C before further analyses. Production yields for inclusion body and powder MAPs were determined as 10.5 g/L (  64% purity) and  0.4 g/L (  93% purity), respectively. 2.2. Measurement of bulk adhesive strength Bulk adhesive strength was measured using wood adherends (5 mm thickness  10 mm width  150 mm length) according to a previously described protocol [25,34]. Briefly, the inclusion body and lyophilized powder MAPs were dissolved in DW for a final concentration of 300 g/L. The samples were applied onto 10  10 mm2 areas of wood surfaces (Fig. 1A) as a single spread method, and the attached wood adherends were directly incubated with compressed clip for 24 h at room temperature (25 73 °C) for sufficient curing. In addition, to test practical wood adhesion, bulk adhesive strength was measured in various conditions, such as incubation time, open assembly time, temperature, and humidity, after 3 days of incubation. A commercially available chloroprene (CR) adhesive (Ogong Bond, Incheon, Korea) was used as a comparative chemical-based wood adhesive. Shear strength was directly measured using a universal material testing machine (Model No. 3344; Instron, Norwood, MA, USA) with a 2000 N load cell (Fig. 1A). Adhesion force in Pascal (Pa) was calculated through dividing the shear force (in Newton) by the adherend overlap area (in m2) following the ASTM D1002 standard method (ASTM

International D1002-05, 2005). Each adhesion measurement was repeated at least 10 times and averaged for a given sample. The statistical significance was analyzed by comparing two groups through the paired Student's t-test: statistical significance is designated by *p o0.05, **p o0.01, and ***p o0.005. The certified determination of the bulk adhesive strength for the inclusion body MAP was conducted at the Korea testing laboratory (KTL, Seoul, Korea), an officially authorized institution, following a protocol almost the same as that described in this work, except the attached wood adherends were stored for 2 weeks at room temperature before testing. Shear strength was measured using a universal material testing machine (Model No. 5589; Instron) at 237 1 °C and 60% relative humidity. 2.3. Harmlessness evaluation of MAP wood furniture adhesive To measure the possible emission of harmful contaminants, 1.2 g of inclusion body MAP was coated on a glass slide with a 63  63 mm2 area, washed with DW, and incubated for 3 h at 100 °C. Then, the extracted sample was taken from the slide and placed in a 20 L chamber at 25 °C and 50% relative humidity with 0.5 times/h air change. VOCs and formaldehyde were analyzed by gas chromatography/mass spectrometry (GC/MS; Shimadzu, Kyoto, Japan) and high-performance liquid chromatography (HPLC; Agilent, Santa Clara, CA, USA), respectively. A harmlessness evaluation was conducted at KTL. In addition, the heavy metal contents (lead (Pb), arsenic (As), cadmium (Cd), mercury (Hg), and chromium (Cr)) from the inclusion body MAP was analyzed by inductively coupled plasma-mass spectrometry (ICP-MS; Agilent) after sample pretreatment using a microwave digestion system.

3. Results and discussion 3.1. Selection of formulation type for wood furniture adhesive For successful use as a practical wood furniture adhesive, the selection of a feasible formulation of MAP is a prerequisite. In the present work, we compared two formulation types (inclusion body and powder) of a MAP wood furniture adhesive that are derived from MAP production processes. Because the recombinant MAP was expressed as an inclusion body in E. coli [20], we evaluated an inclusion body MAP as a candidate for a potential

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Fig. 2. The long-term stability of the MAP wood adhesive. Commercially available chloroprene adhesive was used as a comparative control. Adhesive strength in each incubation time was measured after 24 h of curing of attached wood adherends at room temperature and 60% relative humidity without open assembly time. Data are shown as means and standard deviations (N ¼minimum of 5).

Fig. 3. Bulk adhesive strength of MAP wood adhesive according to open assembly time. Adhesive strengths were measured after each open assembly time and subsequent 24 h of curing of attached wood adherends at room temperature and 60% relative humidity. Data are shown as means and standard deviations (N ¼ minimum of 5).

Table 1 Evaluation of harmlessness of MAP wood adhesive.

3.2. Harmlessness evaluation as environmentally-friendly wood furniture adhesive

Organic compound a

TVOC benzene toluene ethylbenzene xylene styrene formaldehyde a b

mg/m2 h 2  10 N/Db N/D N/D N/D N/D N/D

3

Heavy metal

mg/kg

Pb Cd Hg Cr As

N/D N/D N/D N/D N/D

Total volatile organic compounds. Non-detected.

formulation type of wood furniture adhesive. Soluble lyophilized powder MAP was obtained after the acetate extraction process of the inclusion body. Thus, we also tested powder MAP as another candidate for formulation type. We compared the bulk adhesive strengths of two formulation types of MAP using a wood specimen. We observed that the adhesive strength of inclusion body MAP was 2.85 70.33 MPa, whereas isolated powder MAP showed a higher adhesive strength (3.85 70.23 MPa) (Fig. 1B). The lower adhesive strength of the inclusion body MAP can be explained by its lower purity (  64%) compared to that of the powder MAP ( 93%) because the inclusion body contains other intracellular proteins. For feasible use as a wood furniture adhesive, the production yield, production cost, and adhesive strength should be considered together. Although the inclusion body MAP has a lower adhesive strength due to its lower purity, its production yield (  10.5 g/L) and cost ( 1.5 USD/ g-inclusion body) are superior to those of the powder MAP (yield: 0.4 g/L, cost:  67.5 USD/g-powder) because of its relatively simple production process. Based on the overall consideration of the competitive production yield and cost of the inclusion body MAP with its comparative adhesive strength compared to that of commercial chloroprene adhesive (  1–2 MPa; Fig. 2), we finally selected the inclusion body as the formulation type of MAP wood furniture adhesive. It was also found that the adhesive strengths of other commercial wood adhesives such as urea-formaldehyde resin, phenol-formaldehyde resin, and melamine-formaldehyde resin are around or below 3 MPa [35–44], which are similar strength levels compared to our developed MAP wood adhesive.

Next, we evaluated the harmlessness of the inclusion body MAP to be guaranteed as an environmentally-friendly harmless wood furniture adhesive through measuring the amounts of generating VOCs. We found that five BTXs (benzene, toluene, ethylbenzene, xylene, and styrene) and formaldehyde, which were regarded as environmentally toxic substances, were not detected from the inclusion body-type MAP adhesive (Table 1). The total VOCs generated from the inclusion body MAP was only 2  10  3 mg/m2 h, which was significantly lower than the value (1  10  1 mg/m2 h) of the best rated environmentally-friendly adhesive. In addition, heavy metals such as Pb, Cd, Hg, Cr, and As were not detected from the inclusion body MAP (Table 1). Therefore, we confirmed that the inclusion body MAP can be used as an environmentally-friendly, harmless wood furniture adhesive. 3.3. Evaluation of MAP wood furniture adhesive in various conditions Generally, wood adhesives need to provide stable adhesion under various conditions, such as time, temperature, and humidity. First, we investigated the long-term stability of the MAP wood furniture adhesive for 5 weeks after 24 h curing of the attached wood adherends at room temperature and 60% relative humidity. We observed that the MAP wood adhesive stably maintained its adhesive strength at approximately 3 MPa over 5 weeks (Fig. 2). The commercial chloroprene wood adhesive also showed stable (actually, slightly increased) wood adhesion during the observation period even though its adhesive strength was lower (approximately 1 MPa) than that of the MAP wood adhesive. The testing of open assembly time, which is defined as the elapsed time after the application of adhesive before its strength becomes effective, is a prerequisite for the commercialization of an adhesive product. Thus, we compared adhesive strengths according to the open assembly time. We found that the adhesive strengths of the MAP wood adhesive were not significantly affected within an open assembly time of 5 min (Fig. 3). However, in an open assembly time over 5 min, the adhesive strengths were decreased, which might have been due to the dryness of the adhesive on the wood surface from a long period of exposure in air

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Fig. 4. Bulk adhesive strength of MAP wood adhesive according to incubation conditions: (A) temperature and (B) humidity. Adhesive strengths were measured after 24 h of curing and 3 days of incubation of attached wood adherends at each temperature and relative humidity condition without the open assembly time. Data are shown as means and standard deviations (N¼ minimum of 10).

(data not shown). Thus, we concluded that the developed MAP wood furniture adhesive is capable of maintaining its adhesive property within an open assembly time of 5 min. But, under a humidity condition selected to prevent prompt drying, the MAP wood furniture adhesive would maintain its adhesiveness over a longer open assembly time. A long exposure period of attached wood adherends at high temperature and high humidity can cause wood distortion, which results in the weakening of adhesive strength or the detachment of adhesive from a wood surface. Thus, we evaluated adhesion stability according to various temperatures and humidity levels for a 3-day incubation after a 24-h curing of wood adherends. As expected, the MAP wood adhesive maintained its adhesive strengths at various test temperatures (  20 °C to 45 °C; Fig. 4A) and relative humidity (25% to 90%; Fig. 4B) conditions, although it seemed that the strengths were slightly decreased at 45 °C and 90% relative humidity. Furthermore, through the certified measurement of bulk adhesive strength, we confirmed that our developed MAP wood furniture adhesive has an adhesive strength of 2.47 70.06 MPa, which almost coincides with our measurement (2.85 70.33 MPa). Collectively, from these studies, it should be highlighted that the inclusion body-type MAP could be successfully used as a promising environmentally-friendly, harmless wood furniture adhesive.

4. Conclusions Herein, we evaluated recombinant MAP as a potential wood furniture adhesive. An inclusion body was selected as the formulation type through overall considerations of production yield, production cost, and harmlessness, with no detections of VOCs and heavy metals. The developed MAP wood adhesive showed a strong (  2.5–3 MPa) adhesive strength and good long-term stability on wood adhesion. It also showed stable adhesion under various environmental conditions, including open assembly time (less than 5 min), temperature ( 20 to 45 °C), and humidity (up to 90%). Therefore, the robust and strong adhesion in various environments and the demonstrated harmlessness supports the view that the developed MAP based on an inclusion body type can be potentially used as an environmentally-friendly wood adhesive. However, although the production cost for MAP wood adhesive was significantly reduced in this work, it is still more expensive compared to most commercial chemical-based wood adhesives. Therefore, the developed harmless MAP-based wood adhesive

could be applicable to relatively high value-added areas such as eco-friendly high-quality furniture. In addition, further efforts on cost reduction are needed.

Acknowledgments This work was supported by the Marine Biotechnology Program (Marine BioMaterials Research Center) funded by the Ministry of Oceans and Fisheries, Korea and the Technology Innovation Program (10041466) funded by the Ministry of Trade, Industry & Energy, Korea. We thank the Joongang Institute of Life Science (Namyangju, Korea) for helping with the experimental design and engaging in useful discussion.

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