polymers Article Superabsorbent Polymer Network Degradable by a Human Urinary Enzyme Minji Whang †, Hyeonji Yu † and Jungwook Kim * Department of Chemical and Biomolecular Engineering, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 04107, Korea; [email protected] (M.W.); [email protected] (H.Y.) * Correspondence: [email protected]; Tel.: +82-2704-8793 † These authors contributed equally. Abstract: Owing to its superior water absorption capacity, superabsorbent polymer (SAP) based on a poly (acrylic acid) network is extensively used in industrial products such as diapers, wound dressing, or surgical pads. However, because SAP does not degrade naturally, a massive amount of non-degradable waste is discarded daily, posing serious environmental problems. Considering that diapers are the most widely used end-product of SAP, we created one that is degradable by a human urinary enzyme. We chose three enzyme candidates, all of which have substrates that were modified with polymerizable groups to be examined for cleavable crosslinkers of SAP. We found that the urokinase-type plasminogen activator (uPA) substrate, end-modified with acrylamide groups at sufficient distances from the enzymatic cleavage site, can be successfully used as a cleavable crosslinker of SAP. The resulting SAP slowly degraded over several days in the aqueous solution containing uPA at a physiological concentration found in human urine and became shapeless in ~30 days. Citation: Whang, M.; Yu, H.; Kim, J. Keywords: biodegradable polymer; superabsorbent polymer (SAP); cleavable crosslinker; poly Superabsorbent Polymer Network (acrylic acid) (PAA); urokinase-type plasminogen activator (uPA) Degradable by a Human Urinary Enzyme. Polymers 2021, 13, 929. https://doi.org/10.3390/ polym13060929 1. Introduction Superabsorbent polymer, commonly made of acrylic acid-based crosslinked copoly- Academic Editors: Sandra M. A. Cruz mer, absorbs as much as hundreds of times its weight in water due to high osmotic pressure and Filipa A. M. M. Gonçalves generated by the concentrated charge of the polymer network. Owing to its superior water absorption capacity, fast absorption rate, and low discharge of absorbed water under Received: 5 March 2021 applied pressure, SAP has been used for various industrial products including diapers, Accepted: 16 March 2021 Published: 17 March 2021 wound dressing, surgical pads, fire-retardant gel, and food additives [1]. The global SAP market was about 2.3 million metric tons in 2020 [2], indicating that a massive amount Publisher’s Note: MDPI stays neutral of SAP waste is discarded daily. Because SAP is naturally non-biodegradable, this waste with regard to jurisdictional claims in poses a serious environmental problem, indicating that the development of an eco-friendly published maps and institutional affil- and biodegradable SAP is urgent [3]. When developing a biodegradable SAP, a critical iations. consideration is the ability to synthesize SAP using a radical polymerization (the current industrial method) rather than a step polymerization. Currently, the most commonly used SAP in the industry is a crosslinked network of poly (acrylic acid) (PAA) chains [4]. It has been known that PAA chains of a molecular weight less than 1 kg/mol can be degraded in sewage by soil microorganisms [5]. Therefore, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. a design principle in creating a biodegradable SAP is first to degrade a crosslinked PAA This article is an open access article network into linear PAA chains by cleaving crosslinking sites and then fragmentizing linear distributed under the terms and PAA chains into smaller pieces to be degraded by the microorganisms. Previous works have conditions of the Creative Commons synthesized biodegradable SAP through embedding cleavable moieties into the polymer Attribution (CC BY) license (https:// network [6]; however, these attempts had limited success due to complex synthetic routes, creativecommons.org/licenses/by/ low production yield, and insufficient water absorbency and mechanical properties [7–9]. 4.0/). Instead of a PAA, other polymers that are charged and intrinsically biodegradable were Polymers 2021, 13, 929. https://doi.org/10.3390/polym13060929 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 929 2 of 10 Polymers 2021, 13, 929 2 of 10 properties [7–9]. Instead of a PAA, other polymers that are charged and intrinsically bio- degradable were made into SAPs via crosslinking. For example, a crosslinked network of madebiodegradable into SAPs poly via crosslinking. (aspartic acid) For polymers example, was a crosslinked synthesized network via a condensation of biodegradable polymer- poly (asparticization of acid) L-aspartic polymers acid was [10] synthesized. However, viaits use a condensation in the SAP industry polymerization is limited of L-asparticbecause of acidits weak [10]. mechanical However, its strength use in theand SAP a high industry production is limited cost. because While most of its previous weak mechanically synthe- strengthsized biodegradable and a high production SAPs contained cost. Whilethe ester most groups previously for hydrolytic synthesized degradation, biodegradable these SAPsmethods contained cannot the apply ester to groups the current for hydrolytic industrial degradation, SAP manufacturing these methods process cannot since apply it in- tovolves the current a high- industrialtemperature SAP heating manufacturing step, which process accelerates since it hydrolytic involves a degradation high-temperature. Thus, heatingto realize step, the industrial which accelerates application hydrolytic and to enhance degradation. the long Thus,-term to stability realize during the industrial storage, applicationthe use of a andspecific to enhance enzyme– thesubstrate long-term reaction stability for degradation during storage, is prefer theable use of[11] a. specific enzyme–substrateIn this study, reactionbecause fordiapers degradation are the most is preferable widely used [11]. end-product of SAP, we cre- atedIn one this that study, is degradable because diapers by a arehuman the mosturinary widely enzyme used. end-productIf the enzymatic of SAP, substrates we created are onemodified that is at degradable both ends bywith a human polymerizable urinary enzyme.groups, cleavable If the enzymatic crosslinker substratess that can are partici- modi- fiedpate at in both the radical ends with polymerization polymerizable of groups,acrylic acid cleavable can be crosslinkers created. Because that can the participate end modifi- in thecation radical may polymerization sterically hinder of the acrylic enzymes acid can from be binding created. to Because the cleavage the end sites, modification the enzymatic may stericallysubstrates hinder were theexamined enzymes for from their binding degradability to the cleavageafter modification sites, the enzymatic. We created substrates the SAP wereusing examined the enzymatically for their degradability cleavable cros afterslinker modification. and tested We the created biodegradation the SAP using of the the SAP en- zymaticallyinto linear PAA cleavable chains crosslinker when the and enzyme tested was the biodegradationpresent in a human of the urinary SAP into concentration linear PAA chains(Figure when 1). Among the enzyme various was presentenzymes in found a human in urinaryhuman concentrationurine, such as (Figure oxidoreductases,1). Among varioustransferases, enzymes hydrolases found in, and human lyases urine, [12] such, we as selected oxidoreductases, leucine aminopeptidase transferases, hydrolases, (LAP), β- andglucuronidase lyases [12], (GLU), we selected and urokinase leucine aminopeptidase-type plasminogen (LAP), activatorb-glucuronidase (uPA) (Table (GLU), S1 in S andup- urokinase-typeplementary Material plasminogen), all of which activator are hydrolase (uPA) (Table and S1 are in known Supplementary to cleave Material),a specific chem- all of whichical bond are. hydrolase and are known to cleave a specific chemical bond. FigureFigure 1.1. AA schematicschematic illustratingillustrating thethe enzymaticenzymatic degradation degradation of of the the SAP SAP created created using using the the cleavable cleava- crosslinker.ble crosslinker. 2. Materials and Methods 2. Materials and Methods Leucine aminopeptidase (EC 3.4.1.2) microsomal from porcine kidney (LAP), b- Leucine aminopeptidase (EC 3.4.1.2) microsomal from porcine kidney (LAP), β-glu- glucuronidase (EC 3.2.1.31) from helix pomatia type H-2 (GLU), N-(tert-Butoxycarbonyl) curonidase (EC 3.2.1.31) from helix pomatia type H-2 (GLU), N‐(tert-Butoxycarbonyl)gly- glycine, glycine, triglycine, s-pinene, triphosgene, triethylamine, N-carboxyanhydride cine, glycine, triglycine, σ-pinene, triphosgene, triethylamine, N-carboxyanhydride (NCA), 2-aminoethyl methacrylate hydrochloride (2-AMEA), allylamine, 4-nitrophenyl- b- (NCA), 2-aminoethyl methacrylate hydrochloride (2-AMEA), allylamine, 4-nitrophenyl- D-glucuronide (PNPG), hydroxybenzotriazole (HOBt), N,N’-diisopropylcarboiimide (DIC), β-D-glucuronide (PNPG), hydroxybenzotriazole (HOBt), N,N’‐diisopropylcarboiimide acrylic acid N-hydroxysuccinimide (Ac–NHS ester), deuterium oxide (D2O), dimethylsulfoxide- (DIC), acrylic acid N-hydroxysuccinimide (Ac–NHS ester), deuterium oxide (D2O), dime- d6 (DMSO-d), chloroform-d (CDCl3), 2,2-dihydroxyindane-1,3-dione (Ninhydrin) and tris thylsulfoxide-d6 (DMSO-d), chloroform-d (CDCl3), 2,2-dihydroxyindane-1,3-dione (Nin- (hydroxymethyl)aminomethane hydrochloride (Tris–HCl)