Production and Characterization of Cellulose Nanofiber Slurries and Sheets for Biomedical Applications

Production and Characterization of Cellulose Nanofiber Slurries and Sheets for Biomedical Applications

Overview

This research focuses on the production, characterization, and biomedical potential of Cellulose Nanofiber (CNF) slurries and sheets. Conducted at the University of Maine, the work represents a significant stride toward scalable, Good Manufacturing Practice (GMP)-compliant CNF production. The study outlines a continuous mechanical refining system, housed within a Class 7 cleanroom, that generates 14 liters of reproducible CNF slurry and demonstrates how the material can be formed into sheets suitable for biomedical applications, including implantation.

1. Introduction to Cellulose Nanomaterials

Cellulose nanomaterials (CNMs) are gaining popularity due to their renewable, biodegradable, and biocompatible nature. CNMs include:

  • Cellulose Nanofibers (CNFs): Long, flexible fibers (5–30 nm in diameter) with both crystalline and amorphous domains.

  • Cellulose Nanocrystals (CNCs): Highly crystalline, whisker-shaped particles.

  • Bacterial Nanocellulose (BNC): Ribbon-like nanofibers produced by bacteria.

  • Electrospun Nanocellulose: Produced via spinning cellulose or cellulose-polymer blends.

Each type varies significantly in structure and production method. This paper emphasizes mechanically produced CNFs, particularly relevant due to their fibrous networks, tunable properties, and scalable production.

2. Motivation and Objectives

The primary goal of the study was to develop a scalable, GMP-compliant method for producing high-quality CNF slurry and to transform it into sheets that:

  • Have consistent properties,

  • Are biocompatible and sterilizable,

  • Are suitable for in vivo applications such as nerve regeneration scaffolds.

The production strategy uses bleached softwood kraft pulp as feedstock and mechanical grinding to achieve nanoscale defibrillation.

3. Materials and Methods

3.1 Feedstock and Pulp Preparation

The raw material was bleached softwood kraft pulp (98.3% carbohydrates). It was cut into squares, soaked overnight in deionized water, and then manually separated into fiber aggregates (~1 cm).

3.2 Supermasscolloider (SMC) Setup

A Supermasscolloider (SMC), typically used for food paste production, was adapted for CNF refining:

  • Consists of a rotating silicon carbide/alumina stone (rotor) and a stationary stone (stator).

  • Slurry is passed through the rotor-stator gap multiple times.

  • The system was upgraded for continuous refining with a 14-liter slurry loop including:

    • Feed hopper,

    • Recirculating pump,

    • Overflow loop,

    • Cooling jacket,

    • Sampling/waste valves.

Refinement energy was monitored using a wattmeter, and refining was performed in a cleanroom environment to comply with ISO 7 standards.

3.3 SMC Operation

Key steps:

  • Set rotor speed to ~2,000 rpm.

  • Adjust rotor-stator gap until a specific audible tone indicates optimal proximity.

  • Circulate slurry continuously.

  • At timed intervals, withdraw samples for morphological analysis using the MorFi Analyzer.

  • Track refining progress via % fines content (fibers <200 µm).

The system operated until 90% fines were achieved, yielding a uniform CNF slurry.

4. CNF Sheet Formation and Characterization

4.1 Sheet Casting

The 2 wt% CNF slurry was poured onto stainless steel plates using a casting knife (~1.9 mm thickness). Sheets dried overnight under ambient conditions and were removed carefully with blades.

4.2 Sheet Thickness

Twelve sheets were measured:

  • Average thickness: ~56.2 µm.

  • Deviation: <3% — confirming consistency.

  • Thickness was adjustable by modifying slurry solids content or casting knife height.

4.3 Mechanical Testing: Tensile Strength

Using Instron tensile testing and TAPPI protocols:

  • Young’s modulus (parallel to casting direction): 4.99–5.71 GPa.

  • Perpendicular samples: 4.62–5.10 GPa.

  • Statistically significant directional difference (p < 0.001), attributed to the casting direction.

4.4 Surface Roughness

Measured via a Tencor Alphastep 500 profilometer:

  • Air-dried surface: rougher (Ra ≈ 3.17 µm).

  • Steel-contact side: smoother (Ra ≈ 0.70 µm).

  • Surface direction and plate did not significantly affect results.

4.5 Transparency

CNF sheet transparency was controlled via:

  • Solids content: thinner sheets were more transparent.

  • Calendaring: further improved transparency via hot rolling.

  • Transparency is crucial for applications like ocular membranes or biosensors.

4.6 Air Permeability

Using a Gurley densometer:

  • CNF sheets were “Too dense to read” (air flow time exceeded 50,000 s).

  • Indicates excellent air barrier performance, beneficial for wound dressings or barrier membranes.

4.7 Porosity via Mercury Porosimetry

  • CNF sheets had no through-pores.

  • Small voids were due to surface roughness, not actual porosity.

  • Confirms impermeable sheet formation, aligning with densometer data.

5. CNF Slurry Characterization

5.1 Fines Content via MorFi Analyzer

  • CNF samples diluted and analyzed.

  • Goal: reach ≥90% fines.

  • Calibration curves showed direct correlation between energy input and fines content, enabling process reproducibility.

5.2 Viscosity Behavior

  • As fines increase, viscosity rises exponentially.

  • CNF slurries are shear-thinning, allowing processing at higher viscosities under shear.

  • Care needed to maintain flow and prevent blockages during production.

6. Biomedical Relevance

The resulting CNF materials demonstrated the following traits ideal for biomedical use:

  • Sterilizability (e.g., with ethylene oxide),

  • Mechanical integrity (GPa-range modulus),

  • Low air permeability (barrier functions),

  • Transparency,

  • Surface tunability.

The CNF sheets were successfully:

  • Formed into conduits,

  • Sterilized,

  • Implanted into over 100 mice and several non-human primates,

  • No signs of irritation or inflammation observed.

This confirms the biocompatibility and potential for clinical applications in nerve repair and tissue engineering.

7. Key Contributions of This Study

  1. Novel Continuous Production System

    • First report of a 14-liter continuous SMC-based refining system for CNF.

    • Fully compliant with GMP/GLP standards.

  2. Reproducibility

    • Consistent sheet thickness, fiber morphology, and mechanical performance.

    • Refining energy serves as a process control metric.

  3. Full Physical Characterization

    • Surface roughness, tensile strength, transparency, porosity, and viscosity—all evaluated in detail.

  4. Scalability

    • Production method is easily upscalable, critical for commercialization.

  5. Biomedical Viability

    • Successfully used in in vivo nerve regeneration, supporting translation from lab to clinic.

8. Conclusions

This work demonstrates a breakthrough in CNF production for biomedical use, showcasing a scalable, sterile-compliant method to produce mechanically refined CNF slurries and sheets. With consistent performance in mechanical, surface, and morphological attributes, these materials hold tremendous promise for next-generation biomedical scaffolds, implants, barriers, and regenerative devices.

The method enables flexibility in tuning properties by altering:

  • Solids content,

  • Refining energy,

  • Sheet casting techniques,

  • Post-processing (like calendaring).

The absence of air permeability and tight nanostructure further expand their utility in wound protection, membranes, and even electronics. The results bridge the gap between lab-scale CNF production and real-world clinical applications.

https://nanographenex.com/cellulose-nanofiber-cellulose-nanofibril-nanofibrillated-cellulose-cnfs/

https://nanographenex.com/aqueous-suspension-of-nanocrystalline-cellulose-ncc-5wt/

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