UAS Function & Anatomy
A Ureteral Access Sheath (UAS) is a two-component system — an inner tapered dilator and a braided outer sheath — designed to establish and maintain a patent conduit from the urethral meatus to the renal pelvis during ureteroscopic procedures. The UAS serves several simultaneous functions: providing a stable working channel for the ureteroscope, enabling repeated scope insertion and withdrawal without cumulative ureteral trauma, maximising irrigation outflow to control intrarenal pressure (IRP), and protecting the ureteroscope optics during insertion.
UAS Components
- Inner dilator — tapered, flexible; creates the ureteral pathway and the sheath is advanced co-axially over it
- Outer sheath — reinforced, maintains lumen patency; holds position in the ureter while the scope operates
- Funnel cone / inlet — proximal opening that guides the ureteroscope into the sheath lumen; critical for optic protection
- Radiopaque tip marker — confirms sheath tip position in renal pelvis under fluoroscopy
- Hydrophilic surface — coats dilator and sheath outer surfaces for atraumatic insertion
Clinical Engineering Requirements
- ①Low insertion force — hydrophilic surface + tapered tip allow passage without excessive ureteral force
- ②Kink resistance — sheath must maintain open lumen through ureteral angulation during operation
- ③Maximum irrigation outflow — annular space between scope OD and sheath ID drives IRP
- ④Scope access fidelity — ID tolerance must accommodate scope OD + irrigation flow without binding
- ⑤Atraumatic removal — seamless dilator-to-sheath transition avoids mucosal stripping on withdrawal
The UAS–IRP Relationship
Intrarenal pressure (IRP) is the most clinically critical variable during ureteroscopic stone surgery — exceeding 30 cmH₂O significantly increases the risk of pyelovenous reflux and sepsis. The UAS is the primary IRP management tool: the annular space between the ureteroscope outer diameter and the sheath inner diameter creates the passive outflow path. A wider annular space (larger ID–to–scope-OD differential) allows greater flow, reducing IRP. The engineering trade-off is that a larger sheath OD imposes greater ureteral stretch — hence the dual specification of sheath ID and OD, and why 12F vs. 14F decisions are clinically significant.
Key Design Parameters
The dimensional specification of a UAS is the foundation of its entire performance envelope. Small dimensional changes (±0.1 mm) can have significant clinical consequences for scope compatibility, irrigation flow rate, and insertion force.
Manawa UAS — Engineering Specification Reference
| Parameter | 9.5F Sheath | 12F Sheath |
|---|---|---|
| Sheath outer diameter (OD) | 11.5F (~3.8 mm) | 14F (~4.6 mm) |
| Sheath inner diameter (ID) | ≥ 9.5F (~3.1 mm) | ≥ 12F (~4.0 mm) |
| Maximum scope accepted | 10F | |
| Working length | Standard / Long variants | Standard / Long variants |
| Guidewire compatibility | Up to 0.038" | Up to 0.038" |
| Radiopaque markers | Sheath tip (1 band) | Sheath tip (1 band) |
| Flexibility transition length flex/stiff | Up to 10cm | Up to 10cm |
French (F) to mm Conversion Note
The French (Fr or F) unit is the standard sizing system for urological catheters: 1 French = 0.333 mm diameter. A 12F sheath has an OD of approximately 4.0 mm; a 14F has an OD of approximately 4.67 mm. The difference (~0.67 mm OD) significantly impacts insertion force through the uretero-vesical junction and the degree of ureteral stretch imposed during the procedure. Sheath OD tolerancing to ±0.05 mm is standard manufacturing requirement.
Material Selection
UAS material selection differs fundamentally from balloon catheter materials — the sheath must resist external radial compression (preventing lumen collapse when the ureter contracts) while remaining flexible enough to track the ureteral anatomy. The dilator must be rigid enough to dilate but flexible enough to navigate without causing trauma.
BODY
Pebax® (PEBA) or Polyurethane — Shore 80A up to 72D
The preferred material for UAS sheath bodies. Polyurethanes or Pebax offers a unique combination of flexibility, kink resistance, and biocompatibility. Grade selection (80A to 72D) determines shaft stiffness. The outer jacket of the Manawa sheath uses a combinaison of super soft and medium durometer grade applied over the length of the sheath to provide distal flexibility while maintaining sufficient pushability, with the distal tip transitioning to an even softer formulation for atraumatic positioning in the renal pelvis. The polymer jacket is reflown with PTFE inner liners and compatible with SS coil reinforcement.
LINER
PTFE (Polytetrafluoroethylene)
The inner surface of the sheath lumen is lined with PTFE for two reasons: (1) minimum coefficient of friction against the ureteroscope insertion and withdrawal — PTFE COF ≈ 0.04 dry, enabling repeated scope passage without lumen wear, and (2) chemical inertness — PTFE is unaffected by irrigation fluid, contrast agents, or laser energy emitted through the scope. PTFE tubing is precision-extruded to ±0.02mm ID tolerance for consistent scope clearance.
Polyethylene
The dilator requires stiffness to transmit the axial advancement force necessary for ureteral dilation. Polyethylene provides the necessary column strength while still maintaining sufficient flexibility. The tapered tip is formed by thermal moulding into a rounded, atraumatic geometry — tip stiffness must be measurably lower than the dilator shaft to avoid urothelial injury during advancement.
BODY
SS Coil + Polymer Jacket — Distal Flexible Segment
For the Manawa Suction FANS-UAS, the sheath incorporates a Stainless Steel SS flat-wire coil reinforcement instead of the standard SS round-wire. The flat coil properties allow the sheath to flex sufficiently for lower calyx access — following the 270°+ angle of the ureteroscope — while recovering to its resting straight shape on retraction. This distal flexibility is the key FANS differentiator for lower pole stone access.
Hydrophilic Coating for UAS
The hydrophilic coating on the Manawa UAS is applied to both the outer surface of the sheath (for ureteral insertion) and the outer surface of the dilator (for co-axial advancement within the sheath and the ureter). When wetted, the coating reduces insertion friction by up to 10× compared to an uncoated surface, enabling atraumatic advancement without excessive ureteral dilation force.
Coating Specification for UAS Applications
Sheath outer surface coating
- → Full-length coating from tip to proximal taper
- → PVP cross-linked to Pebax substrate (UV cure)
- → Wet COF: ≤ 0.06 (vs. urothelium surrogate)
- → Durability: 10 insertion cycles at 5N axial force
- → Must survive EO sterilization without delamination
Dilator outer surface coating
- → Applied to dilator body and tip region
- → Must not interfere with seamless dilator-sheath transition
- → Coating thickness: 5–15 µm dry (validated by XRF)
- → Biocompatibility: ISO 10993-5 cytotoxicity pass
- → No coating leaching detectable in simulated use extraction
Seamless Dilator-to-Sheath Transition
A critical engineering detail is the transition zone where the dilator tip meets the sheath inlet. Any step, ridge, or discontinuity at this transition can engage urothelial tissue during insertion, causing mucosal stripping or bleeding. The Manawa design achieves a seamless, flush transition through precision dilator-to-sheath OD matching (ΔOD ≤ 0.05 mm at the transition zone) and a distal sheath geometry that gently flares to meet the dilator taper — eliminating any catch point on insertion or withdrawal.
Tip & Dilator Design — Atraumatic Engineering
The dilator tip is the leading edge of the UAS system and the first component to engage ureteral tissue. Tip design governs the radial dilation force distribution during insertion, the risk of ureteral perforation, and the guidewire tracking characteristics of the system.
Taper Angle Engineering
The dilator taper angle (typically 15–25° half-angle) determines how rapidly the cross-sectional area increases as the device advances. A shallower taper (lower angle) distributes radial dilation force over a longer distance — reducing peak tissue stress. A steeper taper provides shorter device length but concentrates force. For the Manawa dilator, a graduated taper design uses a shallower initial taper (20 mm length) followed by a faster body taper, balancing atraumatic entry with a compact overall device length.
Rounded Tip Geometry
The dilator tip terminates in a rounded, hemispherical or elliptical nose — not a pointed or chisel profile. The rounded geometry cannot penetrate an intact ureteral wall; it can only dilate by progressively expanding the lumen. The tip durometer must be measurably softer than the dilator shaft (typically 15–20 Shore D lower) to allow deformation on contact with tissue before any mucosal injury threshold is reached. Tip stiffness is validated via a 10 g axial load deflection test against a rigid flat surface.
Guidewire Tracking at the Tip
The dilator tip must track coaxially over the guidewire through the uretero-vesical junction — the tightest anatomical point in the urinary tract. The GW lumen exit at the dilator tip must be centred (eccentricity ≤ 0.1 mm) to prevent the guidewire from deflecting the tip off-axis. A PTFE-lined tip inner lumen with a smooth, chamfered entry reduces guidewire binding at the tip — validated by a 0.038" GW pull-through force test (acceptance: ≤ 1N over full dilator length including tip traversal).
Reinforcement Architecture
UAS reinforcement serves a different function to balloon catheter reinforcement. Where a balloon shaft needs braid reinforcement primarily for kink resistance under bending, a UAS sheath needs reinforcement primarily for radial rigidity — resistance to lumen collapse under the compressive load of ureteral peristalsis and sphincter tone.
Sheath Reinforcement Options
| Reinforcement Type | Radial Strength | Kink Resistance | Wall Addition |
|---|---|---|---|
| SS flat-wire braid (45°) | Excellent | Excellent | +0.10–0.15 mm |
| Nitinol coil (distal) | Moderate | Excellent + flexible | +0.08–0.12 mm |
| Stainless steel coil | Good | Good | +0.10–0.18 mm |
| No reinforcement (tip zone) | Low | Low — intentional | No addition |
Braid-to-Tip Transition Engineering
The braid reinforcement must terminate proximal to the sheath tip — the distal 5–10 mm is intentionally unreinforced to allow the tip to flex and conform to the renal pelvis without exerting radial trauma. However, this unreinforced zone creates a potential stress concentration at the braid terminus. The Manawa sheath design uses a tapered braid terminus (progressively reducing picks-per-inch over 15 mm) to create a gradual stiffness transition rather than an abrupt step, eliminating the risk of kinking at the reinforcement endpoint.
Suction-Assisted FANS-UAS Technology
The Manawa Suction FANS-UAS (Flexible Active-suction Navigable Sheath) represents a next-generation UAS architecture that addresses the primary remaining challenge in ureteroscopic stone surgery: intrarenal pressure control in the lower pole. Standard UAS devices rely on passive outflow — the natural hydrostatic drainage through the annular space around the scope. The FANS-UAS adds active suction to the outflow path, dramatically augmenting IRP management.
Standard UAS — Passive Outflow
Irrigation fluid flows in through the scope working channel. Outflow is passive through the annular space between scope OD and sheath ID. IRP is determined by the outflow resistance of this annular space — influenced by scope size, sheath ID, fragment accumulation, and fluid viscosity. At high irrigation rates, passive outflow may be insufficient to maintain IRP < 30 cmH₂O.
FANS-UAS — Active Suction
A dedicated suction port on the sheath hub connects to a vacuum source. The leak-free valve and pressure ventilation slider allow the surgeon to modulate suction intensity continuously. Active suction actively draws fluid from the collecting system, keeping outflow > inflow and maintaining IRP well below the critical threshold — even at high irrigation rates or with stone fragment accumulation.
FANS-UAS Engineering Innovations
Innovative funnel-like cone design
The proximal funnel cone guides scope entry while protecting the flexible ureteroscope optics from impact damage during insertion — a critical feature given the replacement cost of modern flexible ureteroscopes (€25,000–€90,000).
Leak-free valve design
The suction port incorporates a silicone valve that seals around the scope shaft during operation — preventing ambient air entrainment that would reduce suction efficiency. Validated to maintain negative pressure >150 mmHg at the sheath hub with a 10F scope inserted.
Pressure ventilation slider
A precision-machined slider valve in the suction line allows stepless adjustment of suction intensity between 0% and 100% — enabling the surgeon to dial in the exact outflow needed for the case without interrupting the procedure to adjust wall suction settings.
Superior distal flexibility for lower calyx access
The Nitinol-reinforced distal sheath segment flexes to accommodate the acute deflection angle of the ureteroscope into the lower pole — enabling FANS-UAS placement to support lower pole stone management, previously limited to standard (non-FANS) sheaths due to stiffness constraints.
Lumen & Flow Engineering
The annular irrigation outflow space between the ureteroscope OD and the UAS inner lumen ID is the governing hydraulic parameter for passive IRP control. Its cross-sectional area, lumen surface roughness, and length combine to determine the Hagen-Poiseuille flow resistance of the outflow path.
Hagen-Poiseuille Outflow — Key Relationships
Annular area formula
A = π/4 × (ID²sheath − OD²scope)
Flow rate scales with A2 for laminar flow — small increases in annular gap have outsized flow improvement. Going from 10F scope in 12F sheath to 10F scope in 14F sheath doubles the annular area and dramatically reduces outflow resistance.
Practical flow impacts
- → 10.7F sheath + 10F scope: ~limited (tight fit) outflow
- → 12F sheath + 10F scope: ~significantly improved outflow
- → 10.7 sheath + 7.5F scope: ~best in class outflow
- → FANS + any scope: active suction overcomes all passive-flow IRP concerns
PTFE Inner Lining — Surface Roughness & Flow
The PTFE inner liner of the Manawa sheath provides not only low scope-insertion friction but also a smooth hydraulic surface for irrigation outflow. PTFE has a surface roughness (Ra) of approximately 0.1–0.4 µm — significantly lower than uncoated Pebax (~0.8–1.5 µm). This reduces boundary-layer turbulence in the annular outflow space, increasing effective flow rate by approximately 8–12% compared to an unlined sheath of identical geometry at equivalent pressure differentials.
Large Internal Lumen Design — Manawa Engineering Principle
The Manawa range is engineered to the maximum possible internal lumen for each sheath OD size — a conscious design choice prioritising irrigation outflow and scope clearance over wall thickness minimisation. The 12F Manawa accepts up to a 10F ureteroscope; the 14F Manawa accepts up to a 12F ureteroscope. Both specifications are validated on the largest commercially available ureteroscopes in each class, ensuring compatibility without lumen binding during clinical use.
Envaste Manawa™ UAS Range
The Manawa range applies the engineering principles documented in this guide across three device configurations — each optimised for a distinct urological access requirement.
Manawa UAS
Standard reinforced ureteral access sheath. 12F & 14F. Hydrophilic coated, large inner lumen, seamless dilator transition.
View productManawa Suction FANS-UAS
Active suction, flexible distal segment, funnel cone, leak-free valve. IRP management for complex stone cases.
View productManawa Nephrostomy Suction
Percutaneous nephrostomy access with integrated suction. Designed for PCNL working sheath placement.
View product